The following points highlight the six main processes for metabolism of carbohydrates in mammalian organism. The processes are: 1. Glycogenesis 2. Glycogenolysis 3. Glycolysis 4. Citric Acid Cycle In Mitochondri 5. Neoglucogenesis 6. Hexose Monophosphate Shunt.
Carbohydrate Metabolism Process # 1. Glycogenesis:
a. Glucose is first phosphorylated to glucose- 6-phosphate by the enzyme hexokinase and glucokinase in presence of the coenzyme ATP and the activator Mg++. ATP is converted to ADP. Hexokinase has high affinity for glucose. Glucokinase is to remove glucose from the blood following a meal.
b. Glucose-6-phosphate is then converted to glucose-1-phosphate. The reaction is catalyzed by the enzyme phosphoglucomutase with Mg++ and the reaction is reversible. Glucose-1, 6-diphosphate is formed as an intermediate due to the phosphorylation of the enzyme.
c. Glucose-1-phosphate reacts with uridine triphosphate (UTP) to form the active nucleotide (uridine diphosphate glucose (UDPG). The reaction is catalyzed by the enzyme UDPG pyrophosphotylase with the release of inorganic pyrophosphate.
d. The C1 of the activated glucose of UDPG forms a glycosidic bond with the C4 of the terminal glucose residue of glycogen liberating UDP by the enzyme glycogen synthetase (glucosyl transferase). The glycogen primer is formed on a protein primer known as glycogenin which is a protein.
e. By the successive 1, 4 linkages by glucose units to the preexisting glycogen chain, the branches of the glycogen ‘tree’ become elongated.
When the chain has been lengthened to between 6 and 11 glucose residues, a second enzyme, the branching enzyme (amylo 1, 4 → 1, 6 trans-glucosidase), acts on the glycogen. This enzyme transfers a part of the -1, 4-chain (minimum length of 6 glucose residues) to a neighbouring chain to form α-1, 6-linkage which establishes a branch point in the molecule.
Regulation of Glycogenesis:
Glycogen synthase (glycogen synthetase) is the key enzyme which regulates the process of glycogenesis. This enzyme exists in “active” as well as “inactive” forms which are inter-convertible. Glycogen also exerts an inhibition on its own formation and insulin also stimulates glycogen synthesis in muscle by promoting dephosphorylation and activation of inactive glycogen synthase.
The inhibition of glycogenesis enhances net glycogenolysis. Hexokinase stimulates the oxidation of glucose in the muscle whereas glucokinase stimulates the oxidation of glucose in the liver.
Glycogen:
a. It is a branched polysaccharide consisting of α-D-glucose units. These glucose units are connected to one another by glucosidic linkages between the first and fourth carbon atoms except at branch points.
b. The molecular weight varies from 1 million to 5 million or more.
c. In case some of the glucose chains terminate in the interior, a larger molecule is possible.
Glycogen Synthetase:
a. In muscle (and possibly liver), it is present in two inter-convertible forms-synthetase D (dependent) whose activity is dependent on the presence of glucose-6-phosphate and synthetase I (independent) whose Km value for UDPG decreases in the presence of glucose-6-phosphate.
b. Synthetase D is converted to synthetase I by synthetase phosphatase.
c. Synthetase I is phosphorylated to form synthetase D by synthetase kinase (cAMP-de- pendent protein kinase) which is active in the presence of 3′, 5′-cyclic adenylic acid (cAMP). ATP acts as phosphate donor.
d. Glycogen synthetase is stimulated by insulin and glucose but inhibited by cyclic AMP.
Cyclic AMP:
a. It is 3’, 5’ adenylic acid (adenine, ribose and phosphate), the 3rd and 5th carbon atoms of ribose are attached to phosphate group. The structure is given (Fig. 17.3).
b. It is the intracellular intermediate compound through which many hormones act.
c. Adenylate cyclase of cell membranes being activated by the hormones epinephrine, norepinephrine and glucagon form cAMP from ATP.
d. cAMP is destroyed by phosphodiesterase. The normally low level of it is maintained by the activity of phosphodiesterase. In liver, insulin increases the activity of phosphodiesterase lowering the concentration of cAMP.
Functions:
a. cAMP causes inhibition (-) of glycogen synthetase.
b. It stimulates (+) phosphorylase activity.
c. Glycogen synthetase I is phosphorylated to form synthetase D by cAMP dependent protein kinase and glycogen synthetase I is formed from synthetase D by synthetase phosphatase.
Adenylate Cyclase:
a. It is the enzyme occurring in cell membrane.
b. It is activated by epinephrine, norepinephrine and glucagon and the activated enzyme forms cAMP from ATP.
c. Thyroid hormones may increase the synthesis of adenylate cyclase.
d. PG endo-peroxides also stimulate the production of cGMP in platelets and recent evidence indicates that PGG2 activates that soluble guanylate cyclase from platelets.
Cyclic GMP:
a. Cyclic GMP is 3′, 5′-guanylic acid (guanine, ribose and phosphate), the 3rd and 5th carbon atoms of ribose are attached to phosphate.
b. Prolactin causes the elevation of cGMP in mouse mammary gland which may be mediated by PGF2α and PGF2 and also raises the level of cGMP in the rat uterus.
c. PGE5 at high concentration stimulates cGMP accumulation.
d. PG endo-peroxides also stimulate the production of cGMP in platelets and recent evidence indicates the PGG2 activates that soluble guanyalate cyclase from platelets.
UDPG:
a. UDPG has been isolated from animal tissues and from yeast.
b. Uridine diphosphate glucose is an intermediate product of glycogenesis formed from glucose-1-phosphate by the enzyme UDPG pyrophosphorylase in presence of the coenzyme UTP.
c. It is essential for the synthesis of lactose in the mammary gland thus:
d. It can be enzymatically oxidized to UDP- glucuronic acid by UDPG- dehydrogenase and is required for the production of ascorbic acid (vitamin C) by the uronic acid pathway.
Carbohydrate Metabolism Process # 2. Glycogenolysis:
a. Glycogen breakdown in the liver and muscle is initiated by the enzyme phosphoiylase. This enzyme is specific for the phosphorylytic breaking of the α-1, 4-linkages of glycogen to produce glucose-1-phosphate. The removal of -1, 4-glucosyl residues continues until about 4 glucose residues remain on either side of α-1, 6-branch. Cyclic AMP stimulates phosphorylase activity.
The de-branching enzyme (amylo-1, 6-glucosidase) causes the hydrolytic splitting of-1, 6-linkages.
By the combined action of both these enzymes, glycogen is converted to glucose- 1-phosphate.
b. The action of phosphoglucomutase is reversible and glucose-6-phosphate is formed from glucose-1-phosphate.
c. In liver and kidney (but not in muscle), there exists a specific enzyme, glucose-6- phosphatase, which removes phosphate from glucose-6-phosphate, enabling the free glucose to diffuse from the cell into the extracellular spaces including the blood. The overall breakdown is shown (Fig. 17.8).
Regulation of Glycogenolysis:
a. The hormones catecholamine’s (epinephrine, norepinephrine), glucagon and thyroid cause the increase in cAMP level in cells. This cAMP activates protein kinase which stimulates the key enzyme phosphorylase for glycogenolysis.
b. Immediately after the onset of muscle contraction, glycogenolysis is highly increased in muscle by the rapid activation of phosphorylase due to the activation of phosphorylase kinase by Ca++.
c. Calmodulin (A Ca++ dependent regulatory protein) causes further activation of phosphorylase kinase for glycogenolysis.
Phosphorylase:
a. In liver, phosphorylase exists in both active and inactive form. The active phosphorylase (phosphorylase a or phospho-phosphorylase) can be inactivated by phosphorylase phosphatase to dephosphophosphorylase. Reactivation takes place by the enzyme phosphorylase b kinase or dephosphophosphorylase kinase in presence of ATP.
b. In muscle, phosphorylase is present in two forms phosphorylase a (active in the absence of 5′-AMP) and phosphorylase b (active only in the presence of 5′-AMP).
Phosphorylase a is physiologically active. It is a tetramer containing 4 mols of pyridoxal phosphate. It is hydrolytically converted to phosphorylase b, a dimer, by phosphorylase phosphatase. Phosphorylase b contains 2 mols of pyridoxal phosphate.
Phosphorylase b re-condenses to phosphorylase a by phosphorylase kinase b. Conversion of phosphorylase b to phosphorylase a signifies the mechanism for increasing glycogenolysis:
c. Inactive muscle phosphorylase b kinase is converted to active phosphorylase by kinase by the activation of active protein kinase which is being stimulated by cyclic AMP. Epinephrine is involved in cyclic AMP formation through adenylate cyclase.
d. Difference in muscle phosphorylase and liver phosphorylase:
(a) No cleavage in the structure in case of liver phosphorylase.
(b) Muscle phosphorylase is not affected by glucagon.
(c) Active phosphorylase inhibits liver synthetase phosphatase.
Carbohydrate Metabolism Process # 3. Glycolysis:
When a muscle contracts under anaerobic condition, pyruvate and lactate become the principal end product. But if it contracts under aerobic condition, pyruvate and lactate disappear and these are further oxidized to CO2 and H2O. Hence, carbohydrate metabolism is divided into two phases – anaerobic and aerobic. When oxygen is in short supply, NADH is re-oxidized by being coupled to the reduction of pyruvate to lactate.
Sequence of Reactions in Glycolysis:
All the enzyme of the Embden-Meyerhof pathway are found in cytosol.
They catalyze the reactions involved in the glycolysis of glucose to lactate:
a. Glucose is first phosphorylated to glucose- 6-phosphate by the enzyme hexokinase and by an additional enzyme in the liver, glucokinase. The activity of glucokinase is affected by nutritional state. This reaction is accompanied by ATP and Mg++. This is an irreversible reaction.
Hexokinase has a higher affinity for glucose than glucokinase. Its functions is to supply glucose to the tissues even in low blood glucose concentration. It can catalyze the phosphorylation of other hexoses but at a slower rate than glucose. But the function of glucokinase is to remove glucose from the blood following a meal. Both the enzymes are stimulated by insulin.
b. Glucose-6-phosphate is an important compound in the metabolic pathways (glycolysis, gluconeogenesis, hexose-mono-phosphate shunt, glycogenesis, glycogenolysis). It is converted to fructose-phosphate by phospho-hexose isomerase.
c. Fructose-6-phosphate is phosphorylated with ATP by phospho fructokinase to form fructose-1, 6-bisphosphate. This is also an irreversible reaction. The enzyme is also stimulated by insulin.
d. The enzyme aldolase splits fructose-1, 6- bisphosphate into two triose-phosphates: glyceraldehyde-3-phosphate and dihydroxy acetone phosphate.
Both the triose-phosphates are interconverted by phosphotrioseisomerase.
Dihydroxy acetone phosphate is also formed from glycerol which is phosphorylated to glycerol-3-phosphate, then to dihydroxy acetone phosphate.
e. Glyceraldehyde-3-phosphate is oxidized to 1, 3- bisphosphoglycerate by glyceraldehyde-3- phosphate dehydrogenase with the help of the coenzyme NAD. Energy is released in the course of the reaction.
f. 3-phosphoglycerate is formed from 1, 3- bisphosphoglycerate by phosphoglycerate kinase with the production of ATP from ADP.
Since two molecules of triose-phosphates are formed from one molecule of glucose, 2 molecules of ATP are generated in this stage. The reaction is reversible,
g. 3-phosphoglycerate is converted to 2-phosphoglycerate by phosphoglyeerate mutase.
h. 2-phosphoglycerate is catalyzed by enolase to produce phosphoenolpyruvate in presence of Mg++.
i. The high-energy phosphate of phosphoenolpyruvate is transferred to ADP by pyruvate kinase to form ATP.
j. Enolpyruvate formed is converted spontaneously to the keto form of pyruvate. This is an irreversible step.
k. If anaerobic conditions prevail, the re-oxidation of NADH through the respiratory chain to oxygen is prevented. Pyruvate then is reduced to lactate by the NADH. The reaction is catalyzed by lactate dehydrogenase. Glycolysis in erythrocytes even under aerobic condition forms lactate because of the absence of enzymatic machinery for the aerobic oxidation of pyruvate.
Total number of ATP formed by glycolysis under anaerobic conditions up to pyruvic acid:
From one molecule of glucose, two molecules of glyceraldehyde-3-phosphate are formed. After that, in the reactions of glycolysis, each product is 2 molecules.
But in anaerobic conditions, the total number of ATP will be only 2 up to lactic acid.
Because the reduced NAD (NADH) in glyceraldehyde-3-phosphate dehydrogenase is utilized in lactate dehydrogenase, NADH is not oxidized in mitochondria.
The total calories in this case is 2 x 7,600= 15,200 calories.
Regulation of Glycolysis:
a. The enzymes involved in the utilization of glucose are activated on the more availability of glucose whereas the enzymes responsible for gluconeogenesis are inhibited at the same time. The activity of the key enzymes glucokinase, phosphofructokinase-1 and pyruvate kinase is increased by glucose.
b. The hormone insulin stimulates the synthesis of the key enzymes responsible for glycolysis and antagonizes the effects of glucocorticoids and glucagon-stimulated cAMP in enhancing the key enzymes responsible for gluconeogenesis.
c. The hormones epinephrine and glucagon increase cAMP level to activate cAMP- dependent protein kinase which phosphorylates and inactivates the key enzyme pyruvate kinase and thus, inhibit glycolysis.
d. Phosphofructokinase-1 is involved in “feedback” control. This enzyme is activated by AMP and inhibited by ATP and citrate. When ATP is utilized in energy requiring processes, the concentration of AMP is highly increased.
e. In hypoxia, the concentration of ATP in the cell is decreased with the increase in the concentration of AMP which explains clearly the inner significance of increase of glycolysis in absence of oxygen.
Effect of Hormones in Glycolysis:
a. Insulin stimulates hexokinase and glucokinase which catalyze the conversion of glucose to glucose-6-phosphate.
b. Insulin stimulates phosphofructokinase which catalyzes the conversion of fructose-6-phosphate to fructose-1, 6-bisphosphate.
c. Glucagon stimulates liver glucose-6- phosphatase which is involved in the conversion of glucose-6-phosphate to glucose and also fructose-1, 6-bisphosphatase involved in the conversion of fructose-1, 6- bisphosphate to fructose-6 phosphate.
Inhibitors:
a. lodoacetate is the inhibitor of glyceraldehyde-3-phosphate dehydrogenase involved in the conversion of glyceraldehyde-3-phosphate to 1, 3-bisphosphoglycerate.
b. Arsenitc inhibits the synthesis of ATP by accomplishing uncoupling of oxidation and phosphorylation in the conversion of 1, 3-bisphosphoglycerate to 3-phosphoglycerate.
c. Fluoride inhibits enolase involved in the conversion of 2-phosphoglycerate to phosphoenolpyruvate.
Reaction chart (Fig 17.12) of the pathway of glycolysis ℗ PO32-; Pi, H0P032-; (-), inhibition carbon atoms 1-3 of fructose bisphosphate form dihydroxyacetone phosphate, whereas carbons 4-6 form glyceraldehyde-3-phosphate. The term bis, as in bisphosphate, indicates that the phosphate groups are separated, whereas diphosphate, as in adenosine diphosphate, indicates that they are joined.
Carbohydrate Metabolism Process # 4. Citric Acid Cycle In Mitochondri:
This cycle consists of a series of reactions in mitochondria which catabolizes the oxidation of acetyl- CoA to CO2 and H2O in aerobic condition. The acetyl-CoA combines with a 4-carbon dicarboxylic acid, oxaloacetate to form 6-carbon tricarboxylic acid citrate. In the course of the reactions 2 molecules of CO2 are lost and oxaloacetate is regenerated. Oxaloacetate plays an important catalytic role.
Reactions of the Citric Acid Cycle:
a. Acetyl-CoA first combines with oxaloacetate to form citrate being catalyzed by citrate synthetase.
b. Citrate is converted to isocitrate by the enzyme aconitase (aconitate hydratase) which contains Fe++. This conversion takes place in two steps-dehydration to cisaconitate and rehydration to isocitrate. The reaction is inhibited by fiuoroacctitfe.
c. Isocitrate undergoes dehydrogenation in the presence of isocitrate dehydrogenase to form oxalosuccinate.
d. The decarboxylation of oxalosuccinate 7 produces α-ketoglutarate by the catalytic action of isocitrate dehydrogenase in presence of Mn++.
e. α-ketoglutarate undergoes oxidative de- carboxylation in similar manner as that occurred in pyruvate. The reaction is catalyzed by α-ketoglutarate dehydrogenase complex which requires coenzymes TPP, lipoate, NAD\FAD+ and CoA forming succinyl-CoA.
f. Succinyl-CoA is then converted to succinate by the enzyme succinate thiokinase (succinyl-CoA synthetase). The reaction requires GDP or IDP which is converted to GTP or ITP by the presence of inorganic phosphate.
g. Succinate is further metabolized undergoing dehydrogenation catalyzed by succinate dehydrogenase. The reaction requires FAD. Fumarate is formed. Fumarate, under the influence of fumarase, is converted to malate by the addition of water.
h. Malate is converted to oxaloacetate by malate dehydrogenase which requires NAD+.
Energetics of the citric acid cycle:
Total number of ATP in the complete oxidation of one molecule of glucose:
Number of molecule of CO2 and H2O formed in the oxidation of pyruvic acid
Inhibitors:
a. Fluoroacetate inhibits the enzyme aconitase and prevents the conversion of citrate to isocitrate.
b. Arsenite inhibits α-ketoglutarate dehydrogenase and causes α-ketoglutarate to accumulate.
c. Malonate or oxaloacetate inhibits succinate dehydrogenase competitively resulting in succinate accumulation.
Regulation of the Citric Acid Cycle:
a. The main function of TCA cycle is to provide energy through the respiratory chain and oxidative phosphorylation. The activity of this cycle fully depends on NAD+ and the availability of ADP for the formation of ATP.
b. The rate of respiration and the activity of the citric acid cycle are determined by the adequate O2 and the rate of doing work through the use of ATP.
c. The key enzymes of this cycle are citrate synthase, isocitrate dehydrogenase, and α- ketoglutarate dehydrogenase. All these dehydrogenases are activated by Ca++, which increases in concentration during muscular contraction and secretion in case of increased energy demand. These enzymes are responsive to the energy status as expressed by the [ATP]/[ADP] and [NADH]/[NAD+] ratios.
d. Citrate synthase is allosterically inhibited by ATP and long chain fatty acyl-CoA. Mitochondrial NAD-dependent isocitrate dehydrogenase is allosterically activated by ADP and is inhibited by ATP and NADH.
α-ketoglutarate dehydrogenase complex is under the same condition of pyruvate dehydrogenase.
Succinate dehydrogenase is inhibited by oxaloacetate and the availability of oxaloacetate is controlled by malate dehydrogenase which depends on [NADH]/[NAD+] ratio.
Significance of the Citric Acid Cycle:
a. The major significance of the citric acid cycle is that it acts as the common metabolic pathway for the oxidation of carbohydrate, lipids and proteins because glucose, fatty acids and many amino acids are metabolized to acetyl-CoA which is finally oxidized in the citric acid cycle.
b. The reducing equivalents in the forms of hydrogen or of electrons are formed by the activity of specific dehydrogenases during the oxidation of acetyl-CoA in the cycle. These reducing equivalents then enter the respiratory chain, where large amounts of high-energy phosphate are generated by the oxidative phosphorylation.
c. The enzymes of the citric acid cycle are located in the mitochondrial matrix, either free or attached to the inner surface of the inner mitochondrial membrane which facilitates the transfer of reducing equivalents to the adjacent enzymes of the respiratory chain which is also situated in the inner mitochondrial membrane.
d. The citric acid cycle is amphibolic (dual) in nature which is the source for anabolic processes such as fatty acid and amino acid synthesis and gluconeogenesis.
Carbohydrate Metabolism Process # 5. Neoglucogenesis:
a. The formation of glucose from non-carbohydrate substances such as lactic acid, amino acids and glycerol is called gluconeogenesis or neoglucogenesis.
b. When the carbohydrate is insufficient in the diet, gluconeogenesis meets the needs of the body for glucose.
c. A continued supply of glucose is necessary as a source of energy and glucose is the only fuel which supplies energy to skeletal muscle under anaerobic conditions.
d. There is always a certain basal requirement for glucose even when fat is supplied to the caloric requirement of the organism.
e. In mammals, the liver and kidney are the principal organs responsible for gluconeogenesis. Gluconeogenesis is essentially a reversal of glycolysis. Therefore, the glycolytic activity of liver and kidney is low when there is active gluconeogenesis.
Metabolic Pathways in Gluconeogenesis:
a. The metabolic pathways in connection with gluconeogenesis are the modifications or the Embden-Meyerhof pathways and the citric acid cycle.
b. They are concerned with the conversion of glucogenic amino acids, lactate, glycerol, propionate (in ruminants) to glucoses or glycogen.
c. The energy barriers obstruct a simple reversal of glycolysis:
(i) Between pyruvate and phosphoenolpyruvate,
(ii) Between fructose-1,6-bisphosphate and fructose 6- phosphate,
(iii) Between glucoses-phosphate and glucose,
(iv) Between glucose- 1-phosphate and glycogen.
These barriers are overcome by the following reactions:
(i) The enzyme pywate ouboxytsm present in mitochondria converts pyruvate to oxaloacetate in presence of ATP, biotin and CO2. A second enzyme phosphoenolpyruvate carboxykinase present in the extra-mitochondrial part of the cell converts oxaloacetate to phosphoenolpyruvate in presence of GTP. Lactate with the help of these two enzymes and lactate dehydrogenase is converted to phosphoenolpyruvate.
But, oxaloacetate does not diffuse readily from mitochondria. Alternative means are applied to convert oxaloacetate to malate which is readily diffused from mitochondria. Malate is then converted to oxaloacetate in the extra-mitochondrial portion of the cell.
(ii) The conversion of fructose-1, 6- bisphosphate to fructose-6-phosphate catalyzed by another enzyme fructose-1. 6-bisphosphatase. This enzyme is present in liver, kidney and striated muscle but absent from adipose tissue, heart muscle and smooth muscle.
(iii) Glucose-6-phosphate is converted to glucose by glucose-6-phosphatase which is present in intestine, liver and kidney but absent from muscle and adipose tissue.
(iv) The conversion of glucose-1-phosphate to glycogen is through UDPG and glycogen synthetase.
Glucogenic amino acids after transamination or deamination form either pyruvic acid or members of the citric acid cycle. Therefore, the glucogenic amino acids and lactate can be converted to glucose or glycogen. Propionate in ruminants enters the main glucogenic pathway via the citric acid cycle being converted to succinyl-CoA.
Conversion of Propionate to Succinyl-CoA:
a. Propionate is first activated by thiokinase with ATP and CoA to form propionyl-CoA.
b. Propionyl-CoA undergoes CO2 fixation reaction to form D-methyl-malonyl-CoA, catalyzed by propionyl-CoA carboxylase and biotin is required as a coenzyme.
c. D-methyl-malonyl-CoA is converted to L-methyl-malonyl-CoA by methyl-malonyl- CoA racemase.
d. L-methyl-malonyl-CoA is isomerized to succinyl-CoA by methyl-malonyl-CoA isomerase which requires vitamin B12 as a coenzyme.
Conversion of Glycerol:
a. Glycerol is first converted to glycerol-3- phosphate by glycerokinase with ATP in liver and kidney.
b. Glycerol-3-phosphate is oxidized to dihydroxyacetone phosphate by glycerol- 3-phosphate dehydrogenase in presence of NAD+. Dihydroxyacetone phosphate is then converted to glucose.
Conversion of Lactate to Glucose:
a. Lactic acid is the major end product in muscle in anaerobic glycolysis. Muscle tissue is incapable of resynthesizing glucose from lactate. The conversion takes place entirely in the liver.
b. Muscle lactate is transported to the liver by the blood. In the liver, it is converted to glucose and glycogen by the enzymes concerned in gluconeogenesis.
c. Liver glycogen is converted to glucose which is carried back to muscle by blood.
This conversion of muscle lactate to glucose in liver and its reentry into muscle is called “Cori Cycle”.
Conversion of Amino Acids to Glucose:
a. The glucogenic amino acids are converted to the intermediates of citric acid cycle either by transamination or deamination which is given in Fig. 17.19.
b. These intermediates are converted to malate and finally converted to glucose by the enzymes involved in gluconeogenesis.
Conversion of Fatty Acids to Glucose:
Fatty acids are metabolized to acetyl-CoA by β-oxidation. Acetyl-CoA enters the citric acid cycle and then converted to malate. Malate is diffused from the mitochondria to the extra-mitochondrial portion of the cell where it is finally converted to glucose by the enzymes involved in gluconeogenesis.
Acetyl-CoA is not permeable to pass from the mitochondria to the cytosol through the mitochondrial membrane. But citrate is permeable through mitochondrial membrane to pass to the cytosol where it is splitted to acetyl-CoA and oxaloacetate.
Role of fructose 2, 6-Bisphosphate in the Regulation of Glycolysis and Gluconeogenesis in Liver:
a. Fructose 2, 6-bisphosphate is the most potent positive allosteric effector of phosphofructokinase-I and inhibitor of fructose-l, 6-bi-phosphatase in liver.
b. It relieves inhibition of phosphofructokinase-I by ATP and increases affinity for fructose 6-phosphate.
c. It inhibits fructose-1, 6-bisphosphatase by increasing the Km for fructose 1, 6- bi-phosphate.
d. It is formed by phosphorylation of fructose 6-phosphate by phosphofructokinase-2 which is also responsible for its breakdown since it contains fructose-2, 6- bisphosphatase activity. This bi-functional enzyme is under the allosteric control of fructose 6-phosphate.
e. When glucose is short, glucagon stimulates the production of cAMP which inactivates phosphofructokinase-2 and activates fructose-2, 6-bisphosphatase by phosphorylation.
f. In the superfluity of glucose, the concentration of fructose 2, 6-bisphosphate increases stimulating glycolysis by activating phosphofructokinase-I and inhibiting fructose-1, 6-bisphosphatase.
g. In glucose shortage, gluconeogenesis is stimulated by a decrease in the concentration of fructose 2, 6-bisphosphate which deactivates phosphofructokinase-I and de-inhibits fructose-1, 6-bisphosphatase. This mechanism also shows that glucagon stimulation of glycogenolysis in liver results in glucose release rather than glycolysis.
h. Recently, it has been indicated that glucose 1, 6-bisphosphate plays a similar role in some extra-hepatic tissues.
Substrate (Futile) Cycle:
(a) The control points in glycolysis and glycogen metabolism involve a cycle of phosphorylation and dephosphorylation catalyzed by the enzymes like glucokinase and glucose-6-phosphatase; phosphofructokinase-1 and fructose-1, 6- bisphosphatase; pyruvate kinase, pyruvate carboxylase and phosphoenolpyruvate carboxykinase; glycogen synthase and phosphorylase.
If these would be allowed to cycle unchecked, ultimately there would be hydrolysis of ATP. But this cannot happen due to the different control mechanisms which can signify that one limb of the cycle is inhibited as the other is stimulated as per the requirements of the tissue and of the body,
(b) In the phosphofructokinase and fructose- 1, 6-bisphosphatase, the allosteric modifier, fructose 2, 6-bisphosphate, causes a large change in the net effect of metabolites in either direction of the cycle. This fine effect of metabolic control occurs only at the expense of some loss of ATP.
Regulation of Gluconeogenesis:
a. The key enzymes of gluconeogenesis are pyruvate carboxylase, phosphoenolpyruvate carboxykinase, fructose-1, 6-bisphosphatase and glucose-6-phosphatase.
b. The hormones glucagon and glucocorticoids which are secreted during starvation stimulate glucose-6-phosphatase to enhance gluconeogenesis.
c. High carbohydrate diets increase the insulin/glucagon ratio and thus reduce the activities of the key enzymes of gluconeogenesis to minimize gluconeogenesis.
d. ATP and citrate are the activators of fructose-1, 6-bisphosphatase and, hence, gluconeogenesis is increased. But the high level of AMP in liver cells inhibits fructose-1, 6-bisphosphatase and thus reduces gluconeogenesis.
e. In glucose shortage, the more secreted glucagon stimulates gluconeogenesis by decreasing the concentration of fructose-2, 6-bisphosphate which in turn inhibits phosphofructokinase-1 and activates the enzyme fructose-1, 6-bisphosphatase.
f. During starvation the excessive liberated glucagon stimulates the enzyme phosphoenolpyruvate carboxykinase and thus increases gluconeogenesis.
g. The enzyme pyruvate carboxylase is allosterically activated by acelyt-CoA. During starvation increased fatty acid oxidation provides more acetyl-CoA which promotes gluconeogenesis.
h. Increased ADPallosterically inhibits pyruvate carboxylase and thus reduces gluconeogenesis.
i. The hormones glucagon, epinephrine, and glucocorticoids stimulate the synthesis of pyruvate carboxylase and thus enhance gluconeogenesis. But the hormone insulin depresses the enzyme pyruvate carboxylase and thus reduces gluconeogenesis.
Carbohydrate Metabolism Process # 6. Hexose Monophosphate Shunt:
a. This is an alternate aerobic pathway for the oxidation of glucose in the liver, lactating mammary gland and adipose tissue in addition to the Embden-Meyerhof pathway for glycolysis.
b. The enzymes of this pathway are present in the extra-mitochondrial portion of the cell. This pathway is active in liver, adipose tissue, adrenal cortex, thyroid, erythrocytes, testis and lactating mammary gland.
c. In this pathway, 3 molecules of glucose- 6-phosphate yield 3 molecules of CO2 and 3 molecules of 5 carbon residues (pentose sugars). The latter are converted ultimately to 2 molecules of glucose-6-phosphate and one molecule of glyceraldehyde-3- phosphate.
d. NADP, instead of NAD, is used as a hydrogen acceptor in this pathway.
Metabolic Reactions:
The reaction takes place in two phases:
(i) Glucose-6-phosphate by dehydrogenation and decarboxylation gives rise to ribulose-5-phosphate.
(ii) Ribulose-5-phosphate is converted back to glucose-6-phosphate by transketolase and transaldolase.
a. Glucose-6-phosphate is dehydrogenated to 6-phosphogluconate via 6-phosphogluconolactone by glucose-6-phosphate dehydrogenase in presence of NADP and the cofactors Mg++, Mn++ or Ca++. 6-phospho- gluconolactone is acted by gluconolactone hydrolase in presence of Mg++, Mn++ or Ca++. Genetically, the deficiency of glucose-6-phosphate dehydrogenase in erythrocytes is associated with a tendency to hemolysis by primaquine and sulfonamide.
b. 6-phosphogluconate is oxidized by 6- phosphogluconate dehydrogenase in the presence of the coenzyme NADP and cofactors Mg++, Mn++ or Ca++ to 3-keto 6- phosphogluconate which is decarboxylated to form ribulose-5-phosphate.
c. Ribulose-5-phosphate is acted on by ribulose-5-phosphate epimerase which changes the configuration about carbon 3 forming xylulose-5-phosphate and also by the enzyme ribose-5-phosphate ketoisomerase which converts ribulose-5-phosphate to ribose-5-phosphate.
d. Transketolase with the help of TPP and Mg++ transfers carbons 1 and 2 of xylulose-5-phosphate to the ribose-5-phosphate forming sedoheptulose-7-phosphate and glyceraldehyde-3-phosphate.
5. Transaldolase allows the transfer of a 3- carbon moiety from sedoheptulose-7-phosphate to the glyceraldehyde-3-phosphate to form fructose-6-phosphate and erythrose-4-phosphate.
e. Transketolase with the help of TPP and Mg” transfers carbon 1 and 2 from xylulose-5-phosphate to erythrose-4-phosphate producing fructose-6-phosphate and glyceraldehyde-3-phosphate.
The overall reaction of the hexose monophosphate shunt is shown:
Regulation of HMP Shunt:
a. The first reaction of this pathway catalysed by glucose-6-phosphate dehydrogenase is the “rate limiting” step. This is mainly regulated by the cytoplasmic levels of NADP+ and NADPH.
b. The high carbohydrate diets enhance the activities of both dehydrogenases and the rate of the pathway. But reverse occurs in case of starvation and diabetes mellitus.
c. The increased fatty acid synthesis and steroid synthesis re-oxidize NADPH to NADP+ for which cytoplasmic ratio of NADP7 NADPH is increased enabling to enhance the shunt pathway.
d. The hormone insulin stimulates the synthesis of both the dehydrogenases and thus enhances the activity of the pathway.
e. Thyroid hormones also stimulate the activity of glucose-6-phosphate dehydrogenase and thus enhances the shunt pathway.
