Hif 1 Glycogen Phosphorylase

Glucose Homeostasis and Glycogen Metabolism Learning Goals: 1. Understand the importance of glycogen  blood glucose levels, being able to release a quick acting fuel for ATP synthesis. Understand how structure/metabolism of glycogen are optimized to meet these goals. 2. Know the reactions accounting for glycogen synthesis and breakdown. 3. Understand the regulation of glycogen metabolism: short-term effectors, longer-term activators/inhibitors. Multistep signal transduction pathways amplify the effects of hormone action. 4. Understand the distinct roles played by glycogen in skeletal muscle and liver; differences between tissues help meet these goals. 5. Be aware of glycogen storage diseases and know the consequences of defects at particular steps in glycogen metabolism. I.

The Big Picture a. It is important to maintain blood glucose levels even during periods of fasting. i. Fat most efficient energy store but mobilized slowly, cannot be converted to glucose. ii. We store glucose as glycogen, a branched polymer of glucose (skeletal muscle and liver) b. Glucose-6-phosphate = branch point in carbohydrate metabolism. i. Enzymes are distinct (G6P  Glycogen and back) ii. Limited futile cycling iii. Regulation: short term effectors and longer term covalent modifications c. Insuline  glycogen formation, epinephrine/glucagons  glycogen breakdown II. Overview of Glucose metabolism a. Plasma glucose: maintenance is critical aspect of homeostasis (normal = 70-100 mg/dL) i. Below  hypoglycemia ii. Too much glucose increases glycation reactions iii. Tissues rely on glucose: RBCs, brain, renal medulla 1. Cornea, lens, retina, testis, leukocytes, white muscle fibers b. Glucose transport: facilitated transport i. GLUT1 and GLUT 3 1. Basal glucose uptake, all mammalian tissues (esp. neuronal cells, RBCs) 2. Km = 1  always saturated ii. GLUT 2 1. Liver, pancreatic β cells, basolateral side of intestinal cells 2. High Km  uptake proportional to [glucose]  increased uptake when blood glucose levels increase 3. Pancreatic β cells: increased glucose uptake  insulin release 4. Liver: glucose taken up only when levels are high; when glucose levels are normal glucose is taken up by other tissues iii. GLUT 4 1. Skeletal muscle, fat cells 2. Insulin  increased GLUT 4 on plasma membrane  increased glucose uptake in tissues iv. GLUT 5 1. Apical side of sm. Intestine cells 2. Fructose uptake v. SGLT1 1. Sodium/glucose cotransporter 2. Apical side of intestine cells 3. Glucose, galactose uptake **Clinical Correlation: tumors and HIF-1  GLUT1 and GLUT3 c. Trapping of glucose by phosphorylation i. Hexokinase: low Km, use Mg-ATP as substrade, inhibited by G6P ii. Glucokinase: liver, pancreatic β cells- higher Vmax, higher Km, not inhibited by G6P d. Glucose-6-phosphate i. Glycogen to G6P: maintenance of blood glucose, NADPH synthesis, energy production ii. Glycogen = branched polymer, osmotic advantage, only one reducing end and many nonreducing ends

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Pathway for Glycogen Breakdown (Glycogenolysis) a. Glycogen phosphorylase i. Phosphorolytic cleavage of the α(1-4) bonds of glycogen ii. Works on non-reducing ends >4 residues from branch iii. Active site pyridoxal phosphate (general acid-base catalyst) iv. Releases glucose-1-phosphate b. Phosphoglucomutase i. Further metabolism requires glucose-6-phosphate ii. Makes conversion from G1P to G6P iii. Resting enzyme is phosphorylated c. Glucose-6-phosphatase i. Liver: G6P hydrolyzed to glucose ii. Located on luminal surface of ER: G6P transported out to it, glucose transported into cytosol  out to plasma iii. Glucose-6-phosphate + H2O  Glucose + Pi d. Debranching enzyme i. Transfers a block of 3 glucose units from an α(1-6) branch from one outer branch to another ii. Hydrolyzes the remaining α(1-6) linked glucose unit (releases free glucose) Pathway for Glycogen Synthesis a. UDP-Glucose Pyrophosphorylase i. Makes an activated glucose (UDP-glucose) ii. Driven forward by the subsequent hydrolysis of pyrophosphate by pyrophosphatase b. Glycogen synthase i. Catalyzes the elongation of the non-reducing termini of glycogen, forming α(1-4) linkages ii. Adds glucose only to chains at least 4 units long c. Nucleoside diphosphate kinase i. Converts UDP back to UTP (all nucleotide diphosphates) ii. UDP + ATP  UTP + ADP (high ATP/ADP ratio drives forward reaction) d. Glycogenin i. Small protein (dimer) with Tyr residues ii. Glucose is attached in glycosidic linkage iii. Attachment plus 7 glucose units e. Branching enzyme i. Glucose chains = 11 residues  branching enzyme moves a 7 glucose unit chain to a 6 position on a nearby chain Regulation of Glycogen Metabolism a. Glucose: plentiful  glycogen stores increase i. Insulin, ATP, G6P, glucose (liver) b. Glucose: low  liver glycogen degraded to make glucose for release into plasma i. Epinephrine, glucagons, AMP, Ca2+ c. Regulation of glycogen phosphorylase i. Phosophorylated  phosphorylase a, active ii. Not phosphorylated  phosphorylase b, inactive iii. Controlled by hormones which act through phorphorylase kinase and phosphoprotein phosphatase 1 (PP1) 1. Activation of phosphorylase kinase: Phosphorylation of β subunit, Ca2+ binding 2. Epinephrine activates adenylate cyclase which makes cyclic AMP which activates PKA which activates phosphorylase kinase to turn phosphorylase b into phosphorylase a. PKA also converts glycogen synthase a into glycogen synthase b. 3. Insulin triggers a cascade leading to the activation of PP1, which results in the stimulation of glycogen synthesis. Glycogen storage diseases a. Rare; perinatal failulre to thrive, hypoglycemia, muscle weakness b. Von Gierke i. Defective glucose-6-phosphatase or transport system  increased amount of normal glycogen c. Pompe

i. Defective α-1,4-glucosidase  massive increase in amount of normal glycogen d. Cori

i. Defective debranching enzyme  increased glycogen, short outer branches e. McArdle i. Defective phosphorylase  moderately increased amount of normal glycogen