Glycogen Metabolism Lecture 22 PDF

Glycogen Metabolism Lecture 22 Annie Kirby, PhD, RD, LD Associate Professor – Nutrition [email protected] Office 241  Lecture Objectives a. Interpret why the body stores excess glucose as glycogen, rather than just co...

Glycogen Metabolism Lecture 22 Annie Kirby, PhD, RD, LD Associate Professor – Nutrition [email protected] Office 241  Lecture Objectives a. Interpret why the body stores excess glucose as glycogen, rather than just converting all the excess glucose to triacylglycerols (TGs) b. Compare and contrast why muscle and liver have relatively large amounts of glycogen, but other tissues maintain only a small amount of glycogen. c. Recall the structure of glycogen, and identify the role of the α-1,4 and the α-1,6 glucose linkages for the overall structure of glycogen. d. Interpret the biochemical logic of having a highly branched glycogen structure. e. Recall glycogen synthesis from glucose and glycogen breakdown to glucose-1-phosphate units. f. Compare and contrast how the hormones insulin and glucagon affect glycogen synthesis versus degradation pathways. g. Relate why it is necessary to activate glucose-1-phosphate to UDP-glucose for the synthesis of glycogen. h. Interpret why liver cells are unique in their treatment of glucose-6-phosphate produced by glycogen phosphorylase versus glucose-6-phosphate produced in muscle. i. Relate lysosomal degradation of glycogen to glucose and identify what disorder results from a deficiency of lysosomal glycogen degradation. j. Relate how high levels of AMP in muscle will lead to an activation of glycogen phosphorylase and increased glycogen breakdown. Compare this activation to that observed when glycogen phosphorylase is activated by phosphorylation by PKA in response to epinephrine binding. k. Relate the commonly used treatments for glycogen storage diseases and relate the biochemical basis for their efficacy.  Introduction: Why Glycogen? Humans consume about 160 g of glucose per day The blood carries only about 20 g 75% of glucose is used by the brain Most tissues preferentially use fatty acids as their energy source Including liver and muscle After a meal, excess glucose is either: 1. Stored as glycogen in muscle (340g) and liver (120g) 2. Converted to FAs in the liver → stored as TG in adipose tissue Between meals, liver glycogen is hydrolyzed to glucose to be released into the bloodstream to maintain blood glucose Only liver can do this because it contains glucose 6-phosphatase Cleaves the phosphate group off glucose-6-phosphate to make free glucose Glucose is an osmotic molecule, meaning its accumulation can have large impacts on osmotic pressure. Glycogen is non-osmotic.  Why glycogen instead of triglycerides? Liver contains ~12-24 hour supply of glucose from glycogen By 30 hours it’s nearly gone. Muscle glycogen is hydrolyzed to provide glucose 6-phosphate for anaerobic glycolysis Why is glycogen used as a storage fuel in addition to TGs? Muscle metabolizes glycogen much more rapidly than fatty acids Fatty acids are not metabolized under anaerobic conditions Acetyl CoA (produced by FA breakdown) cannot be converted to glucose Free energy of reaction of Acetyl CoA to Pyruvate too largely positive High acetyl CoA negative modulator for PDH  Structure of glycogen Branched glucose polysaccharide Glucosyl units linked by α-1,4 glycosidic bonds with α-1,6 branches every 8-10 residues Only one residue has a reducing end: attached to glycogenin protein Other ends of chains are non-reducing ends  Structure of glycogen Present in all tissues as polymers of very high molecular weight (107 – 108 g/mol) collected together in glycogen particles 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 glycogen synthesis and degradation and some regulatory enzymes are bound to surface of glycogen particles  Glycogenolysis The process of breaking down glycogen Muscle Glycogen is broken down to glucose 1-phosphate (G1P), converted to glucose 6-phosphate (G6P), which enters glycolysis to make ATP Liver As in muscle, then G6P is converted to glucose by glucose 6- phosphatase, which is then transported to the blood to maintain blood glucose levels Other tissues have low glycogen stores, used as emergency reserve (e.g., hypoxia) Glycogen Synthesis Hexokinase (S1) Converts glucose to G6P Phosphoglucomutase Converts G6P to G1P UDP-glucose pyrophosphorylase (S2) Converts G1P to UDP-G Glycogen synthase and branching enzyme (S3) Converts UDP-glucose to glycogen  Glycogen Synthesis Basic principle: Addition of glucose units to existing glycogen molecule Key elements of glycogen synthesis: Formation of α-1,4-glycosidic bonds to link glucose residues Formation of an α-1,6 branch every 8-10 residues Site of attachment: Non-reducing free ends of the molecule Reducing end is attached to glycogenin Glucose is phosphorylated to G6P by Hexokinase (tissues other than the liver) Glucokinase (liver)  UDP-glucose gives the energy to synthesize glycogen Building glycogen is an energy-consuming, anabolic process Glucose 1-phosphate monomers do not have enough energy on their own G1P units are activated by UTP to form UDP-glucose, which is also used in other pathways UDP makes an excellent leaving group  Attaching Glucose Glycogen synthase adds UDP-glucose units to growing glycogen chain in α-1,4 linkages (C1 of UDP- glucose to C4 of non-reducing end) After ~11 glucose units, branching enzyme (4:6- transferase) moves a 6-8 residue portion to another glycosyl residue through an α-1,6 linkage Each branch is lengthened by glycogen synthase and extra branches are created by the branching enzyme Glycogen degradation overview Glycogen phosphorylase and debrancher enzyme (D1) Converts glycogen to G1P Phosphoglucomutase Converts G1P to G6P Glucose 6-phosphatase (D2) Converts G6P to glucose Only in liver  Glycogen Breakdown in Liver Liver: unique function – primary means to maintain blood glucose level G6P liberated during glycogen breakdown is converted to glucose By glucose-6-phosphatase (present only in liver and kidneys) Glucose readily released in response to ↓ Glucose ↑ need due to exercise Tied directly to activation of gluconeogenesis (production of glucose from dietary fuels) Gluconeogenesis also produces G6P Glycolysis also linked to pentose phosphate pathway, and synthesis of other sugars, which intersect at G6P  Degradation of Glycogen Glycogen Degradation by two enzymes: Glycogen phosphorylase The debrancher enzyme has transferase and α-1,6-glucosidase activity 4:4-tranferase, 1,6-glucosidase Glycogen phosphorylase Uses inorganic phosphate to remove G1P, one unit at a time from non-reducing ends Chews glycogen down to 4 residues from branch point Releases G1P, Converted to G6P → enters a variety of pathways by phosphoglucomutase In liver, G6P is dephosphorylated by glucose 6-phosphatase Transported out of the cell by glucose transporter Note: only happens in the liver, which regulates blood glucose levels  Degradation of Glycogen Branch Points Glycogen Phosphorylase cannot act on glycosidic bonds of the 4 glucose residues adjacent to a branch Allosterically hindered Debrancher enzyme has two activities: 4:4-transferase activity: 3 glucose units removed from the branch point Attached to the end of a longer straight chain By an α-1,4 glycosidic bond Glycogen phosphorylase acts on this 1,6-glucosidase activity Cleaves the remaining glucose of the branch Attached by the α-1,6 linkage Yield of glucose at the branch point: 1 free glucose 7-9 G1P residues  Lysosomal Degradation of Glycogen Glycogen particles can become trapped in transport vesicles that fuse with lysosomes Lysosomes break down substances to base units to make metabolic intermediates that intersect key pathways Specific enzyme, lysosomal glucosidase, hydrolyzes glycogen to glucose Clinical relevance: Type II glycogen storage disease, Pompe disease Genetic defect in lysosomal glucosidase Prevents it from functioning Glycogen particles build up in vesicles Primarily affects heart, muscle, and liver, but can be any organ with lysosomes Can be treated with enzyme replacement therapy  Glycogen regulation in Fasting or Fed states Glucagon (fasting) Liver only (muscles lack glucagon receptors) Glycogenolysis (activates glycogen phosphorylase, inactivates glycogen synthase) Insulin (fed) Negates glucagon signal Glycogenesis (inactivates glycogen phosphorylase, activates glycogen synthase) Stimulates GLUT4 glucose transporter in muscle Exercise: stimulates glycogenolysis  Regulation of Glycogen Degradation Glycogen degradation is stimulated, and synthesis is inhibited when the enzymes of glycogen metabolism are phosphorylated. Glucagon acts on liver cells and epinephrine (adrenaline) Hormone signaling activates cAMP activates protein acts on both liver and muscle adenylate cyclase, which kinase A cells to stimulate glycogen converts ATP to cAMP degradation. 1. Protein kinase A phosphorylates glycogen Phosphorylase kinase synthase, causing it to be less Phosphorylase a cleaves phosphorylates active glucose residues from the phosphorylase b, converting nonreducing ends of glycogen 2. Protein kinase A it to phosphorylase a (it’s phosphorylates phosphorylase chains. active form) kinase Regulation of Glycogen Degradation Epinephrine stimulates liver to raise blood glucose Epinephrine, released due to exercise or hypoglycemia, can bind α-receptors on hepatocytes G-proteins transfer signal to Phospholipase C, which cleaves PIP2 to DAG and IP3 IP3 stimulates Ca2+ release from ER DAG and Ca2+ both stimulate PKC Ca2+ binds calmodulin, which activates calmodulin- dependent PK and phosphorylase kinase Phosphorylase kinase activates Glycogen phosphorylase All three kinases inactivate glycogen synthase  Additional regulatory mechanisms of muscle In addition to cAMP-mediated regulation, adenosine monophosphate (AMP) and Ca2+ stimulate glycogen breakdown in muscle Phosphorylase b is activated by a rise in AMP (1) as an allosteric activator Phosphorylase kinase is activated by Ca2+, which is released during muscle contraction: Ca2+ binds to Calmodulin (2) which serves as a subunit of phosphorylase kinase  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 Result: Liver will not be able to produce glucose units for release into blood Hypoglycemia during times of fasting Skeletal muscle will not get the glucose it needs to support function Cramps due to low energy  Glycogen Storage Diseases: Therapy Problem: without proper glycogen metabolism, liver (blood) or muscles will not have glucose to support function For liver-based forms: Patient must maintain proper blood glucose level through diet Small glucose snacks at regular intervals For skeletal muscle-based forms: Reduce exercise and muscle fatigue to avoid painful muscle cramps Or to supplement with higher dietary glucose and amino acids  Summary Glucose units are stored as long, branch-chained glycogen polymer This allows for rapid degradation when glucose demand is high In muscles, exercise and epinephrine signaling stimulate glycogenolysis, providing glucose 6-phosphate monomers for the glycolytic pathway In liver, fasting, exercise, and hypoglycemia stimulate glycogenolysis, but for a different reason: to generate free glucose to enter the blood stream and maintain proper blood glucose levels Individuals with deficiencies in glycogen-related enzymes have glycogen storage disorders, which can range from inconvenient/uncomfortable to deadly  Question The immediate degradation of glycogen under normal conditions gives rise to which one of the following? a. More glucose than glucose-1-phosphate b. More glucose-1-phosphate than glucose c. Equal amounts of glucose and glucose-1-phosphate d. Neither glucose nor glucose-1-phosphate e. Only glucose-1-phosphate  Question A patient has large deposits of liver glycogen, which, after an overnight fast, had shorter-than-normal branches. This abnormality could be caused by a defective form of which one of the following proteins or activities? a. Glycogen phosphorylase b. Glucagon receptor c. Glycogenin d. Amylo-1,6-glucosidase e. Amylo-4,6-transferase  Question What is the purpose for converting glucose 1-phosphate to UDP-glucose prior to glycogen synthesis? A. It allows glycogen formation B. It ensures the 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  Question Consider a person with type 1 diabetes who has neglected to take insulin for the past 72 hours and also has not eaten much. Which one of the following best describes the activity level of hepatic enzymes involved in glycogen metabolism under these conditions? Glycogen Synthase Phosphorylase Kinase Glycogen Phosphorylase A Active Active Active B Active Active Inactive C Active Inactive Inactive D Inactive Inactive Inactive E Inactive Active Inactive F Inactive Active Active  Question A 3-month-old infant was cranky and irritable, became quite lethargic between feedings, and began to develop a potbelly. A physical exam demonstrated an enlarged liver, while blood work taken between feedings demonstrated elevated lactate and uric acid levels, as well as hypoglycemia. This child most likely has a mutation in which one of the following enzymes? a. Liver glycogen phosphorylase b. Glycogen synthase c. Glucose 6-phosphatase d. Muscle glycogen phosphorylase e. Pyruvate kinase