Structure of glycogen * Glycogen: branched-chain homopolysaccharide made of α-D- glucose linked by α-1,4 linkage. After every 8-10 glucose residues there is a branch containing α-1,6 linkage.

Glycogen molecules exist in cytoplasmic granules that contain the enzymes for synthesis and degradation of glycogen.

Glycogen metabolism Glucose is the main source of energy for brain, cells without mitochondria (RBC) and essential source of energy for exercising muscles because it substrate for anaerobic glycolysis 1) Dietary intake of glucose or glucose precursors as starch, monosaccharides (Fructose), disaccharides (Sucrose, maltose, lactose) 2) Glycogen: provide a rapid supply for glucose in the absence of dietary glucose. Glycogen is rapidly degraded into glucose 3) Gluconeogenesis: provide sustained synthesis of glucose stimulated by low blood glucose (slow) Source of Blood glucose Diet Glycogen gluconeogenesis

Structure and function of glycogen The main store of glycogen in the body is liver (100 g) and skeletal muscles (400g) In muscle: glycogen Glucose to produce ATP and energy In liver: Glycogen Maintain blood glucose Why NOT store excess glucose as free glucose instead of glycogen? Fluctuation of glycogen stores -Liver glycogen stores increase during the well-fed state and are depleted during a fast -Muscle glycogen is NOT affected by short periods of fasting and is moderately decreased in prolonged fasting while it is affected by exercise.

Synthesis of Glycogen (Glycogenesis)  α-D- glucose is the monomer  Occurs in the cytosol  Requires energy supplied by ATP and UTP α-D- glucose attached to UDP (glucose-UDP) is the source of all the glycosyl residues that are added to the growing glycogen

UDP-glucose synthesis UDP-glucose is the source of glucosyl groups that used in glycogen synthesis

UDP + ATP nucleoside diphosphate kinase UTP + ADP Glycogen synthase is responsible for making α-1  4 linkage. It can only elongate existing chains and cannot initiate chain synthesis. * Fragment of glycogen can serve as a primer Elongation of glycogen chains by Glycogen Synthase Involve transfer of glucose units from UDP-glucose to the non-reducing end of the growing chain forming α-1  4 glycosidic linkage (anomeric hydroxyl of carbon 1 of the activated glucose and carbon 4 of accepting glucose residue).

* Formation of branches in glycogen * Glycogen is highly branched molecule (every 8 residues)  increase solubility and size compared to the non-branched amylose. * Branching increases the number of non-reducing ends to which glucosyl residues can be added or removed  accelerate the rate of glycogen synthesis or degradation. * Synthesis of branches by glycosyl 4:6 transferase Branches are made by “branching enzyme” that called Amylo-(α1  4)  (α1  6)transglycosylase or Glucosyl(4:6)transferrase This enzyme transfers 5-8 glycosyl residues from the non-reducing end to another residue forming α-1  6 linkage. Old and new non-reducing ends are available to be further elongated by Glycogen Synthase

* Fragment of glycogen can serve as a primer In absence of glycogen, specific protein glycogenin can serve as an acceptor of glucose residues. The OH gp of specific tyrosine side chain is the initial site of attachment * Glycogen initiator synthase: transfer the first molecule of glucose from UDP-glucose to glycogenin. Then additional glucose unit is transferred to form short chain

Degradation of Glycogen (glycogenolysis) Glycogen phosphorylase cleaves α- 1,4 glycosidic bond at the non-reducing ends by phosphorolysis.

* Small amount of glycogen is degraded by lysosomal α- glucosidase * Glycogen phosphorylase sequentially degrades the glycogen chains at their non- reducing ends until 4 glucosyl units remain on each chain before a branch poin, it is called limit dextrin, phosphorylase cannot degrade it. Glucosyl (4:4) transferase

Removal of branches Branches are removed by two enzymic activities: The outer three of four glucosyl residues attached at a branch and transferrs them to the non-reducing end at another chain, thus the new chain is subjected to glycogen phosphorylase. The enzyme Glycosyl (4:4) transferase Remaining single glucose residue attached in an α- 1,6 – linkage is removed by amylo α –(1,6)– glucosidase releasing free glucose * Conversion of glucose 1- phosphate to glucose 6-phosphate by “Phosphoglucomutase”

Remaining single glucose residue attached in an α- 1,6 – linkage is removed by amylo α –(1,6)– glucosidase releasing free glucose Degradation of Glycogen (glycogenolysis)

glycogen synthase aglycogen synthase b Protein kinase Phosphoprotein phosphatase ATPADP Pi H2OH2O Glycogen synthase glycogen synthase a (active: unphosphorylated) glycogen synthase b (less active: phosphorylated) Glycogen phosphorylase Glycogen phosphorylase b (Inactive: Unphosphorylated) Glycogen phosphorylase a (active: phosphorylated) Phosphorylase a phosphatase Phosphatase b kinase Pi H2OH2O ATP ADP * Glycogen synthase and Glycogen phosphorylaseare reciprocally regulated.

* Regulation of Glycogen Synthesis and Degradation To maintain the blood glucose level, the glycogen synthesis and degradation is highly regulated. Well fed state activation of glycogen synthesis Fasting degradation of glucose is accelerated *Both glycogen synthase and phosphorylase are allosterically and hormonally regulated. In skeletal muscle: degradation is activated during exercise while glycogen accumulation activated at the rest

Phosphorylase kinase is responsible for phosphorylation and activation of phosphorylase. And it is regulated by phosphorylation – dephosphorylation mechanism Regulation of glycogen phosphorylase glycogen phosphorylase is subject to allosteric activation by AMP and allosteric inhibition by glucose and ATP

Activation of glycogen degradation by cAMP-directed pathway

* Activation of glycogen degradation in muscle by Ca +2 and AMP A. During muscle contraction  urgent need for ATP… what happens? Nerve impluses cause membrane depolarization  Ca +2 is released from sarcoplasmic reticulum  Ca +2 bind to calmidulin subunit of phosphorylase kinase and activate it with the need for it’s phosphorylation. Phosphorylase kinase is maximally active when both phosphorylated and bound to Ca +2 B. During exercise  ATP is depleted  AMP will increase  AMP bind to inactive form (phosphorylase b and activate it without phosphorylation) Phosphorylase kinase is multisubunits protein, one subunit is Ca +2 - binding regulatory protein (Calmodulin) * Calmodulin also found in free form in the cells and act as Ca +2 receptors. * Binding of Ca +2 to calmodulin part of phosphorylase kinase induce conformational change to make it more active. * Ca +2 bind to both the phosphorylated and unphosphorylated forms of phosphorylase kinase * Maximum activity obtained by phosphorylation and binding of Ca +2 to phosphorylase kinase.

Regulation of glycogen synthase Glycogen synthase needs to be turned on and glycogen phosphorylase turned off during the glycogen synthesis

* Inhibition of glycogen synthesis by cAMP-directed pathway

Epinephrine c-AMP inactive b a active inactive Epinephrine Glucagon

The End

* Regulation of glycogen synthase Glycogen synthase needs to be turned on and glycogen phosphorylase turned off during the glycogen synthesis. * Glycogen synthase a (active form unphosphorylated) and not affected by the presence of G6P * Glycogen synthase b (inactive, phosphorylated) and it’s activity is affected by the presence of G6P G6P is allosterically activator for glycogen synthase b * Glycogen synthase a can be converted into b form with several kinases, these kinases are regulated by second massengers of hormone actions including cAMP, Ca +2, diacylglycerol Different kinase  reflect different sites of phosphorylation

Effector control of glycogen metabolism:- * Glycogen itself –ve feedback on synthesis * Glucose High glucose level in blood  activate glycogenesis by two mechanisms: 1- high glucose level  stimulate insulin release from pancreas insulin inhibit hepatic glycogen phosphorylase and activate glycogen synthase. 2- hormone independent mechanism, direct inhibition of glycogen phosphorylase a by binding the glucose to this enzyme  stimulate the dephosphrylation of this enzyme  inactivation of it. So “phosphorylase a function as glucose receptor” * Glucagon: stimulate glycogen degradation in the liver at response of low blood glucose  glucagon is released  stimulate glucagon mobilization from liver. * Epinephrine: stimulate glycogen degradation, epinephrine is released from adrenal medulla  interact with plasma receptors  activate adenylate cyclase  increase cAMP  activation of glycogenolysis and inhibition of glycogenesis and glycolysis to increase the release of glucose from liver. Epinephrine + β- adrenergic receptor  stimulate adenylate cyclase Epinephrine + α- adrenergic receptor  activation of phospholipase C

* Coordinated regulation of glucagon synthesis and degradation:- - Glycogen synthesis and degradation are regulated by the same hormonal signals. High level of insulin  increase glycogen synthesis, decrease glycogen degradation High glucagon or epinephrine  increase glycogen degradation and decrease glycogen synthesis. * The effect of these hormones are mediated by cAMP level Insulin Glucagon Epinephrin decrease cAMP increase cAMP cAMP activate some protein kinases that phosphorylate the key enzymes  Phosphorylation of the enzymes may activate or inactivate this enzyme

Phosphoprotein phosphatase: reduce the phosphorylation state of both glycogen phosphorylase and phosphorylase kinase * Regulation of phosphoprotein phosphatase is linked to cAMP. * Glucagon and epinephrine increase cAMP  promote activation of phosphorylase kinase and inactivation of phosphoprotein phosphatase. * Insulin exerts opposite effect on phosphorylase by promoting activation of phosphoprotein phosphatase activity.

* Insulin stimulates glycogen synthesis in muscle and liver. High blood glucose level  increase insulin secretion and decrease glucagon Low blood glucose level  decrease insulin and increase glucagon * Glucagon & Insulin have opposite effect on glycogen metabolism * Insulin increase glucose utilization by promoting glycogensis and inhibiting glycogenolysis in muscle & liver * Insulin is essential for the entrance of glucose into muscle cell NOT hepatocyte because hepatocytes have insulin insensitive glucose transport system (GLUT-2) * While muscle cells and adipocytes have glucose transport system (GLUT-4) that is insulin dependent. * Glycogen metabolism disorder Glycogen storage disease !!!