tailieunhanh - Biochemistry, 4th Edition P51

Biochemistry, 4th Edition P51. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | Is the Activity of Some Enzymes Controlled by Both Allosteric Regulation and Covalent Modification 463 Glycogenbinding site FIGURE Structure of the glycogen phosphorylase monomer pdb id 8GPB . tory phosphorylation site is located at Ser14 on each subunit. A glycogen-binding site on each subunit facilitates prior association of glycogen phosphorylase with its substrate and also exerts regulatory control on the enzymatic reaction. Each subunit contributes a tower helix residues 262 to 278 to the subunitsubunit contact interface in glycogen phosphorylase. In the phosphorylase dimer the tower helices extend from their respective subunits and pack against each other in an antiparallel manner. Glycogen Phosphorylase Activity Is Regulated Allosterically Muscle Glycogen Phosphorylase Shows Cooperativity in Substrate Binding The binding of the substrate inorganic phosphate Pi to muscle glycogen phosphorylase is highly cooperative Figure which allows the enzyme activity to increase markedly over a rather narrow range of substrate concentration. FIGURE v versus S curves for glycogen phosphorylase. a The response to the concentration of the substrate phosphate Pi . b ATP and glucose-6-P are feedback inhibitors. c AMP is a positive effector. It binds at the same site as ATP. 464 Chapter 15 Enzyme Regulation ATP and Glucose-6-P Are Allosteric Inhibitors of Glycogen Phosphorylase ATP can be viewed as the end product of glycogen phosphorylase action in that the glucose-1-P liberated by glycogen phosphorylase is degraded in muscle via metabolic pathways whose purpose is energy ATP production. Glucose-1-P is readily converted into glucose-6-P to feed such pathways. In the liver glucose-1-P from glycogen is converted to glucose and released into the bloodstream to raise blood glucose levels. Thus feedback inhibition of glycogen phosphorylase by ATP and glucose-6-P provides a very effective way to regulate glycogen breakdown. Both ATP and glucose-6-P act by .