Biochemistry - 31 - Glycogen 2023.pdf

Glycogen Metabolism Lecture 31 Reference: Lieberman and Peet, Chapter 26 Bluefield University – VCOM Campus MABS Program Biochemistry Jim Mahaney, PhD 1 Lecture Objectives a. Relate the primary source of blood glucose during the fed, fasted and starved states. b. Interpret why muscle and liver ha...

Glycogen Metabolism Lecture 31 Reference: Lieberman and Peet, Chapter 26 Bluefield University – VCOM Campus MABS Program Biochemistry Jim Mahaney, PhD 1 Lecture Objectives a. Relate the primary source of blood glucose during the fed, fasted and starved states. b. Interpret why muscle and liver have relatively large amounts of glycogen, but other tissues maintain only a small amount of glycogen. c. Interpret the role of liver glycogen as a reservoir of glucose to be used during fasting, and the role of muscle glycogen as a reservoir of glucose for times of exertion. d. Recall the structure of glycogen and interpret the advantage of having a highly branched glycogen structure. e. Identify the α-1,4 and the α-1,6 glucose linkages in glycogen and relate how they contribute to the structure and function of glycogen. f. Recall the glycogen synthesis and degradation pathways at the level of detail provided on the slides. In particular, compare and contrast the fate of glucose-6-phosphate produced in liver versus skeletal muscle. g. Relate the need to activate glucose-1-phosphate to UDP-glucose prior to glycogen synthesis h. Relate the regulation of liver glycogen degradation by glucagon signaling. Why would epinephrine signaling have the same effect as glucagon on glycogen degradation in liver? i. Relate the regulation of skeletal muscle glycogen degradation by AMP and catecholamines. j. Compare and contrast the effects of catecholamine binding to liver α receptors and β receptors on liver glycogen breakdown, and relate how these two pathways lead to net glucose production by liver. k. Recall lysosomal degradation of glycogen to glucose. 2 2 Objectives A, C Blood Glucose in the Fed, Fasted and Starved States • Humans need ~200 grams glucose per day for optimal function. • 75% used by brain (about 150 g) • Basal blood glucose level… 80-100 mg glucose per ml of blood to meet body needs • about 4 g total glucose in blood (4 L) at any given time Where does the glucose come from during the fed, fasted and starved states? • Fed State: glucose brought in through diet (Fed state) • Glucose enters blood stream à blood glucose much higher than 80-100 mg/dL à Insulin à glucose uptake à blood glucose returns to basal 80-100 mg/dL • Fasting: blood glucose must be maintained at 80-100 mg/dL to support tissues that depend on glucose as a primary energy source. • Short fasts (6-12 hours) liver GLYCOGEN is the source of glucose • Longer fasts (24 hours +) liver GLUCONEOGENESIS is source of glucose • In between short and long glycogenolysis decreases and gluconeogenesis increases. 28.2 3 3 Objective A Blood Glucose • Ingested glucose is cleared within 4 hrs after a meal. • Glycogenolysis supplies glucose in the short term, but does not last much more than 18-24 hrs • Gluconeogenesis supplies glucose to the blood over the long term, as long as precursors are available. • Notice the transition from ingested glucose to glycogen-supplied glucose to gluconeogenesis supplied glucose. • Goal is to maintain blood glucose at 80-100 mg/dL. 4 4 Objective B Glycogen Glycogen is STORED glucose that can be used in times of need. LIVER: (100-120 g glycogen) • Liver is the main agent that monitors and controls the blood glucose level. • Liver is directed by insulin and glucagon, which determine liver action • After a meal (insulin), excess glucose is stored as glycogen in liver. • Additional excess glucose is converted to FAs and TG in the liver and exported to adipose tissue for storage. • Between meals (i.e., during fasting), liver glycogen is degraded to glucose to be released into the bloodstream to maintain blood glucose. Large supply of glycogen allows liver to regulate blood glucose for ~18 hours. MUSCLE: (300-400 g glycogen) • Muscle also uses glycogen – but it keeps the glycogen for its own use (not released to blood) • Under insulin signal, muscle accumulates glucose for energy – but also stores glucose as glycogen • Muscle glycogen is used to support muscle metabolism at all times, fasting or fed. Large amount of glycogen in fast twitch muscles to support fast-twitch function. Less in slow twitch muscles, but still there. 5 5 Glucose versus Fatty Acids during Fasting Liver: Degrades glycogen to produce glucose for release into the blood – liver will not use glucose for energy under these conditions. Liver will rely on fatty acids as primary energy source to support liver function during fasting Muscle: Muscle contraction occurs during the fed state, fasting state and starved state Fast twitch skeletal muscle – during energy bursts: Glycolysis / ATP much faster than fatty acid oxidation / OxPhos ATP Glycolysis /ATP anaerobic and can operate even when pO2 is low All muscles: Fatty acids are a major energy source for muscle overall – especially cardiac / slow twitch Fast twitch skeletal muscle will use fatty acids for ATP production when at rest, during recovery 6 6 Glycogen: a Reservoir of Glucose Units for ATP Generation from Glycolysis Objective C • Glycogen in all tissues is degraded to glucose 1-phosphate, which is then converted to glucose 6-phosphate. • LIVER dephosphorylates glucose 6-phosphate to free glucose and exports the free glucose out into the blood • Skeletal muscle: glucose-6-phosphate is used immediately by glycolysis for ATP production. Is not dephosphorylated to glucose. • Other tissues similar to muscle: low amount of glycogen makes it an emergency reserve • E.g., Absence of oxygen or restricted blood flow 7 7 Objective D Structure of Glycogen • Branched glucose polysaccharide • glucosyl units linked by α-1,4 glycosidic bonds with α-1,6 branches every 8-10 residues • present in all tissues as very high molecular weight polymers (107 – 108 g/mol) • collected together in glycogen particles • Only one residue has a reducing end: attached to glycogenin protein • Other ends of chains are non-reducing ends • Branched structure allows for: • tight packing of glucose • rapid degradation and rapid synthesis • enzymes can work on several branches at the same time • enzymes involved in synthesis and degradation and some regulatory enzymes are bound to surface of glycogen particles 8 8 Objective D, E Glycogen Protein at the core is Glycogenin. New glycogen molecules are synthesized onto this core protein. 9 9 Objective F Glycogen Synthesis (S) and Degradation (D) 10 10 Objective F Glycogen Synthesis • Consists mostly of adding to glucose units to existing glycogen molecules • Key is the formation of α-1,4-glycosidic bonds to link glucose residues in long chains, and the formation of an α-1,6 branch every 8-10 residues • Site of attachment is the non-reducing free ends of the molecule • Reducing end is attached to glycogenin • Glucose is phosphorylated to glucose 6-phosphate by hexokinase (glucokinase in liver) as it enters the cell • Glucose 6-phosphate is converted to glucose 1-phosphate 11 11 Objective F Glucose Phosphorylation • Glucose is phosphorylated to glucose 6-phosphate by hexokinase (glucokinase in liver) as it enters the cell • Traps glucose inside cell • Glucose 6-phosphate is converted to glucose 1-phosphate in preparation for attachment to glycogen 12 12 Objective G Activation of Glucose 1-P • Glucose 1-phosphate must be activated first: UTP is utilized • Phosphate on glucose position 1 attaches to the α phosphate on UTP, displacing PPi, results in UDP-glucose = activated glucose • Energy for UDP-glucose formation from hydrolysis of PPi • UDP-glucose is also a precursor for other pathways 13 13 Objective F Attaching Glucose • The anomeric carbon of glucose on UDP-glucose forms an α1,4 linkage with carbon C-4 on the glucose at the nonreducing end of the glycogen chain, • displaces UDP and increases the chain length. • When the chain is about 11 residues long, a 6-8 residue piece is cleaved from the end of the chain and reattached to a glucosyl unit by an α-1,6 bond, forming a branch point. • Each branch is lengthened until they are cleaved to form new branches, etc. 14 14 Objective F Glycogen Degradation: Liver • Liver: unique function – primary means to maintain blood glucose level • Glucagon signaling activates glycogenolysis – MAJOR factor. • Epinephrine/norepinephrine signaling also activates glycogenolysis • Glucose 6-phosphate liberated during glycogen breakdown is converted to glucose by glucose 6-phosphatase (present only in liver and kidneys) • Glucose readily released in response to lower levels or increased need due to exercise, etc. • Tied directly to activation of gluconeogenesis (production of glucose from dietary fuels). Gluconeogenesis also produces glucose 6-phosphate. • Glycolysis also linked to pentose phosphate pathway, and synthesis of other sugars, which intersect at glucose 6-phosphate 15 15 Objective F Glycogen Degradation: Muscle • Glycogen is broken down to glucose 1-phosphate by (glycogen phosphorylase) which is converted to glucose 6-phosphate • Skeletal muscle does not have glucose 6-phosphatase, so it can’t generate free glucose that can enter the blood stream • Rather, glucose 6-phosphate enters the glycolytic pathway to make ATP within the muscle cell. • AMP is the MAJOR metabolic signal that activates glycogen degradation in muscle under normal conditions. • Muscle not responsive to glucagon • Epinephrine/Norepinephrine also major activator of glycogenolysis in muscle 16 16 Objective F Degradation of Glycogen • Glycogen phosphorylase and the debrancher enzyme • Glycogen phosphorylase cleaves of single glucose units by transferring a phosphate ion to the anomeric carbon of the glucose, breaking the α-1,4 glycosidic linkage. (note: phosphate not from ATP) • Releases glucose 1-phosphate, which is converted to glucose 6phosphate by phosphoglucosemutase: this then enters a variety of pathways • In liver, glucose 6-phosphate is dephosphorylated by glucose 6phosphatase and transported out of the cell by glucose transporter. • In muscle, glucose 6-phosphate enters into glycolysis 17 17 Objective F Glycogen Branch Points • Glycogen phosphorylase can’t act on glycosidic bonds of the four glucoses adjacent to a branch • branch does not fit properly into the active site. • Debrancher enzyme has two activities to handle branch points: • 4:4 transferase activity - a three glucose unit is removed from the 4 glucoses at the branch point, and it is attached to the end of a longer straight chain by an α-1,4 glycosidic bond (glycogen phosphorylase operates on this) • 1,6-glucosidase activity – the single remaining glucose reside of the branch attached by the α-1,6 linkage is cleaved forming free glucose. • Yield of glucose at the branch point: 1 glucose and 7-9 glucose 1-phosphate residues 18 18 Objective H Glucagon Regulation of Glycogen Breakdown: Liver • Low blood glucose level: glucagon is high and insulin is low Glucagon signals the fasting state: stimulates glycogen breakdown • Glycogen phosphorylase is phosphorylated by cAMP-dependent PKA Shuts down glycogen synthesis X • Glycogen sythase phosphorylated = inhibited X • After a high carbohydrate meal, blood glucose will increase, Insulin will be released – high insulin to glucagon ratio. Glucagon signaling stops, PKA activity inhibited Insulin signaling activates protein phosphatase 1 which reverses the effects of PKA. glycogen phosphoylase is inhibited glycogen synthase activated 19 19 Objective H Glycogen Metabolism is Regulated by Insulin and Glucagon Glucagon PKA PP1 PKA PP1 Insulin • Glucagon stimulates protein kinase A (PKA(, which phosphorylates both enzymes. • Glycogen phosphorylase turns “ON” • Glycogen synthase turns “OFF” • Insulin activates phosphatases (PP1), which reverse the phosphorylation effects. 20 20 Objective I Glycogen and Exercise • Muscles use lots of ATP, need lots of energy to keep ATP production going. Fast twitch skeletal muscles rely heavily on glycolysis for ATP production during times of exertion. • Muscle contraction uses ATP for energy, producing ADP as a product. The enzyme adenlyate kinase converts two ADP to 1 ATP and 1 AMP. Over time, AMP concentration increases in the muscle cell. • AMP is an allosteric activator for the muscle isoform of glycogen phosphorylase • As AMP levels rise, glycogen phosphorylase activity greatly increases à more glucose produced for more ATP production by glycolysis. • Skeletal muscle glycogen phosphorylase can be phosphorylated (activated) in response to catecholamine (epinephrine/norepinephrine) signaling. • Sympathetic stimulation of cardiovascular system also stimulates skeletal muscle performance • Fight or Flight signaling activates skeletal muscle for increased performance. 21 21 Objective I Regulation of Glycogen Phosphorylase in Muscle Skeletal Muscle Where does the AMP come from? • Two major control mechanisms for glycogen phosphorylase activity: • AMP, which is produced by muscle metabolism (normal mechanism) • Phosphorylation, which IN MUSCLE is stimulated by catecholamines • Sympathetic nervous stimulation increases glucose to increase skeletal muscle exercise performance 22 22 Objectives I, J Muscle Glycogen Phosphorylase: Three Modes of Regulation • Both AMP and phosphorylation affect glycogen phosphorylase during exercise. • AMP: allosteric activator for the enzyme (previous slide) • Phosphorylation: Calmodulin is activated by high Ca2+ levels in the cell. Calmodulin activates phosphorylase kinase, which activates glycogen phosphorylase itself. Epinephrine: fight or flight. Muscles need lots of glucose for rapid ATP production, quick action. 23 23 Objective J Liver Glycogen Phosphorylase: Catecholamines • The main “signal” acting on liver for glycogen degradation is glucagon signaling • Glucagon receptor, G-s protein, adenylate cyclase, cAMP, Protein Kinase A, targets • Liver β-receptors also bind catacholamines (Epinephrine), but it has a much smaller overall effect on glycogen degradation than glucagon. • Catacholamine receptor – very similar to glucagon receptor and effects…stimulate glycogen breakdown and inhibit glycogen synthesis • Catecholamine binding to α-recptors has major additive role of activating glycogen breakdown and shutting down glycogen synthesis • Receptor, G protein, PLC, IP3, cell calcium increase, calmodulin, kinase targets à additional phosphorylation of glycogen synthase (OFF) and more stimulation of phosphoryase kinase (ON) à activate glycogen phosphorylase à glycogen breakdown. 24 24 Objective J Epinephrine Binding to α-Receptors in the Liver Fig. 26.8 Epinephrine (catecholamine) signaling results in large amounts of glucose being liberated in blood by glygcogenolysis. But you don’t want this glucose to stimulate glycogen synthesis in the liver (or other glucose conversion pathways). Catecholamines strongly inhibit glycogen synthesis, so glucose stays in a free form for tissue utilization. 25 25 Objective K Lysosomal Degradation of Glycogen • Glycogen particles can become trapped in transport vesicles that fuse with lysosomes • break down substances to base units – does not to make metabolic intermediates that intersect key pathways • Specific enzyme, lysosomal glucosidase, hydrolyzes glycogen to glucose • type II glycogen storage disease: genetic defect in lysosomal glucosidase • prevents it from functioning • glycogen particles build up in vesicles • disrupts heart and liver function. • Large number of glycogen storage diseases arise from enzyme deficiencies and tissue dependent problems in liver versus muscle (Pathology) 26 26 Glycogen Storage Diseases • The inability to synthesize or breakdown glycogen by normal mechanisms. • Classified by number according to which enzyme is deficient. • O, I, III, IV, VI affect the liver • V and VII affect skeletal muscle • End result is that patient’s liver will not be able to produce glucose units for release into blood = hypoglycemia during times of fasting • OR…skeletal muscle will not get the glucose it needs to support function = cramps due to low energy. 27 27 Question Which of the following leads to an increased rate of glycogen breakdown in skeletal muscle? A. AMP and glucagon B. Epinephrine and insulin C. Epinephrine and AMP D. Glucagon E. Insulin 28 28 Question What is the purpose for converting glucose 1-phosphate to UDPglucose prior to glycogen synthesis? A. It allows glycogen formation to be reversible. B. It ensures that multiple glucose carbons don’t react with the glycogen molecule. C. It increases the reactivity of the glucose molecule. D. It makes glycogen formation irreversible. E. It results from insulin stimulation of the glycogen synthase. 29 29 Thank You! 30 30