Glycogen metabolism Gluconeogenesis Marco Y W Zaki Ph

Glycogen metabolism & Gluconeogenesis Marco Y. W. Zaki; Ph. D, M. Sc, B. Sc

Glycogen metabolism Structure of glycogen: • Glycogen is homopolysaccharide formed of branched α D glucose units (a 1, 4 and α 1, 6). • The main glycosidic bond is α 1 -4 -linkage. Only at the branching point, the chain is attached by α 1 -6 linkage. • Each branch is made of 12 -14 glucose units. • Glycogen is present mainly in cytosol of liver and muscles.

Glycogen metabolism Liver glycogen Muscle glycogen Source 1. Blood glucose. 2. Other hexoses (fructose). 3. Non-carbohydrate. Blood glucose. Amount 120 gm maximally (depends on liver weight). 400 gm maximally (depends on muscle mass). Concentration 6%. 1%. Function Source for blood glucose for the body especially the brain at times of need. After 12 -18 hours fasting, liver glycogen is depleted. Source of energy for contracting muscles and produce lactate. End product of glycogenolysis Glucose. Lactate. Hormonal regulation Adrenaline, noradrenaline and glucagon stimulate glycogenolysis and inhibit glycogenesis and insulin has an opposite effect. Adrenaline and noradrenaline stimulates glycogenolysis and inhibits glycogenesis and insulin has an opposite effect. Glucagon has no effect.

Glycogen metabolism, Synthesis of glycogen (glycogenesis) Definition: It is the formation of glycogen in liver and muscles. Substrates for glycogen synthesis: In liver: a) Blood glucose. b) Other hexoses: fructose and galactose. c) Non-carbohydrate sources: (gluconeogenesis) e. g. glycerol and lactate. These are converted first to glucose, then to glycogen. In muscles: Blood glucose only.

Glycogen metabolism, Synthesis of glycogen (glycogenesis)

Glycogen metabolism, Synthesis of glycogen (glycogenesis); steps Glucose molecules are the first activated to uridine diphosphate glucose (UDP-G). Then these UDP-G molecules are added to a glycogen primer to form glycogen. 1. Formation of UDP-Glucose (UDP-G) 2. Formation of glycogen: a) UDP-Glucose reacts with glycogen primer, which may be: 1) Few molecules of glucose linked together by α. 1 -4 linkage. 2) A protein called glycogenin. UDP-G molecules react with -OH of tyrosine of that protein to initiate glycogen synthesis. b) Glycogen synthase enzyme: By the action of glycogen synthase (key enzyme of glycogenesis), UDP-G molecules are added to glycogen primer causing elongation of the α 1 -4 branches up to 12 -14 glucose units. c) Branching enzyme: It transfers parts of the elongated chains (5 -8 glucose residues) to the next chain forming a new α I-6 glycosidic bond. The new branches are elongated by the glycogen synthase and the process is repeated.

Glycogen metabolism, Breakdown of glycogen (Glycogenolysis) Definition It is the breakdown of glycogen into glucose (in liver) and lactic acid (in muscles). Steps • Phosphorylase (the key enzyme of glycogenolysis) acts on α. 1 -4 bonds, breaking it down by phosphorolysis (i. e. breaking down by addition of inorganic phosphate "Pi"). Therefore, it removes glucose units in the form of glucose-1 -phosphate. • Phosphorylase enzyme acts on the branches containing more than 4 glucosyl units. • When the branch contains 4 glucose units, 3 of them are transferred to a next branch by transferase enzyme, leaving the last one. • The last glucose unit that is attached to the original branch by α 1 -6 bond is removed by debranching enzyme by hydrolysis (i. e. breaking the bond down by addition of H 20). • Glucose-1 -phosphate molecules are converted to glucose-6 - phosphate, by mutase enzyme.

Glycogen metabolism, Breakdown of glycogen (Glycogenolysis) Fate of glucose-6 -phosphate • In liver: glucose-6 -phosphate is converted to glucose by glucose-6 -phosphatase. • In muscles: there is no glucose 6 -phosphatase, so glucose-6 phosphate enters glycolysis to give lactate.

Glycogen metabolism; regulation • There is a coordinated regulation of glycogenesis and glycogenolysis i. e. conditions leading to stimulation of glycogenolysis, inhibiting at the same time glycogenesis and vise versa. • During fasting, glycogenolysis is stimulated and glycogenesis is inhibited. This provides blood glucose from liver glycogen. • After meal, part of absorbed glucose (40%) goes to general circulation to be utilized. The remaining (60%) is converted into glycogen in liver. So after meal, glycogenesis is stimulated and glycogenolysis is inhibited. • The principle enzymes controlling glycogen metabolism are glycogen synthase and phosphorylase. During fasting • The binding of hormones, such as glucagon or epinephrine, to membrane receptors signals the need for glycogen to be degraded—either to elevate blood glucose levels or to provide energy for exercising muscle. • Activation of protein kinase: Binding of glucagon or epinephrine to their specific cell-membrane receptors results in the c. AMP-mediated activation of c. AMP-dependent protein kinase. This enzyme is a tetramer, having two regulatory subunits (R) and two catalytic subunits (C). c. AMP binds to the regulatory subunit dimer, releasing individual catalytic subunits that are active.

Glycogen metabolism; regulation • Activation of phosphorylase kinase: Phosphorylase kinase exists in two forms: an inactive “b” form and an active “a” form. Active c. AMP-dependent protein kinase phosphorylates the inactive “b” form of phosphorylase kinase, producing the active “a” form. • Activation of glycogen phosphorylase: Glycogen phosphorylase also exists in two forms: the dephosphorylated, inactive “b” form and the phosphorylated, active “a” form. Active phosphorylase kinase phosphorylates glycogen phosphorylase b to its active “a” form, which then begins glycogen breakdown. Phosphorylase a is reconverted to phosphorylase b by the hydrolysis of its phosphate by protein phosphatase 1.

Glycogen metabolism; regulation • Inactivation of Glycogen synthase: The regulated enzyme in glycogen synthesis is glycogen synthase. It also exists in two forms, the active a form and the inactive b form. However, for glycogen synthase, the active form is dephosphorylated while the inactive form is phosphorylated. Glycogen synthase a is converted to the b form (and, therefore, is inactivated) by phosphorylation at several sites on the enzyme, with the level of inactivation proportional to its degree of phosphorylation. This conversion process is catalyzed by several different protein kinases that are regulated by c. AMP or other signaling mechanisms. The binding of the hormones glucagon or epinephrine to the hepatocyte receptors, or of epinephrine to muscle cell receptors, results in the activation of adenylyl cyclase, mediated by a G protein. This enzyme catalyzes the synthesis of c. AMP, which activates c. AMP-dependent protein kinase A, as described. Protein kinase A then phosphorylates and, thereby, inactivates glycogen synthase. Glycogen synthase b can be transformed back to the a form by protein phosphatase 1, which removes the phosphate groups hydrolytically.

Glycogen metabolism; regulation After meal: • Blood glucose level tends to increase stimulating the release of Insulin hormone. • Insulin stimulates the phosphodiesterase enzyme converting c. AMP into AMP abolishing its glycogen-degrading effect. • Insulin also stimulates phosphatase enzyme that removes phosphate group from Glycogen synthase enzyme (thus activating it) and phosphorylase enzyme (thus inhibiting it). • As a consequence, glycogenolysis decreases and glycogenesis is induced.

Glycogen metabolism; Glycogen storage diseases Definition: These are group of inherited disorders characterized by deposition of abnormal type or quantity of glycogen in the tissues. Causes: They are mainly due to deficiency of one of enzymes of glycogen metabolism. Types: (8 types) l. Type one (I): Von Gierk' s disease: It is due to deficiency of glucose-6 -pbosphatase and is the commonest type. It is characterized by: • Accumulation of large amount of glycogen in liver leading to disturbance of liver functions and hepatomegaly. • Fasting hypoglycemia. • Ketosis and hyperlipidemia. • Hyperuricemia (gout): decrease glucose-6 -phosphatase increase Glucose-6 -phosphate stimulate Pentose phosphate pathway increase Ribose production increase Uric acid Gout.

Glycogen metabolism; Glycogen storage diseases II. Type two (II): Pompe’s disease. III. Type three (Ill): Limit dextrosis (Cori’s disease): a) Due to deficiency of debranching enzymes in liver, muscles and heart. b) Glycogen has many short branches.

Gluconeogenesis Definition Gluconeogenesis is the formation of glucose from non-carbohydrate sources. These sources include: Lactate, pyruvate, glycerol, some amino acids (Glucogenic amino acids) and propionate (in ruminants only). Functions • Gluconeogenesis supplies the body with glucose: I. Glucose is the only source of energy for nervous tissues, RBCs and skeletal muscles during exercises. 2. Glucose is the precursor of milk sugar (lactose) in mammary gland. 3. Glucose is important during low carbohydrate diet or when liver glycogen is depleted (liver glycogen is depleted after 12 -18 hours). • Gluconeogenesis clears the blood from the waste products of other tissues as lactate (produced by muscles and RBCs). Location Intracellular location: cytosol and mitochondria. Organ location: Liver (90%) and Kidney (10%).

Gluconeogenesis Major pathways and regulation of gluconeogenesis and glycolysis in the liver. • Entry points of glucogenic amino acids after transamination are indicated by arrows extended from circles. • The key gluconeogenic enzymes are shown in double bordered boxes. • The ATP required for gluconeogenesis is supplied by the oxidation of fatty acids. • Propionate is important only in ruminants. • Arrows with wavy shafts signify allosteric effects; dashshafted arrows, covalent modification by reversible phosphorylation. • High concentrations of alanine act as a “gluconeogenic signal” by inhibiting glycolysis at the pyruvate kinase step.

Gluconeogenesis, steps The steps of gluconeogenesis are mainly the reversal of glycolysis, except for the three irreversible kinases which are replaced by the following enzymes: Pyruvate & Phosphoenolpyruvate • Reversal of the reaction catalyzed by pyruvate kinase in glycolysis involves two endothermic reactions. • Mitochondrial pyruvate carboxylase catalyzes the carboxylation of pyruvate to oxaloacetate, an ATP-requiring reaction in which the vitamin biotin is the coenzyme. Biotin binds CO 2 from bicarbonate as carboxybiotin prior to the addition of the CO 2 to pyruvate. • The resultant oxaloacetate is reduced to malate, exported from the mitochondrion into the cytosol and there oxidized back to oxaloacetate. • A second enzyme, phosphoenolpyruvate carboxykinase, catalyzes the decarboxylation and phosphorylation of oxaloacetate to phosphoenolpyruvate using GTP as the phosphate donor. • In liver and kidney, the reaction of succinate thiokinase in the citric acid cycle produces GTP (rather than ATP as in other tissues), and this GTP is used for the reaction of phosphoenolpyruvate carboxykinase, thus providing a link between citric acid cycle activity and gluconeogenesis, to prevent excessive removal of oxaloacetate for gluconeogenesis, which would impair citric acid cycle activity.

Gluconeogenesis; steps Fructose 1, 6 -Bisphosphate & Fructose-6 -Phosphate • The conversion of fructose 1, 6 -bisphosphate to fructose-6 -phosphate, for the reversal of glycolysis, is catalyzed by fructose 1, 6 -bisphosphatase. • Its presence determines whether a tissue is capable of synthesizing glucose (or glycogen) not only from pyruvate but also from triose phosphates. Glucose-6 -Phosphate & Glucose • The conversion of glucose-6 -phosphate to glucose is catalyzed by glucose-6 -phosphatase. • It is present in liver and kidney, but absent from muscle, which, therefore, cannot export glucose into the bloodstream.

Blood glucose is derived from the diet, gluconeogenesis, & glycogenolysis • The digestible dietary carbohydrates yield glucose, galactose, and fructose that are transported to the liver via the hepatic portal vein. Galactose and fructose are readily converted to glucose in the liver. • Glucose is formed from two groups of compounds that undergo gluconeogenesis: (1) those that involve a direct net conversion to glucose, including most amino acids and propionate and (2) those that are the products of the metabolism of glucose in tissues. Thus, lactate, formed by glycolysis in skeletal muscle and erythrocytes, is transported to the liver and kidney where it reforms glucose, which again becomes available via the circulation for oxidation in the tissues. This process is known as the Cori cycle, or the lactic acid cycle. • In the fasting state, there is a considerable output of alanine from skeletal muscle, far in excess of the amount in the muscle proteins that are being catabolized. • It is formed by transamination of pyruvate produced by glycolysis of muscle glycogen, and is exported to the liver, where, after transamination back to pyruvate, it is a substrate for gluconeogenesis. This glucose-alanine cycle provides an indirect way of utilizing muscle glycogen to maintain blood glucose in the fasting state. The ATP required for the hepatic synthesis of glucose from pyruvate is fromed by the oxidation of fatty acids. • Glucose is also formed from liver glycogen by glycogenolysis.

Blood glucose is derived from the diet, gluconeogenesis, & glycogenolysis

Gluconeogenesis; regulation Hormonal regulation: 1. Glucocorticocoids e. g. cortisol: stimulate gluconeogenesis by the following mechanisms: a) They Induce the synthesis of gluconeogenesis enzymes b) Glucocorticoids stimulate protein catabolism by tissues this leads to increase glucogenic amino acids available for gluconeogenesis. 2. Glucagon: Stimulates gluconeogenesis by lowering the level of fructose 2, 6 bisphosphate. 3. Insulin: Inhibits gluconeogenesis. It acts as inhibitor for synthesis of enzymes of gluconeogenesis. Allosteric effectors (Acetvl Co. A and ATP): 1. Stimulate gluconeogenesis by inhibiting glycolysis (through inhibiting phosphofructokinase-1) and stimulate gluconeogenesis (by stimulating fructose 1, 6 bisphosphatase). 2. Acetyl Co. A also stimulates pyruvate carboxylase (gluconeogenesis) and inhibits pyruvate dehydrogenase (oxidation).

Gluconeogenesis; energy •