Biochemistry, 4th Edition P89. 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 There a Good Index of Cellular Energy Status 843 is at equilibrium is a dead cell. Living cells break down energy-yielding nutrient molecules to generate ATP. These catabolic reactions proceed with a very large overall decrease in free energy. Kinetic controls over the rates of the catabolic pathways are designed to ensure that the ATP ADP Pi ratio is maintained very high. The cell by employing kinetic conl-rols over the rates of metabolic pathways maintains a very high ATP ADP P ratio so that ATP hydrolysis can serve as the driving force for virtually all biochemical events. ATP Has Two Metabolic Roles The role of ATP in metabolism is twofold 1. It serves in a stoichiometric role to establish large equilibrium constants for metabolic conversions and to render metabolic sequences thermodynamically favorable. This is the role referred to when we call ATP the energy currency of the cell. 2. ATP also serves as an important allosteric effector in the kinetic regulation of metabolism. Its concentration relative to those of ADP and AMP is an index of the energy status of the cell and determines the rates of regulatory enzymes situated at key points in metabolism such as PFK in glycolysis and FBPase in gluconeogenesis. Is There a Good Index of Cellular Energy Status Energy transduction and energy storage in the adenylate system ATP ADP and AMP lie at the very heart of metabolism. The amount of ATP a cell uses per minute is roughly equivalent to the steady-state amount of ATP it contains. Thus the metabolic lifetime of an ATP molecule is brief. ATP ADP and AMP are all important effectors in exerting kinetic control on regulatory enzymes situated at key points in metabolism so uncontrolled changes in their concentrations could have drastic consequences. The regulation of metabolism by adenylates in turn requires close control of the relative concentrations of ATP ADP and AMP. Some ATP-consuming reactions produce ADP PFK and hexokinase are examples. Others lead
