Biochemistry, 4th Edition P50. 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. | What Factors Influence Enzymatic Activity 453 of enzyme synthesis are important mechanisms for the regulation of metabolism. By controlling the amount of an enzyme that is present at any moment cells can either activate or terminate various metabolic routes. Genetic controls over enzyme levels have a response time ranging from minutes in rapidly dividing bacteria to hours or longer in higher eukaryotes. Once synthesized the enzyme may also be degraded either through normal turnover of the protein or through specific decay mechanisms that target the enzyme for destruction. These mechanisms are discussed in detail in Chapter 31. Enzyme Activity Can Be Regulated Allosterically Enzymatic activity can also be activated or inhibited through noncovalent interaction of the enzyme with small molecules metabolites other than the substrate. This form of control is termed allosteric regulation because the activator or inhibitor binds to the enzyme at a site other than allo means other the active site. Furthermore such allosteric regulators or effector molecules are often quite different ster-ically from the substrate. Because this form of regulation results simply from reversible binding of regulatory ligands to the enzyme the cellular response time can be virtually instantaneous. Enzyme Activity Can Be Regulated Through Covalent Modification Enzymes can be regulated by covalent modification the reversible covalent attachment of a chemical group. Enzymes susceptible to such regulation are called interconvertible enzymes because they can be reversibly converted between two forms. Thus a fully active enzyme can be converted into an inactive form simply by the covalent attachment of a functional group. For example protein kinases are enzymes that act in covalent modification by attaching a phosphoryl moiety to target proteins Figure . Protein kinases catalyze the ATP-dependent phosphorylation of OH groups on Ser Thr or Tyr side chains. Removal of the phosphate group by
