Carbohydrates: Glycogen Metabolism PDF

BIOCHEMISTRY CARBOHYDRATES: Glycogen Metabolism Brendo Jandoc, M.D. - extensively degraded in exercising muscle to provide Topic Outli...

BIOCHEMISTRY CARBOHYDRATES: Glycogen Metabolism Brendo Jandoc, M.D. - extensively degraded in exercising muscle to provide Topic Outline important energy source I. Overview - produces glucose 1-phosphate as the major product, II. Structure and Function of Glycogen but free glucose is also formed. A. Amounts of Liver and Muscle glycogen 3. Gluconeogenesis B. Structure of glycogen - process of synthesizing glucose from C. Fluctuation of glycogen stores noncarbohydrate precursors III. Synthesis of Glycogen (Glycogenesis) - provide sustained synthesis of glucose A. Synthesis of UDP-glucose - slow in responding to falling glucose level B. Synthesis of a primer to initiate glycogen synthesis B. Glycogenesis and Glycogenolysis  occur in separate ways C. Elongation of glycogen chains by glycogen  are regulated in liver by hormonal changes that signal the synthase need for blood glucose D. Formation of branches in glycogen 1. Function IV. Degradation of Glycogen (Glycogenolysis) - helps maintain glucose homeostasis by forming A. Shortening of chains (glycogenesis) or breaking down (glycogenolysis) glycogen B. Removal of branches - crucial for the storage of energy derived from carbohydrate C. Conversion of glucose 1-phosphate to glucose metabolism 6-phosphate 2. Location D. Lysosomal degradation of glycogen a. Glycogenesis – takes place when blood glucose levels are V. Regulation of Glycogen Synthesis and Degradation sufficiently high A. Activation of glycogen degradation by cAMP- - Liver directed pathway - Muscle B. Inhibition of glycogen synthesis by a cAMP- b. Glycogenolysis - occurs primarily in the liver and is directed pathway stimulated by the hormones glucagon and epinephrine C. Allosteric regulation of glycogen synthesis and (adrenaline) degradation - Heart VI. Glycogen storage disease - Liver I. OVERVIEW - Muscle (skeletal muscle)  Glycogen is the storage form of glucose found in most types of cells. It is composed of glucosyl units linked by α-1,4-glycosidic bonds, with α-1,6-branches occurring roughly every 8 to 10 glucosyl units.  rapidly released from liver and kidney glycogen in the absence of a dietary source of glucose  extensively degraded in exercising muscle to provide important energy source A. Glucose  preferred energy source of the brain  required energy source for cells with few or no mitochondria (mature RBCs)  energy source in exercising muscle (substrate for anaerobic glycolysis) Three Primary Sources of Glucose 1. Diet - dietary intake of glucose and glucose precursors: starch, monosaccharides (fructose), disaccharides (lactose, maltose, sucrose) 2. Glycogen degradation - rapidly released from liver and kidney glycogen in the absence of glucose dietary source ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 1 of 8  BIOCHEMISTRY CARBOHYDRATES: Glycogen Metabolism Figure 1. Glycogenesis and Glycogenolysis Pathway in the Liver - Adequate for about 12 hours without the support of gluconeogenic Insulin decreases the level of cAMP only after it has been raised by pathways glucagon or epinephrine · The release of insulin causes an increased movement of glucose into the cells and increased glucose metabolism. Glucagon acts on heart muscle but not in skeletal muscle and is the primary hormone responsible for increasing glucose levels. Glucan Transferase and Debranching Enzyme - two separate activities Glycogenolysis in skeletal muscle and of the same enzyme liver. Glycogen stores serve different functions in muscle cells and liver. In skeletal muscle and other cell types, the G6P enters the glycolytic pathway. In the liver, the G6P that is generated from glycogen degradation is hydrolyzed to glucose (blood glucose). A. Amounts of Liver and Muscle Glycogen  400 g of glycogen → 1 - 2 % of fresh weight of resting muscle  100 g of glycogen → 6 - 8 % of fresh weight of a well-fed adult liver B. Structure of Glycogen  highly branched chain of homopolysaccharide made from α- D-glucose  exist in discrete cytoplasmic granules that contain most of the enzymes necessary for glycogen synthesis and degradation 1. Primary Glycosidic Bond - α-1,4 linkage - the linkages between glucose residues 2. Branch Point - every 8-10 glucosyl residues Figure 2. The gluconeogenic pathway is almost the reverse of the glycolytic pathway, except for the reaction sequences. At these three steps, the reactions are catalyzed by the different enzymes. The energy requirements of these reactions differ, and one pathway II. STRUCTURE AND FUNCTION OF GLYCOGEN 1. Main Stores of Glycogen a. Muscle Glycogen (Skeletal muscle) - Fuel reserve for the synthesis of ATP during muscle contraction (for the generation of ATP in the absence of oxygen or during restricted blood flow) - the G6P enters the glycolytic pathway b. Liver Glycogen - maintain blood glucose concentration during fasting or exercise - α-1, 6 linkages - first and immediate source of glucose for the maintenance - more frequent in the interior of the molecule of blood glucose levels - the G6P that is generated from glycogen degradation is hydrolyzed to glucose 2. Duration of Glycogen Storage ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 2 of 8  BIOCHEMISTRY CARBOHYDRATES: Glycogen Metabolism The anomeric carbon of one glucosyl residue that is not attached to - The enzyme itself is phosphorylated, and the another glucosyl residue (the reducing end) is attached to the protein phosphate group takes part in a reversible glycogenin by a glycosidic bond. reaction in which glucose 1,6- bisphosphate is an intermediate C. Fluctuation of Glycogen Stores 1. Increased - well-fed state 2. Depleted - fasting state 3. Muscle Glycogen - not affected by short periods of fasting (days) - moderately decreased during prolonged fasting (weeks) - stores are replenished after exercise III. GLYCOGEN SYNTHESIS (GLYCOGENESIS)  energy-requiring pathway;  high-energy phosphate from UTP is used to activate the Formation of uridine diphosphate (UDP)-glucose from glucose. glucosyl residues to uridine diphosphate glucose (UDP-G) ADP, adenosine diphosphate; ATP, adenosine triphosphate; PPi, inorganic pyrophosphate. 1. Precursor B. Synthesis of Primer to Initiate Glycogen Synthesis - synthesized from α-D-glucose molecules  Glycogen synthesis requires the formation of α-1,4-glycosidic 2. Location bonds to link glucosyl residues in long chains and the - occurs in the cytosol formation of an α-1,6-branch every 8 to 10 residues. 3. Energy Supplied by  Most of glycogen synthesis occurs through the lengthening of - ATP (for glucose phosphorylation) the polysaccharide chains of a pre-existing glycogen molecule - UTP (Uridine triphosphate) (a glycogen primer). 1. Glycogen Synthase A. UDP-Glucose Synthesis - enzyme that attaches the glucosyl residues in α-1,4- 1. UDP-Glucose glycosidic bonds (makes the α-1,4 linkages) - Immediate precursor of glycogen (α-D-Glucose) - can only elongate pre-existing chains of glucose - activated form of glucose - rate-limiting step of glycogen synthesis - carbon 1 of glucosyl unit is esterified to the - regulatory enzyme for glycogen synthesis diphosphate moiety of UDP - transfers glucose residues from UDP-glucose to the - Glucose enters cells and is phosphorylated to glucose 6- nonreducing ends of a glycogen primer phosphate by the enzyme hexokinase (or by 2. Glycogen Fragment - serve as primer, in cells whose glycogen stores are not glucokinase in the liver) depleted, in the elongation process 3. Glycogenin  protein to which glycogen is attached  in the absence of glycogen fragment, it serves as acceptor of glucose residues  hydroxyl group of a specific tyrosine side chain serve as the site of initial glucosyl unit attachment  catalyzes the transfer of the first few glucose molecules by α-1,4 linkages  stays associated with the completed glycogen molecule  found in the center of the completed glycogen molecule 4. Glycogen Initiator Synthase 2. Enzymes  catalyzes the transfer of the initial UDP-glucosyl unit to a. UDP-Glucose Pyrophosphorylase glycogenin - reversible reaction b. Pyrophosphatase Note: Branching of glycogen serves two major roles: increased - hydrolyze inorganic pyrophosphate (PPi) → sites for synthesis and degradation, and enhancing the solubility makes the overall reaction irreversible of the molecule. c. Phosphoglucomutase C. Elongation of Glycogen Chains by Glycogen Synthase - converts glucose 6-phosphate to glucose 1-  transfer of glucose from UDP-glucose to the nonreducing phosphate. end of the growing chain, forming a new glycosidic bond ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 3 of 8  BIOCHEMISTRY CARBOHYDRATES: Glycogen Metabolism between the anomeric hydroxyl of carbon 1 of the  Glycogen degradation is a phosphorolysis reaction (breaking of activated glucose to carbon 4 of accepting glucosyl residue a bond using a phosphate ion as a nucleophile).  UDP - released after formation of new α-1,4 linkage  Glycogen is degraded by two enzymes: glycogen phosphorylase - converted back to UTP by nucleoside diphosphate kinase and the debranching enzyme UTP + ATP ↔ UTP + ADP  not a reversal of the synthetic reactions, instead, a separate set D. Formation of Branches in Glycogen of cytosolic enzymes is  Branching Involves detachment of existing Glycogen chains required  Oligomer - 6 to 8 glucose residues in length, removed from the  Products: nonreducing end of the chain. - primary product:  Amylose - linear molecule of unbranched glucosyl residues glucose 1-phosphate, obtained attached by α-1,4 linkages and is much less soluble than by breaking α-1,4 glycosidic glycogen bonds  Glycogen - branches located every 8 glucosyl residues (average) - free glucose: → tree-like structure released from each α-1,6 - branching also increases the number of nonreducing ends linked glucosyl residue to which new glucosyl residues can be added (and from A. Shortening of Chains which residues are removed) → accelerated synthesis  Glycogen (and degradation) Phosphorylase - different isoenzymes 1. Synthesis of Branches are present in different tissues - Glucosyl 4:6 Transferase [Amylo-α-(1,4) → α-(1,6)- - catalyzes the rate- transglucosidase] – branching enzyme responsible for making limiting step in branches glycogenolysis—the - breaks an a-1,4 bond of 6-8 glucosyl residues from the phosphorolytic cleavage of the nonreducing end of the glycogen chain to another residue α-1,4 linkages of glycogen to yield glucose-1-phosphate and forms an α-1,6 bond and attaches it to a non-terminal - requires pyridoxal phosphate as its coenzyme (the glucosyl residue by an α-1,6 linkage phosphate group is catalytically active) - resulting new and old nonreducing end can now be further - continue to hydrolyze a-1,4 linkages until it reaches a elongated by glycogen synthase point four glucose units from an α-1,6 branch - residues remain before a branch point → resulting 2. Synthesis of Additional Branches structure is limit dextrin (phosphorylase cannot degrade it - After elongation of these two ends has been accomplished by further) glycogen synthase, their terminal 6-8 glucosyl residues can be Disorders due to Genetic Defects of the Enzyme removed and used to make additional branches. a. Type IV Glycogen Storage Disease (McArdle Disease) - Muscle phosphorylase deficiency; type V GSD o Manifestations  skeletal muscle cramps and pain secondary to rhabdomyolysis  low blood lactate level during exercise b. Type VI Glycogen Storage Disease (Hers’ Disease) - genetic deficiency of liver phosphorylase; type VI GSD - patients typically have partial deficiency of the protein o Manifestations  Hepatomegaly: extreme enlargement of the liver  moderate hypoglycemia  mild acidosis  growth retardation The new branch points are at least 4 residues and an average of B. Removal of Branches by Debranching Enzyme System 7 to 11 residues from previously existing branch points.  Branches are removed by the two enzymatic activities of a single bifunctional protein, the debranching enzyme which has IV. DEGRADATION OF GLYCOGEN (GLYCOGENOLYSIS) both glucosyl 4:4 transferase and a-1,6-glucosidase activity. ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 4 of 8  BIOCHEMISTRY CARBOHYDRATES: Glycogen Metabolism  Two Enzymatic Activities: a. Oligo - α(1,4) → α(1,4) - Glucantransferase [Glucosyl (4:4) V. REGULATION OF GLYCOGEN SYNTHESIS AND DEGRADATION Transferase] 1. The regulation of glycogen synthesis in different tissues  removes the outer 3 of the 4 of the glucosyl residues attached matches the function of glycogen in each tissue. at a branch → transfers them to the nonreducing end of 1. Glycogen another chain (α-1,4 is broken and α-1,4 is made) a. In the liver: b. Amylo-α-(1,6)-Glucosidase (Amylo-6-Glucosidase) - glycogenesis accelerates during periods well fed state,  removes the remaining single glucose residue hydrolytically whereas glycogenolysis accelerates during periods of releasing free glucose, glucosyl chain is now available again for fasting. degradation by glycogen phosphorylase b. In skeletal muscle: Cori disease, a type III GSD, results from a deficiency of debranching - glycogenolysis occurs during active exercise, and enzyme. Type IIIa is a deficiency of both liver and muscle enzymes. glycogenesis begins as soon as the muscle is again at rest. Manifestations: – cardiac and pulmonary problems 2. Two Levels of Regulation – stunted growth a. Hormonal Regulation – hepatomegaly - glycogen synthase and glycogen phosphorylase are – hypoglycemia during fasting and myopathy hormonally regulated to meet the needs of the body as a – acidosis whole b. Allosteric Control C. Conversion of Glucose 1-Phosphate to Glucose 6-Phosphate - glycogen synthesis and degradation are allosterically 1. In the liver, glycogen is degraded to maintain blood glucose. controlled to meet the needs of a particular tissue a. Phosphoglucomutase converts glucose 1-phosphate to A. Activation of glycogen degradation by cAMP-directed pathway glucose 6-phosphate. During fasting: b. Glucose 6-phosphate translocase transports G6P into the 1. Binding of glucagon or epinephrine to receptors initially endoplasmic reticulum (transported out of the ER to the activating G protein which activates adenylyl cyclase which cytosol by GLUT-7) converts ATP to 30,50-cyclic adenosine monophosphate (cAMP). c. Glucose 1,6-bisphosphate is released from G6P by glucose 2. As cAMP levels rise, it binds to the regulatory (inhibitory) 6-phosphatase – this enzyme also acts in subunits activating protein kinase A, releasing the catalytic gluconeogenesis. subunits in an active form 2. In muscle, glycogen is degraded to provide energy for 3. Protein kinase A phosphorylates glycogen synthase, causing it to contraction. be less active, thus decreasing glycogen synthesis. a. Glucose 6-phosphate enters the pathway of glycolysis and 4. Phosphorylated phosphorylase kinase phosphorylates glycogen is converted either to lactate or to CO2 leading to the phosphorylase. generation of ATP via oxidative phosphorylation. 5. Phosphorylated glycogen phosphorylase catalyzes the b. Muscle does not contain glucose 6-phosphatase phosphorolysis of glycogen, producing glucose 1-phosphate. therefore glucose 6-phosphate cannot be 6. Phosphorylase a cleaves glucose residues from the nonreducing dephosphorylated. ends of glycogen chains, producing glucose 1-phosphate, which D. Lysosomal Degradation of Glycogen  the purpose of this pathway is unknown CHECK POINT!  1-3% of Glycogen is degraded by an α-1,4 glucosidase located in The degradation lysosomesof glycogen normally produces which of the following?  A.a deficiency More glucose than of this glucose enzyme 1-phosphate causes accumulation of glycogen in B.vacuoles More inglucose 1-phosphate than glucose the lysosomes C. disease, Pompe Equal amounts a type of glucose II GSD, is and glucose 1-phosphate a lysosomal storage disease. The D. Neither glucose or glucose 1-phosphate deficiency of the lysosomal α-1,4 α-1,6 glucosidase (acid maltase) E. leads enzyme Only glucose 1-phosphate of glycogen within the vacuoles. to the accumulation Manifestations: – CNS → psychomotor retardation – cardiomegaly → cardiac failure – pulmonary failure is oxidized or, in the liver, converted to blood glucose. B. Inhibition of Glycogen synthesis by cAMP-directed pathway ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 5 of 8  BIOCHEMISTRY CARBOHYDRATES: Glycogen Metabolism  The regulated enzyme in glycogenesis is glycogen synthase - phosphorylation cascade, retardation of glycogenesis  exists in two forms, the active “a” form and the inactive “b” ii. Insulin form - increases glycogenesis, decreases glycogenolysis i. “a” Form - heightens glucose entry into muscle cells - independent - phosphodiesterase activation → increased cAMP - dephosphorylated form destruction → decreased cAMP levels - most active form b. In Liver - does not require G6P for activity i. Glucagon ii. “b” Form - adenylate cyclase activation in liver cell membranes - dependent → turns on glycogenolysis, reduces glycogenesis - phosphorylated form ii. Insulin - inactive - increased activity of glycogen synthase → increased - allosteric enzyme form glycogenesis - activated by high concentrations of G6P iii. Glucagon: Insulin Ratio  Glycogen synthase a is converted to the inactive “b” form by - more important than absolute levels of either phosphorylation catalyzed by several different cAMP hormones regulated protein kinases (level of inactivation is - glycogen metabolism is strongly influenced by the proportional to the degree of phosphorylation) predominant hormone - insulin domination → glycogen storage after a meal C. Allosteric regulation of Glycogen synthesis and degradation - glucagon domination → mobilization of glycogen stores 1. Regulation of glycogen synthesis and degradation in the well-fed state: - glycogen synthase b in both liver and muscle is allosterically activated by glucose 6-phosphate - glycogen phosphorylase a is allosterically inhibited by glucose 6- phosphate, as well as by ATP [Note: Ca2+ indirectly activates phosphorylase in both muscle and liver by directly activating phosphorylase kinase.] 2. Activation of glycogen degradation by calcium: a. Calcium activation of muscle phosphorylase kinase: 2. cAMP Levels Fluctuate in Response to Hormonal Stimuli 2+  Ca binds calmodulin and the complex activates - muscle phosphorylase kinase b synthase active form predominates, low phosphorylase b. Calcium activation of liver phosphorylase kinase: kinase active form → increased glycogenesis, decreased 2+  Ca -calmodulin complex forms and activates glycogenolysis hepatic phosphorylase kinase b - glucagon and epinephrine in liver and epinephrine in 3. Activation of glycogen degradation in muscle by AMP: muscle → adenylate cyclase activation → increased cAMP  AMP binds to glycogen phosphorylase b, causing its activation without phosphorylation phosphorylase active form → limited glycogenesis, increased glycogenolysis D. Coordinated Regulation of Glycogen Synthesis and Degradation Summary 3. Key Enzymes are Phosphorylated by a Family of Kinases (Some 1. Glycogen Synthesis and Degradation are Regulated by the same are cAMP-Dependent) Hormonal Signals - phosphorylation of enzyme → conformational change that a. In Muscle affects the active site → increased or decreased enzyme i. Epinephrine activity - promotes glycogenolysis, inhibits glycogenesis - adenylate cyclase activation → increased cAMP levels E. Phosphate Group Removal by Protein Phosphatase - emergency situation → epinephrine release →  hydrolytic cleavage glycogenolysis activation via ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 6 of 8  BIOCHEMISTRY CARBOHYDRATES: Glycogen Metabolism  protein phosphatase activation opposes that of the protein kinases  if the activity of protein kinase > protein phosphatase → more of the regulated enzyme is in the phosphorylated form VI. GLYCOGEN STORAGE DISEASES  group of genetic diseases that result from a defect in an enzyme (enzyme deficiencies) required for glycogen synthesis or degradation Result to: 1. formation of glycogen that has an abnormal structure 2. accumulation of excessive amounts of normal glycogen in specific tissues as a result of impaired degradation B. Cause and Characteristics A. Deficiencies in PFK and Glucose-6-Phosphatase (G6Pase) REFERENCES - G6P accumulation → excessive glycogen accumulation  Harvey RA, Ferrier DR. Lippincott’s Illustrated Reviews: th Biochemistry. 5 Edition (2011). Philadelphia, PA 19103.  Lieberman M, Marks AD, Peet A. Marks’ Basic Medical th Biochemistry A Clinical Approach. 4 Edition (2013). Philadelphia, PA 19106. ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 7 of 8  BIOCHEMISTRY CARBOHYDRATES: Glycogen Metabolism  Rodwell VW, Bender DA, Botham KM, Kenelly PJ, Weil PA. Harper’s Illustrated st Biochemistry. 31 Edition (2018). McGraw-Hill Education.  Swanson TA, Kim SI, Glucksman MJ, Lieberman. Biochemistry, Molecular Biology th & Genetics. 5 Edition (2010). Wolter Kluwer Lippincott Williams & Wilkins. Philadelphia, PA 19106. ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 8 of 8