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In the light-independent reaction of photosynthesis, one of the products is glyceraldehyde 3-phosphate, and the Wikipedia page on the light-independent reactions states that 6 of these can be used to form glucose. However, the Wikipedia article on gluconeogenesis does not mention this, and the Wikibooks article on gluconeogenesis only mentions the molecule glycerinaldehyde 3-phosphate (which I'm not sure if it's a typo, because I can't find any information on glycerinaldehyde).
So how does G3P actually become glucose (and is there a good reason this information is not available on those Wikipedia pages, plus the page on G3P)?
Advice to students of biochemistry
This site is concerned with biology, not with biological entries in Wikipedia. Wikipedia is a voluntary effort to which anyone may contribute, and is full of errors and omissions. It's structure means that it is focussed on individual small topics, rather than presenting an integrated account of various areas of science. The student who wishes a balanced integrated account of a topic that has been subjected to editorial review should consult a text book. Those whose resources do not permit this should try searching on NCBI Bookshelf, which provides free on-line search-only access to old editions of texts. For biochemistry, Berg et al. is recommended.
The generation of glucose from triose produced in the dark reaction is well understood
The answer to the question can be found in e.g. Berg et al. 20.1.3:
The 3-phosphoglycerate product of rubisco is next converted into three forms of hexose phosphate: glucose 1-phosphate, glucose 6-phosphate, and fructose 6-phosphate… The steps in this conversion (Figure 20.9) are like those of the gluconeogenic pathway (Section 16.3.1), except that glyceraldehyde 3-phosphate dehydrogenase in chloroplasts, which generates glyceraldehyde 3-phosphate (GAP), is specific for NADPH rather than NADH. Alternatively, the glyceraldehyde 3-phosphate can be transported to the cytosol for glucose synthesis.
Berg et al. Fig. 20.9
The Section 16.3.1 referred to is a general treatment of gluconeogenesis, which is the same in all organisms or cells that possess the enzymes to catalyse its unique steps. (Some cells may not have - or need to have - the initial steps from pyruvate. In fact, Fig. 20.9 shows the steps to G 6-P - the only step missing is the phosphatase step that generates glucose.) There is therefore no reason to expect an account specific for plants.
Glucose is made from the trioses (3-carbon sugars) in plants according to the usual gluconeogenesis pathway. That is, glyceraldehyde phosphate is converted to fructose-1,6-diphosphate by triose phosphate isomerase and aldolase, and then dephosphorylated to obtain hexose phosphates. Free glucose is not usually the end product in plants though; instead, glucose is coupled to ADP for use in synthesis of starch.
I'm afraid I did not find any good open-access references on plant gluconeogenesis, but it should be covered in most biochemistry textbooks.
Step 1: Hexokinase
The first step in glycolysis is the conversion of D-glucose into glucose-6-phosphate. The enzyme that catalyzes this reaction is hexokinase.
Here, the glucose ring is phosphorylated. Phosphorylation is the process of adding a phosphate group to a molecule derived from ATP. As a result, at this point in glycolysis, 1 molecule of ATP has been consumed.
The reaction occurs with the help of the enzyme hexokinase, an enzyme that catalyzes the phosphorylation of many six-membered glucose-like ring structures. Atomic magnesium (Mg) is also involved to help shield the negative charges from the phosphate groups on the ATP molecule. The result of this phosphorylation is a molecule called glucose-6-phosphate (G6P), thusly called because the 6′ carbon of the glucose acquires the phosphate group.
The 10 Steps of Glycolysis
There are 10 steps of glycolysis, each involving a different enzyme. Steps 1 – 5 make up the energy-requiring phaseof glycolysis and use up two molecules of ATP. Steps 6 – 10 are the energy-releasing phase,which produces four molecules of ATP and two molecules of NADPH. The net products of glycolysis are two molecules of pyruvate, two molecules are ATP, and two molecules of NADPH.
Metabolism of Carbohydrates: Catabolism and Anabolism (With Diagram)
Let us make an in-depth study of the metabolism of carbohydrates. The metabolism of carbohydrates is done through two processes: A. Catabolic Processes and B. Anabolic Processes. The catabolic processes of carbohydrates include: 1. Glycolysis 2. Citric Acid Cycle 3. Glycogenolysis 4. HMP Pathway or Pentose Phosphate Pathway and 5. Uronic Acid Pathway. The anabolic processes of carbohydrates include: 1. Glycogenesis and 2. Gluconeogenesis.
Metabolism of carbohydrates in the cell:
Metabolism is a complex process of breakdown and syn­thesis of the biomolecules inside the cell. Breakdown of molecules is known as catabolism and synthesis is termed as anabolism.
The catabolic processes of carbohydrates includes:
(4) Hexose monophosphate pathway and
The anabolic processes of carbohydrates include:
A. Catabolic Processes:
Glycolysis is the breakdown (lysis) of glucose to pyruvic acid under aerobic conditions and to lactic acid under anaerobic conditions.
Anaerobic glycolysis is also termed as Embden-Meyerhof pathway (EMP), after the scientists who proposed it. Glycolysis occurs in the cytosol of the cell and is initiated when the ATP level of the cell is low.
It can be divided into two stages viz.:
In stage one, one molecule of glucose in converted into two molecules of D-glyceraldehyde-3-phosphate. Glucose is either cleaved from the glycogen molecule or enters the cell individually and is phosphorylated to glucose-6-phosphate by converting ATP to ADP with the help of the enzyme hexokinase/glucokinase.
The phosphorylation of glucose serves two purposes. First, it makes the glucose molecule more reactive and ready for other reactions. Second, because phosphorylated compounds cannot pass through the cell membrane, phosphorylation keeps the glucose inside the cell. The six carbons in glucose- 6-phosphate structure need to be rearranged to form fructose-6-phosphate so that it can split into two structures of 3 carbons each.
The new compound, fructose-6-phosphate is phosphorylated again so that each of the 2, three carbon units have a phosphate group attached to them. The conversion of fructose- 6-phosphate to fructose-6-disphosphate via phosphofructokinase is the primary regulation point of glycolysis. The final step of stage one is the splitting of fructose-6-disphosphate into 2 molecules of glyceraldehyde-3-phosphate.
Stage 2 of glycolysis is designed to liberate inorganic phosphate for the synthesis of ATP and to convert the glyceraldehyde’s into pyruvate. Glyceraldehyde is oxidized, in other words a hydrogen atom is removed from it, and phosphorylated to produce 1,3-diphosphoglycerate.
The NADH carries the hydrogen to the electron transfer system for the production of 3 ATPs. In the next four reactions, four additional ATPs are synthesized (two each from both the three carbon compounds), before the final product of glycolysis i.e. pyruvate is formed. The three-carbon structure of pyruvate has several fates depending upon the energy state of the cell.
In anaerobic glycolysis, NADH + H + is not oxidized through the electron transport chain, instead is oxidized by lactate dehydrogenase, hence no production of 6 ATPs i.e., ATPs are produced less in number.
ATPs produced in anaerobic glycolysis = 4 (7 th & 10 th step)
ATPs utilized in anaerobic glycolysis = 2 (1 st & 3 rd step)
Therefore, only two (2) ATPs are produced in anaerobic glycolysis of glucose.
Overall reaction stoichiometry/chemical summary of reaction:
Glucose + 2ATP + 2P, + 2ADP + 2NAD + → 2Pyruvate + 2NADH + 2H + + 4ATP + 2H2O
Glucose + 2Pi+ 2ADP + 2NAD + 2Pyruvate + 2NADH + 2H + + 2ATP + 2H2O
Under anaerobic conditions:
Glucose + 2 Pi + 2ADP 2 Lactate+ 2 ATP + 2H2O
(Regenerates NAD + allowing reaction to continue in the absence of oxygen)
Anaerobic glycolysis generating lactate vs complete oxidation of glucose:
Salient features of glycolysis:
It is the main route for glucose metabolism. It occurs in all the cells of the body. Brain and RBC depend only on glucose for oxidation and production of energy. In brain aerobic glycolysis occurs whereas in RBC there is always anaerobic glycolysis (due to the absence of mitochondria), leading to the production of lactic acid.
In skeletal muscle aerobic glycolysis occurs in normal conditions but during vigorous muscular contraction, anaerobic glycolysis is the major pathway for energy production. Although glycolysis can occur either aerobically or anaerobically, humans use aerobic glycolysis for about 90% of the time. Glycolysis can be initiated via glucose entering the cell from the blood or glucose arising from the breakdown of glycogen.
In human muscle, glycolysis is almost always initiated from the breakdown of glycogen. Since the human brain does not store glycogen, glycolysis is initiated in this tissue from blood glucose. The initiation of glycolysis is regulated by the ATP concentration in the cytoplasm. When the concentration of ATP is high and ADP is low, glycolysis is inhibited. Specifically, the enzyme phosphofructokinase is inhibited by large ATP/ADP ratio. When the concentration of ATP is low and ADP is high, glycolysis is stimulated.
Glycolysis in RBC—The Rapaport-Lumbering cycle:
Erythrocytes metabolize excessive amounts of glucose by the glycolytic pathway. This generates much ATP which is not required and cannot be used by erythrocyte.
Thus if ATP production by substrate phosphorylation is prevented by taking diversion pathway, it will:
(1) Reduce the production of ATP and
(2) Supply 2, 3-diphosphoglycerate required for the haemoglobin function which helps in de-loading of oxygen in the tissues.
Hence 1, 3-diphosphoglycerate formed in normal glycolysis is not converted to 3-phosphoglycerate, instead it takes a bypass route through 2,3-diphosphoglycerate, as under—
Pyruvate is an important regulatory point for energy production. The ultimate fate of pyruvate depends on the energy state of the cell and the degree of oxidative phosphorylation taking place. When the energy state of the cell is low (high ADP low ATP), pyruvate enters the TCA cycle as acetyl-CoA via the pyruvate dehydrogenase complex and oxidized completely to CO2 & H2O to yield energy.
The pyruvate dehydrogenase complex is one of the most complex proteins in the body and consists of more than 60 subunits. When the energy state of the cell is high, the regulator of glycolysis is the enzyme phospho­fructokinase, and thus there is limited pyruvate in the cell.
However, if pyruvate is present during the time of high-energy states, such as the liver metabolism of fructose, pyruvate is transformed into acetyl- CoA and is packaged as lipid. If oxygen to the cell is limiting, such as during intensive exercise, glycolysis proceeds anaerobically and pyruvate is converted to lactate by the lactate dehydrogenase enzyme. Finally, pyruvate can be converted into the amino acid alanine via transamination.
Pyruvate dehydrogenase complex is a multi-enzyme complex made up of 3 enzymes viz:
(2) Dihydrolipoyl Transacetylase and
(3) Dihydrolipoyl Dehydrogenase.
This reaction requires five coenzymes viz.:
(iv) Flavin adenine dinucleotide (FAD) and
(v) Nicotinamide adenine dinucleotide (NAD + ).
Acetyl-CoA formed in the above reaction may take part either in its oxidation to carbon dioxide and water, through TCA cycle, or formation of lipids, or synthesis of cholesterol etc. etc., which depends upon the nutritional state of the body and the type of the cell where it is formed.
2. Krebs’s Cycle/Citric Acid Cycle/TCA Cycle:
Citric acid cycle also known as tricarboxylic acid (TCA) cycle is named after the scientist Sir Hans Krebs (1900-1981) who discovered it. He proposed the key elements of this pathway in 1937 and was awarded the Nobel Prize in Medicine for the discovery in 1953.
Krebs’s cycle is a set of continuous reactions (8 steps) occurring in a cyclic manner in the mitochondrial matrix in eukaryotes and within the cytoplasm in prokaryotes. Acetyl-CoA, the fuel of TCA cycle, enters the citric acid cycle inside the mitochondrial matrix, and gets oxidized to CO2 and H2O while at the same time reducing NAD to NADH and FAD to FADH2. The NADH and FADH2 can be used by the electron transport chain to create ATP.
In step 1, the two-carbon compound, acetyl-S-CoA, participates in a condensa­tion reaction with the four-carbon compound, oxaloacetate, to produce citrate, a six carbon com­pound catalysed by the enzyme citrate synthase. This is the first stable tricarboxylic acid in the cycle and hence the name TCA cycle.
Isomerization of citrate:
Step 2 involves moving the hydroxyl group in the citrate molecule so that it can later form an α-keto acid. This process involves a sequential dehydration and hydration reaction, to form the D-isocitrate isomer (with the hydroxyl group now in the desired α-location), with cis-aconitase as the intermediate. A single enzyme, aconitase performs this two-step process.
Generation of CO2 by an NAD linked enzyme:
Oxidative decarboxylation takes place in the next reaction. The reaction is catalysed by the enzyme isocitrate dehydrogenase. The reaction in­volves dehydrogenation to oxalosuccinate, an unstable intermediate which spontaneously decarboxylates to give α-ketoglutarate. In addition to decarboxylation, this step produces a reduced nicotina­mide adenine dinucleotide (NADH) co factor, or a reduced nicotinamide adenine dinucleotide phos­phate (NADPH) cofactor.
A second oxidative decarboxylation step:
This step is performed by a multi-enzyme complex, the α-ketoglutarate dehydrogenation complex. The multi-step reaction performed by the α-ketogl­utarate dehydrogenation complex is analogous to the pyruvate dehydrogenase complex, i.e. an α-keto acid undergoes oxidative decarboxylation with formation of an acyl-CoA i.e. succinyl-CoA.
Succinyl-CoA is a high potential energy molecule. The energy stored in this molecule is used to form a high energy phosphate bond in a guanine nucleotide diphos­phate (GDP) molecule. Most of the GTP formed is used in the formation of ATP, by the action of nucleoside di-phosphokinase.
The succinate produced by succinyl CoA-synthetase in the prior reaction needs to be converted to oxaloacetate to complete the Krebs’s cycle. The first step in the conversion is the dehydrogenation of succinate to yield fumarate facilitated by the enzyme succinate dehydrogenase. FAD is covalently bound to the enzyme (via a histidine residue) which is converted to FADH2 that is oxidized through the ETC producing 2 ATPs.
Hydration of a carbon-carbon double bond:
Fumarate undergoes a stereo-specific hydration of the C=C double bond, catalysed by fumarate hydratase (also known as fumarase), to produce L- malate.
Dehydrogenation reaction that will regenerate oxaloacetate:
L-malate (malate) is dehydrogenated to produce oxaloacetate by the enzyme malate dehydrogenase during which one molecule of NAD + is converted to NADH 4- H + . The formation of oxaloacetate completes the Krebs’s cycle
The sum of all reactions in the citric acid cycle is
Acetyl-CoA + 2H2O + 3NAD + + Pi + GDP + FAD à 2CO2 + 3NADH + GTP + CoASH + FADH2 + 2H +
Number of ATP’s produced in one TCA cycle:
The TCA cycle produces 3 NADH + H + and one FADH2, these are known as the reducing equivalents. These reducing equivalents are oxidized through the electron transport chain. When NADH is oxidized through ETC it produces 3 ATPs and oxidation of FADH2 through ETC produces 2 ATPs.
Regulation of TCA cycle:
The regulation of the TCA cycle is largely determined by substrate availability and product inhibition.
i. NADH, a product of dehydrogenases in the TCA cycle, inhibits pyruvate dehydrogenase, isoci­trate dehydrogenase and α-ketoglutarate dehydrogenase and also citrate synthase.
ii. Succinyl-CoA inhibits succinyl-CoA synthase and citrate synthase. ATP inhibits citrate synthase and α-ketoglutarate dehydrogenase.
iii. Calcium is used as a regulator, it activates isocitrate dehydrogenase and a-ketoglutarate dehydro­genase. This increases the reaction rate of many of the steps in the cycle, and therefore increases flux throughout the pathway.
Importance of citric acid cycle or amphibolic role of TCA cycle:
TCA cycle is the common pathway for the oxidation of carbohydrates, fats and proteins (catabolic role). The anabolic role is synthesis of various carbohydrates, amino acids and fats. As it takes part both in anabolism and catabolism, it is said to be amphibolic pathway of metabolism.
It is the replenishment of the depleted intermediates of TCA cycle. As the TCA cycle takes part in the anabolic reactions, the intermediates of TCA cycle are utilized for the synthesis of various compounds. This results in the deficiency of one or more of the TCA cycle intermediates.
In order to continue the TCA cycle, those intermediates, which are deficient, must be filled up by some other process and this process is known as anaplerosis. For example oxaloacetate is utilized for the synthesis of the amino acid aspartic acid and oxaloacetate is replaced via anaplerosis by carboxylation of pyruvate to oxaloacetate by the enzyme pyruvate carboxykinase.
Total number of ATPs produced when glucose is completely oxidized to CO2 and H2O
(2) 2 pyruvate 2 acetyl-CoA 2 NADH → 3ࡨ = 6 ATP
(3) 2 cycles of citric acid cycle for the 2 acetyl-CoA → 12ࡨ = 24 ATP
(a) Total 38 ATPs are formed when one molecule of glucose is completely oxidized to CO2 and H2O.
(b) Net gain of 36 ATP is seen when NADH produced in glycolysis in the step catalysed by glyceraldehyde-3-phosphate dehydrogenase in the cytosol is transported to the mitochondria for oxidation in ETC, facilitated by glycerol phosphate shuttle instead of malate-aspartate shuttle.
(c) Net gain of 39 ATP does occur when glucose present in the glycogen is directly oxidized.
There are some reactions that take place in the cytosol which produce NADH. These NADH have to be oxidized through the electron transport chain situated in the inner mitochon­drial membrane. NADH is not permeable to the mitochondrial membrane therefore shuttle systems operate for its transport.
There are three shuttle mechanisms:
(1) Glycerophosphate shuttle
(2) Malate- Aspartate shuttle and
Glycogen is a polysaccharide made up of glucose. It is the storage form of glucose in the body. Glucose requires more water for storage, but glycogen can be stored with much less amount of water hence glucose is stored as glycogen in the cell.
The largest amount of glycogen is stored in the liver and muscle. Liver glycogen provides glucose to other cells and maintains the blood glucose level in normal amounts. Muscle glycogen serves as readily available source of glucose during vigorous exercise, for glycolysis in the muscle itself. Glycogen metab­olism includes glycogenesis and glycogenolysis.
Breakdown of glycogen to glucose is known as glycogenolysis.
Glycogen phosphorylase is the key enzyme of glycogenolysis. It acts only on α-1 → 4 glycosidic linkages and thus releases glucose units one by one from the linear chain, till two or three or four glucose units near the branching point are left over.
The remaining three glucose units linked by α-1 → 4 linkages are transferred to another linear chain by the enzyme glucan transferase, thus leaving one glucose residue linked with α-1 → 6 glycosidic linkage, which is acted upon by de-branching enzyme (amylo-l,6-glycosi- dase) and thus releasing free glucose. If glycogen is subjected to the action of phosphorylase alone, it will result in the formation of a glycogen molecule with each branch having only 4 glucose units which is called the ‘limit dextrin’.
Regulation of glycogen metabolism:
Glycogen metabolism is reciprocally regulated, mainly by the action of hormones. At the time of shock and excitement, epinephrine stimulates glycogenolysis, both in muscle and liver, whereas glucagon stimulates glycogenolysis only in the liver under hypoglycemic conditions. Insulin inhibits glycogenolysis and promotes glycogenesis.
Glycogen storage diseases:
Glycogen storage diseases are a group of inherited disorders characterized by deficient mobilization of glycogen and deposition of abnormal forms of glycogen.
4. HMP Pathway or Pentose Phosphate Pathway:
Hexose monophosphate shunt pathway or the HMP pathway is an alternative pathway for glucose oxidation. It neither utilizes nor produces ATP.
The main purpose or significance of this pathway is:
I. It produces the reducing equivalents NADPH + H + , for the synthesis of lipids (fatty acids and steroids) and keeps glutathione in reduced state in RBC.
II. It generates ribose sugar (pentose phosphate) for the formation of nucleic acids.
The organs in which HMP pathway occurs are those which are actively concerned with lipid synthesis, like the adipose tissue, kidney, lactating mammary gland, liver, RBC, thyroid and gonads. It takes place in the cytosol.
The steps involved in this pathway are:
Transfer of 2-carbon moiety i.e., active glycelaldehyde is known as transketolation. It is catalysed by the enzyme transketolase and the coenzyme is Thaimine pyrophosphate (TPP). In thiamine deficiency (also in pernicious anemia) transketolase activity is decreased in blood.
Transfer of 3-carbon moiety i.e., active dihydroacetone is known as transaldoladon. It is catalysed by the enzyme transaldolase.
5. Uronic Acid Pathway:
This is a synthetic pathway for the various uronic acids.
1. It produces glucuronic acid which takes part in detoxification of bile pigments, phenols, aromatic acids and steroid hormones.
2. It provides glucuronic acid and galacturonic acid for the formation of glycoproteins.
3. In lower animals this pathway leads to synthesis of ascorbic acids (vitamin C).
Metabolism of Fructose:
In the diet fructose is obtained from fruits, honey and table sugar (sucrose). In human body it is the sugar of the semen and amniotic fluid.
It is a genetic defect in which there is excretion of fructose in the urine due to the lack of the enzyme fructokinase.
A person shows disliking towards fruits and fructose rich diets due to the deficiency of the enzyme aldolase-B.
Metabolism of Galactose and Synthesis of Lactose:
In the diet, galactose is mainly derived from the milk sugar lactose. In the body it is converted to glycogen or may take part in the synthesis of the milk sugar lactose in lactating mammary gland.
Lactose intolerance type-II:
This is due to the deficiency of the enzyme galactose-1-phosphate uridyl transferase, which results in the accumulation of galactose in the blood i.e. galactosemia and excretion in the urine i.e. galactosuria. Such infants are intolerant to lactose and hence to milk. They show symptoms like diarrhoea and vomiting on giving milk. Lactose free milk is the only remedy.
B. Anabolic Processes:
The anabolic processes of carbohydrates are given below:
Synthesis of glycogen from glucose is known as glycogenesis. Glucose entrapped in the cell as glucoses-phosphate is mutated to glucose-1-phosphate by the enzyme phosphoglucomutase, which in turn is at­tached to UTP by the enzyme glucose-1-phosphate uridyl transferase (pyrophosphorylase) forming UDP- glucose.
Glycogen synthase adds glucose (the activated UDP-glucose) to the glycogen primer (preformed gly­cogen with a few glucose units) by making α-1 → 4 glycosidic linkages and thus forms a linear chain of 10 to 12 glucose residues, all linked by α-1 → 4 glycosidic linkage.
At this time another enzyme i.e. branching enzyme (glycosyl-(4 → 6) transferase) removes 6 to 7 glucose units from the linear chain and transfers them to the other chain and attaches by α-1 → 6 linkage, therefore creating a branching point. The process of addition of glucose by glycogen synthase to the linear chain and branching enzyme creating the branching points is repeated and thus glycogenesis is completed.
Gluconeogenesis is the formation of glucose from non-carbohydrate sources. Gluconeogenesis helps to maintain the glucose level in the blood, so that the brain, RBC and muscle can extract glucose from it to meet their metabolic demands when dietary glucose is low. This process is very much necessary in the body because brain and RBC utilizes only glucose as energy fuel.
The major non-carbohydrate precursors of glucose are lactate, glucogenic amino acids (all except leucine) and glycerol. Lactate is formed by RBC in glycolysis because mitochondria are absent. Lactate is also formed by active skeletal muscle when the rate of glycolysis exceeds the rate of TCA cycle, the pyruvate formed is converted to lactate.
Amino acids are derived from proteins in the diet and during starvation, from the breakdown of proteins in skeletal muscle.
Glycerol is derived from the hydrolysis of triacylglycerol’s (TAG).
Gluconeogenesis occurs mainly in liver and kidney. It also occurs in brain and muscle to some extent.
Gluconeogenesis occurs during:
(2) To clear lactate formed in RBC and muscle,
(3) When carbohydrates in the diet are low,
Gluconeogenesis is almost the reversal of glycolysis excepting at three steps which are irreversible in glycolysis. These steps are reversed by enzymes known as the key enzymes of gluconeogenesis i.e. those enzymes specific for gluconeogenesis only but not for any other pathway.
The key enzymes of gluco­neogenesis are:
1. Pyruvate carboxylase (or carboxykinase)
2. Phosphoenol pyruvate carboxykinase
The process of gluconeogenesis is as follows:
Role of 2, 6-Bio-phosphate in gluconeogenesis:
Fructose 2, 6-bisphosphate (or fructose 2, 6-diphosphate), is a metabolite which allosterically affects the activity of the enzymes phosphofructokinase 1 (PFK-1) and fructose 1, 6-bisphosphatase (FBPase-1) to regulate glycolysis and gluconeogenesis. Fructose 2, 6-bisphosphate is synthesized and broken down by the bi-functional enzyme, phosphofructokinase 2/fructose 2, 6-bisphosphatase (PFK-2/FBPase-2).
The synthesis of Fructose 2, 6-bisphosphate is performed through the phosphorylation of fructose 6-phosphate using ATP by the PFK-2 portion of the enzyme. The breakdown of Fructose 2, 6-bisphosphate is caused by dephosphorylation, catalyzed by FBPase-2 to produce Fructose 6-phosphate and Pi. Fructose 2, 6-bisphosphate stimulates glucose breakdown further through reduction of gluconeogenesis through allosteric inhibition of fructose 1, 6-bisphosphatase.
Hormones that regulate gluconeogenesis:
Gluconeogenesis is stimulated by:
4. Epinephrine also stimulates but to a lesser extent
Gluconeogenesis is inhibited by:
Conversion of muscle glycogen to liver glycogen through blood lactate and back to muscle glycogen through blood glucose is known as Cori’s cycle.
Glucose first converts to glucose-6-phosphate by hexokinase or glucokinase, using ATP, with the addition of a phosphate group. Glucokinase is a subtype of hexokinase found in humans. Glucokinase has a reduced affinity for glucose and is found only in the pancreas and liver, whereas hexokinase is present in all cells. Glucose 6-phosphate is then converted to fructose-6-phosphate, an isomer, by phosphoglucose isomerase. Phosphofructose-kinase then produces fructose-1,6-bisphosphate, using another ATP molecule. Dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate are then created from fructose-1,6-bisphosphate by fructose bisphosphate aldolase. DHAP will be converted to glyceraldehyde-3-phosphate by triosephosphate isomerase, where now the two glyceraldehyde-3-phosphate molecules will continue down the same pathway. Glyceraldehyde-3-phosphate will become oxidized in an exergonic reaction into 1,3-bisphosphoglycerate, with the reduction of an NAD+ molecule to NADH and H+. 1,3-bisphosphoglycerate will then turn into 3-phosphoglycerate with the help of phosphoglycerate kinase, along with the production of the first ATP molecule from glycolysis. 3-phosphoglycerate will then convert, with the help of phosphoglycerate mutase, into 2-phosphoglycerate. Enolase, with the release of one molecule of H2O, will make phosphoenolpyruvate (PEP) from 2-phosphoglycerate. Due to the unstable state of PEP, pyruvate kinase will facilitate its loss of a phosphate group to create the second ATP in glycolysis. Thus, PEP will then undergo conversion to pyruvate.
Glycolysis occurs in the cytosol of the cell. It is a metabolic pathway that creates ATP without the use of oxygen but can occur in the presence of oxygenਊs well. In cells that use aerobic respiration as the primary source of energy, the pyruvate formed from the pathway can be used in the citric acid cycle and go through oxidative phosphorylation to undergo oxidation into carbon dioxide and water. Even if cells primarily use oxidative phosphorylation, glycolysis can serve as an emergency backup for energy or serve as the preparation step before oxidative phosphorylation. In highly oxidative tissue, such as the heart, the production of pyruvate is essential for acetyl-CoA synthesis and L-malate synthesis. It serves as a precursor to many molecules, such as lactate, alanine, and oxaloacetate.
Glycolysis precedes lactic acid fermentation the pyruvate made in the former process serves as the prerequisite for the lactate made in the latter process. Lactic acid fermentation is the primary source of ATP in animal tissues with low metabolic requirements and little to no mitochondria. In erythrocytes, lactic acid fermentation is the sole source of ATP, as they lack mitochondria and mature red blood cells have little demand for ATP. Another part of the body which relies entirely or almost entirely on anaerobic glycolysis is the lens of the eye, which is devoid of mitochondria, as their presence would lead to light scattering.
Though skeletal muscles prefer to catalyze glucose into carbon dioxide and water during heavy exercise where the amount of oxygen is inadequate, the muscles simultaneously undergo anaerobic glycolysis along with oxidative phosphorylation.
The amount of glucose available for the process regulates glycolysis, which becomes available primarily in two ways: regulation of glucose reuptake or regulation of the breakdown of glycogen. Glucose transporters (GLUT) transport glucose from the outside of the cell to the inside. Cells that contain GLUT can increase the number of GLUT in the plasma membrane of the cell from the intracellular matrix, therefore increasing the uptake of glucose and the supply of glucose available for glycolysis. There are five types of GLUTs. GLUT1 is present in RBCs, blood-brain barrier,ਊnd blood-placental barrier. GLUT2 is in the liver, beta-cells of the pancreas, kidney, and gastrointestinal (GI) tract. GLUT3 is present in neurons. GLUT4 is in adipocytes, heart, and skeletal muscle. GLUT5 specifically transports fructose into cells. Another form of regulation is the breakdown of glycogen. Cells can store extra glucose in the form of glycogen when glucose levels are high in the cell plasma. Conversely, when levels are low, glycogen can be converted back into glucose. Two enzymes control the breakdown of glycogen: glycogen phosphorylase and glycogen synthase. The enzymes can be regulated through feedback loops of glucose or glucose 1-phosphate, or via allosteric regulation by metabolites, or from phosphorylation/dephosphorylation control.
Allosteric Regulators and Oxygen
As described before, many enzymes are involved in the glycolytic pathway by converting one intermediate to another. Control of these enzymes, such as hexokinase, phosphofructokinase, glyceraldehyde-3-phosphate dehydrogenase, and pyruvate kinase, can regulate glycolysis. The amount of oxygen available can also regulate glycolysis. The “Pasteur effect” describes how the availability of oxygen diminishes the effect of glycolysis, and decreased availability leads to an acceleration of glycolysis, at least initially. The mechanisms responsible for this effect include the involvement of allosteric regulators of glycolysis (enzymes such as hexokinase). The “Pasteur effect” appears to mostly occur in tissue with high mitochondrial capacities, such as myocytes or hepatocytes, but this effect is not universal in oxidative tissue, such as pancreatic cells.
Another mechanism for controlling glycolytic rates is transcriptional control of glycolytic enzymes. Altering the concentration of key enzymes allows the cell to change and adapt to alterations in hormonal status. For example, increasing glucose and insulin levels can increase the activity of hexokinase and pyruvate kinase, therefore increasing the production of pyruvate.
Fructose 2,6-bisphosphate is an allosteric regulator of PFK-1. High levels of fructose 2,6-bisphosphate increase the activity of PFK-1. Its production occurs through the action of phosphofructokinase-2 (PFK-2). PFK-2 has both kinase and phosphorylase activity and can transform fructose 6 phosphates to fructose 2,6-bisphosphate and vice versa. Insulin dephosphorylates PFK-2, and this activates its kinase activity, which increases levels of fructose 2,6-bisphosphate, which subsequently goes on to activate PFK-1. Glucagon can also phosphorylate PFK-2, and this activates phosphatase, which transforms fructose 2,6-bisphosphate back to fructose 6-phosphate. This reaction decreases fructose 2,6-bisphosphate levels and decreases PFK-1 activity.
10 Steps of Glycolysis, Enzymes involved and Regulatory Enzymes of Glycolysis
Glycolysis (Glyco=Glucose lysis= splitting) is the oxidation of glucose (C 6) to 2 pyruvate (3 C) with the formation of ATP and NADH.
- It is also called as the Embden-Meyerhof Pathway
- Glycolysis is a universal pathway present in all organisms:
- from yeast to mammals.
- It is a universal anaerobic process where oxygen is not required
- First phase of cellular reparation in aerobic organisms
- It occurs in the cytosol of cell cytoplasm in both eukaryotes and prokaryotes
In the presence of O2, pyruvate is further oxidized to CO2.
In the absence of O2, pyruvate can be fermented to lactate or ethanol.
Glucose + 2NAD+ + 2 Pi + 2 ADP = 2 pyruvate + 2 ATP + 2NADH + 2 H2O
Here is the video that explains 10 Steps of Glycolysis
2 stages of Glycolysis
First phase: Preparatory Phase or investment phase Phosphorylation of Glucose and its conversion to Glyceraldehyde 3-phosphate. 2 ATP used in this pahse
Second phase: Payoff phase
Oxidative conversion of Glyceraldehyde 3-phosphate to pyruvic acid
(4 ATP and 2 NADH produced)
This reaction requires energy and so it is coupled to the hydrolysis of ATP to ADP and Pi.
Enzyme: hexokinase (regulatory step). It has a low Km for glucose hexokinase phosphorylates glucose that enters the cell
Irreversible step. So the phosphorylated glucose gets trapped inside thecell. Glucose transporters transport only free glucose
Reaction 2 : Isomerization of glucose-6-phosphate to fructose 6-phosphate. The aldose sugar is converted into the keto isoform.
This is a reversible reaction. The fructose-6-phosphate is quickly consumed and the forward reaction is favored.
Reaction 3 : is another kinase reaction. Phosphorylation of the hydroxyl group on C1 forming fructose-1,6- bisphosphate.
Enzyme: phosphofructokinase. This allosteric enzyme regulates the pace of glycolysis (rate limiting step).
ATP is used
Second irreversible reaction of the glycolytic pathway.
Reaction 4: fructose-1,6-bisphosphate splits into 2 3-carbon molecules, one aldehyde and one ketone: dihyroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP).
The enzyme is aldolase.
Up to this step 2 ATP is used
Second phase: Payoff phase
2 GAP molecules generated from each glucose, therefore each of the remaining reactions occur twice for each glucose molecule being oxidized.
Reaction 6: GAP is dehydrogenated by the enzyme glyceraldehyde 3-phosphate dehydrogenase (GAPDH). In the process, NAD+ is reduced to NADH + H+ from NAD. Oxidation is coupled to the phosphorylation of the C1
1,3-bisphosphoglycerate is formed
Reaction 7 : This high energy bond of BPG at C-1 is hydrolyzed to a carboxylic acid and the energy released is used to generate ATP from ADP.
Reaction 8 : The phosphate group shifts from C3 to C2 to form 2-phosphoglycerate.
Reaction 9: Dehydration reaction catalyzed by enolase (a lyase). A water molecule is removed to form phosphoenolpyruvate which has a double bond between C2 and C3.
Reaction 10: Enolphosphate is a high energy bond. It is hydrolyzed to form the enolic form of pyruvate with the synthesis of ATP. Irreversible step
The route of ethanol formation in Zymomonas mobilis
1. Enzymic evidence supporting the operation of the Entner-Doudoroff pathway in the anaerobic conversion of glucose into ethanol and carbon dioxide by Zymomonas mobilis is presented. 2. Cell extracts catalysed the formation of equimolar amounts of pyruvate and glyceraldehyde 3-phosphate from 6-phosphogluconate. Evidence that 3-deoxy-2-oxo-6-phosphogluconate is an intermediate in this conversion was obtained. 3. Cell extracts of the organism contained the following enzymes: glucose 6-phosphate dehydrogenase (active with NAD and NADP), ethanol dehydrogenase (active with NAD), glyceraldehyde 3-phosphate dehydrogenase (active with NAD), hexokinase, gluconokinase, glucose dehydrogenase and pyruvate decarboxylase. Extracts also catalysed the overall conversion of glycerate 3-phosphate into pyruvate in the presence of ADP. 4. Gluconate dehydrogenase, fructose 1,6-diphosphate aldolase and NAD-NADP transhydrogenase were not detected. 5. It is suggested that NAD is the physiological electron carrier in the balanced oxidation-reduction involved in ethanol formation.
Step 1. Glucose is phosphorylated to give glucose-6-phosphate. Thephosphorylation of glucose is an endergonic reaction.
Glucose + Pi - > Glucose-6-phosphate + H2O
∆G° ' = 13.8 kJ mol –1 = 3.3 kcal mol –1
The hydrolysis of ATP is exergonic.
∆G° ' = –30.5 kJ mol –1 = –7.3 kcal mol –1
These two reactions are coupled, so the overall reaction is the sum of the two and is exergonic.
Glucose + ATP - > Glucose-6-phosphate + ADP
∆G° ' = (13.8 + –30.5) kJ mol –1 = –16.7 kJ mol –1 = –4.0 kcal mol –1
Recall that ∆G°' is calculated under standard states with the concentration of all reactants and products at 1 M except hydrogen ion. If we look at the actual ΔΓ G in the cell, the number varies depending on cell type and metabolic state,but a typical value for this reaction is –33.9 kJ mol –1 or –8.12 kcal mol –1 . Thus the reaction is typically even more favorable under cellular conditions. Table 17.1 gives the ∆G° ' and G values for all the reactions of anaerobic glycolysis in erythrocytes.
This reaction illustrates the use of chemical energy originally produced by the oxidation of nutrients and ultimately trapped by phosphorylation of ADP to ATP. Recall that ATP does not represent stored energy, just as an electric current does not represent stored energy. The chemical energy of nutrients is released by oxidation and is made available for immediate use on demand by being trapped as ATP.
The enzyme that catalyzes this reaction is hexokinase. The term kinase is applied to the class of ATP-dependent enzymes that transfer a phosphate group from ATP to a substrate. The substrate of hexokinase is not necessarily glucose rather, it can be any one of a number of hexoses, such as glucose, fructose, and mannose. Glucose-6-phosphate inhibits the activity of hexokinase this is a control point in the pathway. Some organisms or tissues contain multiple isozymes of hexokinase. One isoform of hexokinase found in the human liver, called glucokinase, lowers blood glucose levels after one has eaten a meal. Liver glucokinase requires a much higher substrate level to achieve saturation than hexokinase does. Because of this, when glucose levels are high, the liver can metabolize glucose via glycolysis preferentially over the other tissues. When glu-cose levels are low, hexokinase is still active in all tissues.
A large conformational change takes place in hexokinase when substrate is bound. It has been shown by X-ray crystallography that, in the absence of substrate, two lobes of the enzyme that surround the binding site are quite far apart. When glucose is bound, the two lobes move closer together, and the glu-cose becomes almost completely surrounded by protein (Figure 17.4).
This type of behavior is consistent with the induced-fit theory of enzyme action. In all kinases for which the structure is known, a cleft closes when substrate is bound.
Step 2. Glucose-6-phosphate isomerizes to give fructose-6-phosphate.Glucosephosphate isomerase is the enzyme that catalyzes this reaction. TheC-1 aldehyde group of glucose-6-phosphate is reduced to a hydroxyl group, and the C-2 hydroxyl group is oxidized to give the ketone group of fructose-6-phosphate, with no net oxidation or reduction. (Recall that glucose is an aldose, a sugar whose open-chain, noncyclic structure contains an aldehyde group, while fructose is a ketose, a sugar whose corresponding structure contains a ketone group.) The phosphorylated forms, glucose-6-phosphate and fructose-6-phosphate, are an aldose and a ketose, respectively.
Step 3. Fructose-6-phosphate is further phosphorylated, producing fructose- 1,6-bisphosphate.
As in the reaction in Step 1, the endergonic reaction of phosphorylation of fructose-6-phosphate is coupled to the exergonic reaction of hydrolysis of ATP, and the overall reaction is exergonic. See Table 17.1.
The reaction in which fructose-6-phosphate is phosphorylated to give fructose-1,6-bisphosphate is the one in which the sugar is committed to gly-colysis. Glucose-6-phosphate and fructose-6-phosphate can play roles in other pathways, but fructose-1,6-bisphosphate does not. After fructose-1,6-bisphos-phate is formed from the original sugar, no other pathways are available, and the molecule must undergo the rest of the reactions of glycolysis. The phos-phorylation of fructose-6-phosphate is highly exergonic and irreversible, and phosphofructokinase, the enzyme that catalyzes it, is the key regulatory enzymein glycolysis.
Phosphofructokinase is a tetramer that is subject to allosteric feedback regu-lation of the type we discussed. There are two types of subunits, designated M and L, that can combine into tetramers to give different per-mutations (M4, M3L, M2L2, ML3, and L4). These combinations of subunits are referred to as isozymes, and they have subtle physical and kinetic differences (Figure 17.5). The subunits differ slightly in amino acid composition, so the two isozymes can be separated from each other by electrophoresis. The tetrameric form that occurs in muscle is designated M4, while that in liver is designated L4. In red blood cells, several of the combinations can be found. Individuals who lack the gene that directs the synthesis of the M form of the enzyme can carry on glycolysis in their livers but experience muscle weakness because they lack the enzyme in muscle.
When the rate of the phosphofructokinase reaction is observed at varying concentrations of substrate (fructose-6-phosphate), the sigmoidal curve typical of allosteric enzymes is obtained. ATP is an allosteric effector in the reaction. High levels of ATP depress the rate of the reaction, and low levels of ATP stimulate the reaction (Figure 17.6). When there is a high level of ATP in the cell, a good deal of chemical energy is immediately available from hydrolysis of ATP. The cell does not need to metabolize glucose for energy, so the presence of ATP inhibits the glycolytic pathway at this point. There is also another, more potent, allosteric effector of phosphofructokinase. This effector is fructose-2,6-bisphosphate we shall discuss its mode of action when we consider general control mechanisms in carbohydrate metabolism.
Step 4. Fructose-1,6-bisphosphate is split into two three-carbon fragments. Thecleavage reaction here is the reverse of an aldol condensation the enzyme that catalyzes it is called aldolase. In the enzyme isolated from most animal sources (the one from muscle is the most extensively studied), the basic side chain of an essential lysine residue plays the key role in catalyzing this reaction. The thiol group of a cysteine also acts as a base here.
Step 5. The dihydroxyacetone phosphate is converted to glyceraldehyde-3- phosphate.
The enzyme that catalyzes this reaction is triosephosphate isomerase. (Both dihydroxyacetone and glyceraldehyde are trioses.)
One molecule of glyceraldehyde-3-phosphate has already been produced by the aldolase reaction we now have a second molecule of glyceraldehyde-3-phosphate, produced by the triosephosphate isomerase reaction. The original molecule of glucose, which contains six carbon atoms, has now been converted to two molecules of glyceraldehyde-3-phosphate, each of which contains three carbon atoms.
The ∆ G value for this reaction under physiological conditions is slightly positive (+2.41 kJ mol –1 or +0.58 kcal mol –1 ). It might be tempting to think that the reaction would not occur and that glycolysis would be halted at this step. We must remember that just as coupled reactions involving ATP hydrolysis add their G values together for the overall reaction, glycolysis is composed of many reactions that have very negative G values that can drive the reaction to completion. A few reactions in glycolysis have small, positive ∆ G values (see Table 17.1), but four reactions have very large, negative values, so that the ∆ G for the whole process is negative.
Essay on Metabolism (For School and College Students) | Biology
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1. Essay on the Introduction to Metabolism:
The term metabolism is defined as ‘the chemical processes by which nutritive material is built up into living matter, or by which complex molecules are broken down into simpler substances during the performance of special functions’. The various reactions which involve the synthesis of complex molecules are grouped under anabolism, whereas the breakdown of complex molecules is known as catabolism.
Both anabolic and catabolic proc­esses include a vast number of different chemical reactions, but there are number of common features. Most of the metabolic processes occur inside the cells of the body, mainly in the cytoplasm, but also inside intracellular organelles such as the mitochondria. Anabolic and catabolic reactions involve the action of enzymes and the utilization of energy.
Metabolism, a vital process for all life forms, is a constant process that begins when an organism being conceived and ends when it dies. In case the metabolism stops, results in death. The process of metabolism is really a balancing act involving two kinds of activities that go on at the same time the building up of body tissues and energy stores (anabolism or constructive metabolism) and the breaking down of energy stores to generate more fuel for body functions (catabolism or destructive metabolism).
Almost all of the chemical reactions in the living body require the expenditure of energy, which is made available mainly by the catabolism of the ‘macronutrients’ fats and carbohydrates (particularly glucose), and proteins (to a small extent). According to the law of conservation of energy, the total energy of a system remains constant, though energy may transform into another form. In the body’s metabolism, the energy released from the oxidation of the macronutrients is used for a series of chemical reactions, instead of being released only as heat.
A fundamental feature of both anabolic and catabolic processes is the utilization of energy. The ultimate source of energy for all living system is solar energy. Thus, the meta­bolic process on earth begins with the producers, the plants. First, a green plant takes energy from sunlight. The plant uses this energy and the molecule chlorophyll (which gives plants their green color) to build sugars from water and carbon dioxide in a process known as photosynthesis.
The men and the animals when eat the plants, they take this energy (in the form of sugar), along with other vital cell-building chemicals. The body’s next step is to break the sugar down so that the energy released can be distributed to, and used as fuel by the body’s cells. These reactions are made easy by biological catalysts, (enzymes) and they break down proteins into amino acids, fats into fatty acids and carbohydrates into simple sugars (e.g., glucose).
During these processes, the energy from these compounds can be re­leased by the body for use or stored in body tissues, especially the liver, muscles, and body fat. During anabolism, small molecules are changed into larger, more complex molecules of carbohydrate, protein and fat.
2. Essay on Carbohydrate Metabolism:
In animals, especially in human the major source of dietary carbohydrate is starch from con­sumed plant material and a small amount of glycogen from animal tissue as well as disaccharides such as sucrose from products containing refined sugar and lactose in milk. Digestion in the gut converts all carbohydrate to monosaccharides which are transported to the liver and converted to glucose. The liver has a central role in the storage and distribution within the body of all fuels, including glucose.
Carbohydrate metabolism begins with digestion in the small intestine here monosaccharides are absorbed into the blood stream.
Blood sugar concentrations are controlled by three hormones: .
When the concentration of glucose in the blood increases, insulin is secreted by the pancreas, which stimulates the transfer of glucose into the cells, especially in the liver and muscles, although other organs are also able to metabolize glucose.
Glucose in the body undergoes catabolism in all peripheral tissues, particularly in brain, muscle and kidney to produce ATP. Excess glucose is changed into glycogen by the process of glycogenesis (anabolism) and stored as glycogen in liver and muscle or converted to fatty acids and is stored in adipose tissue as triglycerides. Eqinephrine and glucagon hormones are secreted to stimulate the conversion of glycogen to glucose when blood glucose level be­comes low. This process is called glycogenolysis (catabolism).
Glucose metabolism begins with the process called glycolysis (catabolism). The end products of glycolysis are pyruvic acid and ATP. Since glycolysis releases relatively little ATP, further reactions continue to convert pyruvic acid to acetyl CoA and then citric acid in the citric acid cycle. The majority of the ATPs are made from oxidations in the citric acid cycle in connection with the electron transport chain. During strenuous muscular activity, pyruvic acid is converted into lactic acid rather than acetyl CoA. During the resting period, the lactic acid is converted back to pyruvic acid. The pyruvic acid in turn is converted back to glucose by the process called gluconeogenesis (anabolism).
3. Essay on Glycolysis (Catabolism):
Glycolysis (Embden-Meyerhof pathway) is the initial metabolic pathway of carbohydrate catabolism. It is the most universal process by which cells of all types derive energy from sugars. Glucose is oxidized by all tissues to synthesize ATP. The first pathway which begins the complete oxidation of glucose is called glycolysis. This pathway cleaves the six carbon glucose molecule (C6H12O6) into two molecules of the three carbon compound pyruvate (C3H3O3 – ). This oxidation is coupled to the net production of two molecules of ATP per glu­cose. Glycolysis converts one molecule of glucose into two molecules of pyruvate, along with “reducing equivalents” in the form of the coenzyme NADH.
The global reaction of gly­colysis is:
Glucose + 2 NAD + + 2 ADP + 2 Pi –> 2 NADH+ 2 pyruvate + 2 ATP + 2 H2O + 4 H +
In eukaryotes, glycolysis takes place within the of the cell. Glucose gets into the cell through facilitated diffusion. The first step in glycolysis is phosphorylation of glucose by hexokinase (in liver the most important hexokinase is glucokinase). This reaction con­sumes 1 ATP molecule. Although the cell membrane is permeable to glucose because of the presence of glucose transport proteins, it is impermeable to glucose 6-phosphate.
Glucose 6- phosphate is then rearranged into fructose 6-phosphate by phospho-glucose isomerase. (Fructose can also enter the glycolytic pathway at this point.). Phosphofructokinase-1 then consumes 1 ATP to form fructose 1, 6-bisphosphate. The energy expenditure in this step is justified in 2 ways- the glycolytic process is now irreversible, and the energy supplied to the molecule allows the ring to be split by aldolase into 2 molecules – dihydroxyacetone phos­phate and glyceraldehyde 3-phosphate. (Triosephosphate isomerase converts the molecule of di-hydroxy-acetone phosphate into a molecule of glyceraldehyde 3-phosphate.) Each mole­cule of glyceraldehyde 3-phosphate is then oxidized by a molecule of NAD + in the presence of glyceraldehyde 3-phosphate dehydrogenase, forming 1, 3-bisphosphoglycerate.
Phosphoglycerate kinase then generates a molecule of ATP while forming 3- phosphoglycerate. At this step, glycolysis has reached the break-even point- 2 molecules of ATP were consumed and 2 new molecules have been synthesized. Phosphoglyceromutase then forms 2-phosphoglycerate enolase then forms phosphoenolpyruvate and another sub- strate-level phosphorylation later forms a molecule of pyruvate and a molecule of ATP by means of the enzyme pyruvate kinase (Fig. 3.45).
NAD is used as the electron acceptor in the oxidation reaction. This cofactor is present only in limited amounts and once reduced to NADH, as in this reaction, it must be reoxidised to NAD to permit continuation of the pathway.
Methods of Glycolysis:
This re-oxidation occurs by one of two methods:
(i) Anaerobic Glycolysis:
In the absence of oxygen, pyruvate is reduced to lactate that is ideally suited to utilization in heavily exercising muscles where oxygen supply is often insufficient to meet the demands of aerobic metabolism. The reduction of pyruvate to lactate is coupled to the oxidation of NADH to NAD.
The lactate formed is transported to other tissues and dealt with by one of the two mecha­nisms such as converted back to pyruvate or converted back to glucose in the liver. The process of conversion of lactate to glucose is called gluconeogenesis, uses some of the reac­tions of glycolysis (but in the reverse direction) and some reactions unique to this pathway to re-synthesize glucose.
The majority of the enzymes responsible for gluconeogenesis are found in the cytoplasm the exception is pyruvate carboxylase which is located in the mito­chondria. This pathway requires ATP but has the role of maintaining a circulating glucose concentration in the bloodstream (even in the absence of dietary supply) and also maintain­ing a glucose supply to fast twitch muscle fibres.
The Cori cycle, named after its discoverers, Carl Cori and Gerty Cori, refers to the meta­bolic pathway in which lactate produced by anaerobic glycolysis in the muscles moves to the liver and is converted to glucose, which then returns to the muscles and is converted back to lactate (Fig. 3.46). It can be shown by a complex calculation of energy yields that this proc­ess of partially oxidizing glucose to lactate in muscle, transporting it to the liver for conver­sion back to glucose and then re-supplying it to muscle, actually has a much higher energy yield than the 2 ATP/glucose produced by glycolysis alone.
(ii) Aerobic Glycolysis:
In aerobic condition pyruvate is transported inside mitochondria and oxidized to acetyl coen­zyme A (abbreviated to “ acetyl CoA “). This is an oxidation reaction and uses NAD as an electron acceptor. Further, acetyl CoA is oxidized ultimately to CO2 by citric acid cycle. These reactions are coupled to a process known as the electron transport chain which has the role of harnessing chemical bond energy through a series of oxidation/reduction reactions to the synthesis of ATP and simultaneously re-oxidizing NADH to NAD.
The Krebs cycle, also known as the tri-carboxylic acid cycle (TCA), was first recognized in 1937 by the man for whom it is named, German biochemist Hans Adolph Krebs, the winner of Nobel Prize in 1953. In short, the Krebs cycle constitutes the discovery of the major source of energy in all living organisms. The Krebs cycle reactions take place in the matrix of the mitochondria. Some of the final steps of intermediate metabolism take place there, as well.
For example, in the matrix as well as the cytoplasm, glutamate (the amino acid glu­tamic acid) loses its amino group and is oxidized to alpha-ketoglutarate. Under aerobic con­ditions the end product of glycolysis is pyruvic acid converted to acetyl coenzyme A (acetyl CoA) which is the initiator of the citric acid cycle. In carbohydrate metabolism, acetyl CoA is the link between glycolysis and the citric acid cycle. The citric acid cycle contains the final oxidation reactions, coupled to the electron transport chain, which produce the majority of the ATP in the body.
For each glucose molecule that enters glycolysis, two pyruvate molecules are produced and have gained two NADH and two ATPs, while in the Calvin cycle approximately 54 ATPs are utilized by the plant to synthesize one glucose molecule. ATP is generated by breaking the bonds in glucose and capturing as much as possible of the energy stored in that molecule. The CA cycle produces very little ATP directly, but generates many molecules of reduced coenzymes NAD and FAD as NADH and FADH2.
The Krebs cycle begins with oxalo-acetate and combines with Acetyl CoA to cycle through one complete turn. After Acetyl CoA is oxidized to CO2 and H2O, the electrons drive proton pumps which generate ATP that is greatly needed by the cell. Remember that the NADH molecules are important because they contain extracted electrons which ultimately reduce NAD + .
However, when the electrons do not have enough energy to reduce NAD + , they are stored temporarily in the FADH2 molecule. Each NADH molecule is responsible for the production of three ATP molecules, while FADH2 is responsible for the production of two ATP molecules. In prokaryotic cells, the citric acid cycle occurs in the cytoplasm in eukaryotic cells the citric acid cycle takes place in the matrix of the mitochon­dria.
The overall reaction for the citric acid cycle is:
2 acetyl groups + 6 NAD + + 2 FAD + 2 ADP + 2 Pi –> 4 CO2 + 6 NADH + 6H + + 2 FADH2 + 2 ATP
The citric acid cycle provides a series of intermediate compounds that donate protons and electrons to the electron transport chain by way of the reduced coenzymes NADH and FADH2. The electron transport chain then generates additional ATPs by oxidative phos­phorylation.
The TCA cycle involves 8 distinct steps, each catalyzed by a unique enzyme:
i. The citric acid cycle begins when Coenzyme A transfers its 2-carbon acetyl group to the 4- carbon compound, oxalo-acetate, to form the 6-carbon molecule, citrate.
ii. The citrate is rearranged to form an isomeric form, isocitrate (Fig. 3.47).
iii. The 6-carbon isocitrate is oxidized and a molecule of CO2 is removed producing the 5- carbon molecule α-ketoglutarate. During this oxidation, NAD + is reduced to NADH + H + .
iv. Alpha-ketoglutarate is oxidized, carbon dioxide is removed, and coenzyme A is added to form the 4-carbon compound succinyl-CoA. During this oxidation, NAD + is reduced to NADH + H +
v. CoA is removed from succinyl-CoA to produce succinate. The energy released is used to make guanosine triphosphate (GTP) from guanosine diphosphate (GDP) and Pi by sub- strate-level phosphorylation. GTP can then be used to make ATP.
vi. Succinate is oxidized to fumarate. During this oxidation, FAD is reduced to FADH2.
vii. Water is added to fumarate to form malate.
viii. Malate is oxidized to produce oxaloacetate, the starting compound of the citric acid cycle. During this oxidation, NAD + is reduced to NADH + H +
In addition to their roles in generating ATP by catabolism, the citric acid cycle also sup­plies precursor metabolites (anabolic) for various biosynthetic pathways (Fig. 3.48).
(b) Electron Transport Chain:
It is the final part of the phase-II of aerobic respiration. In respiration, oxidation of the sub­strate occurs by dehydrogenation (i.e., removal) of hydrogen atoms (2H) from the substrate. Most of these hydrogen atoms are accepted by NAD to form reduced co-enzyme NADH. In the aerobic respiration 10 NADH2 are formed (2NADH2 in glycolysis + 8 NADH2 in Krebs cycle) from one molecule of glucose. Also, in Krebs cycle, hydrogen is accepted by FAD to form FADH2 in one step a total of 2 FADH2 are formed by aerobic respiration of each glucose molecule.
Each molecule of reduced co-enzyme thus formed in aerobic respiration (glycolysis and Krebs cycle) is finally oxidized by the free molecular oxygen through a process called termi­nal oxidation (Fig. 3.49).
The respiratory chain (or the ETS) is present in the inner membrane of mitochondrion (i.e., in the cristae membrane). It consists of various enzymes and co-enzymes which act as electron carriers. Embedded in the inner membrane are proteins and complexes of molecules that are involved in the process called electron transport. The electron transport system (ETS), as it is called, accepts energy from carriers in the matrix and stores it to a form that can be used to phosphorylate ADP.
Two energy carriers are known to donate energy to the ETS, namely nicotine adenine di-nucleotide (NAD) and flavin adenine di-nucleotide (FAD). NADH binds to complex -I. It binds to a prosthetic group called flavin mononucleotide (FMN), and is immediately re-oxidized to NAD. NAD is recycled, acting as an energy shuttle. FMN receives the hydrogen from the NADH and two electrons. It also picks up a proton from the matrix. In this reduced form, it passes the electrons to iron-sulfur clusters that are part of the complex, and forces two protons into the inter-membrane space. Reduced NAD carries energy to complex I (NADH-Coenzyme Q Reductase) of the electron transport chain. FAD is a bound part of the succinate dehydrogenase complex (complex II).
Electrons cannot pass through complex-l without accomplishing proton translocation. Electron transport carriers are specific, in which each carrier accepts electrons (and associ­ated free energy) from a specific type of preceding carrier. Electrons pass from complex I to a carrier (Coenzyme Q) embedded by itself in the membrane. From Coenzyme Q electrons are passed to a complex -III which is associated with another proton translocation event.
Complex-II, the succinate dehydrogenase complex, is a separate starting point, and is not a part of the NADH pathway. From succinate, the sequence is Complex II to Coenzyme Q to Complex III to cytochrome C to Complex IV. Thus, there is a common electron transport pathway beyond the entry point, either Complex I or Complex II. Protons are not translocated at Complex II. There is not sufficient free energy available from the succinate dehydrogenase reaction to reduce NAD or to pump protons at more than two sites. From Complex III the pathway moves to cytochrome C then to a Complex IV (cytochrome oxi­dase complex). More protons are translocated by Complex IV, and it is at this site that oxy­gen binds, along with protons, and using the electron pair and remaining free energy, oxygen is reduced to water.
Oxygen serves as an electron acceptor, clearing the way for carriers in the sequence to be re-oxidized so that electron transport can continue. The purpose of elec­tron transport is to conserve energy in the form of a chemiosmotic gradient. The gradient, in turn, can be exploited for the phosphorylation of ADP as well as for other purposes. With the cessation of aerobic metabolism cell is damaged immediately and irreversibly.