SUMMARY

• 8.1 A Few Basic Catalytic Strategies Are Used by Many Enzymes

  Although the detailed catalytic mechanisms of enzymes vary, many enzymes use one or more common strategies. They include (1) covalent catalysis, in which the enzyme becomes temporarily covalently modified; (2) general acid–base catalysis, in which some molecule other than water accepts or donates a proton; (3) metal ion catalysis, which functions in a variety of ways; and (4) catalysis by approximation and orientation, in which substrates are brought into proximity and oriented to facilitate the reaction.

• 8.2 Enzyme Activity Can Be Modulated by Temperature, pH, and Inhibitory Molecules

  Specific small molecules or ions can inhibit even nonallosteric enzymes. Reversible inhibition is characterized by a rapid equilibrium between enzyme and inhibitor. A competitive inhibitor prevents the substrate from binding to the active site. It reduces the reaction velocity by diminishing the proportion of enzyme molecules that are bound to substrate. Competitive inhibition can be overcome by raising the substrate concentration. In uncompetitive inhibition, the inhibitor combines only with the enzyme–substrate complex. In noncompetitive inhibition, the inhibitor decreases the turnover number. Uncompetitive and noncompetitive inhibition cannot be overcome by raising the substrate concentration. In irreversible inhibition, the inhibitor is covalently linked to the enzyme or bound so tightly that its dissociation from the enzyme is very slow. Covalent inhibitors provide a means of mapping an enzyme’s active site.

• 8.3 Chymotrypsin Illustrates Basic Principles of Catalysis and Inhibition

  The cleavage of peptide bonds by chymotrypsin is initiated by the attack by a serine residue on the peptide carbonyl group. The attacking hydroxyl group is activated by interaction with the imidazole group of a histidine residue, which is, in turn, linked to an aspartate residue. This Ser-His-Asp catalytic triad generates a powerful nucleophile. The product of this initial reaction is a covalent intermediate formed by the enzyme and an acyl group derived from the bound substrate. The hydrolysis of this acyl-enzyme intermediate completes the cleavage process. The tetrahedral intermediates for these reactions have a negative charge on the peptide carbonyl oxygen atom. This negative charge is stabilized by interactions with peptide NH groups in a region on the enzyme termed the oxyanion hole.