Chapter 8 - Enzymes

Abstract

Enzymes (E) are catalysts that allow reactions to proceed at fast rates under the mild conditions of temperature, pH, and pressure of cells. Most E are proteins, some are ribonucleic acid (RNA) molecules (ribozymes). The substrate (S) is the substance on which an enzyme acts. E are able to decrease the activation energy of a reaction. During the reaction, E binds to S and forms a transient enzyme-substrate complex (ES). At the end of the reaction, the product(s) are formed, and the enzyme remains unchanged. Active or catalytic site is the specific place in the E where the S binds. E are classified into six categories: oxidoreductases, transferases, hydrolases, lyases, ligases, and isomerases. Coenzymes are small molecules associated with some enzymes. They undergo changes to compensate for the transformations occurring in S. Zymogens or proenzymes are inactive enzyme precursors. They become active after hydrolysis of a portion of their molecule. E activity can be determined by measuring the amount of product formed or substrate consumed during the catalyzed reaction in a given time. The factors that influence an enzymatic reaction include the amount of E present in the sample, the S concentration, temperature, and pH. Allosteric factors are regulators that bind to a place in the E different from the active site. Km or Michaelis constant is the S concentration at which the reaction rate reaches a value equal to half the maximum. E inhibitors are compounds that interfere with E function. They can be irreversible or reversible. The last ones include (1) competitive inhibitors, which increase the Km but not the maximal velocity (Vmax); (2) noncompetitive, which bind to the enzyme in a site different from the catalytic center; they decrease Vmax, do not change Km, and are not influenced by [S]; and (3) anticompetitive, which reduce Km and Vmax. Isozymes are proteins with different structures that have the same enzymatic activity.