Enzyme Mechanisms

Inhibitor A molecule that by binding to the enzyme lowers its activity (i.e., its ability to process the substrate). Intermediate A molecular species usually bound to the enzyme that exists transiently in the course of converting the substrate of the enzyme to its product. Product A molecule that results from a chemical transformation of its […]

Enzyme Mechanisms

THE IMPETUS for understanding how enzymes function is inspired by their enormous catalytic efficiency and their exquisite substrate stereospecificity. With the advent of the determination of enzyme structure and the application of physical organic tools to examine the reaction coordinate for the enzymatic transformation of the substrate, penetrating insights have been gained as to the […]


Enzymes are biological molecules which accelerate the rate, and often direct the specificity, of a chemical reaction. Like all catalysts, they are not themselves consumed in the reactions in which they participate but are regenerated to take part in multiple cycles. Transformations which are very slow, such as the breakdown of DNA, can be accelerated […]


The study of the rates of enzyme-catalyzed transformations provides invaluable information as to the number of steps and their magnitude in the catalytic process. The most common method is to use steady-state conditions in which the enzyme is at <10_8 M concentration and the substrate(s) \xM or higher. In the simplest case of the conwhere […]

ILLUSTRATIVE EXAMPLES A. a-Chymotrypsin Part 1 (Enzyme Mechanisms)

Alpha-chymotrypsin (Fig. 2) catalyzes the facile hydrolysis of peptide bonds, in particular those adjacent to the carboxyl group of aromatic amino acids (tryptophan, ty-rosine, phenylalanine) as well as a variety of esters derived from similar N-acylated amino acids. The enzyme has been the subject of intensive mechanistic study, most of which occurred well before a […]

ILLUSTRATIVE EXAMPLES A. a-Chymotrypsin Part 2 (Enzyme Mechanisms)

D. Triosephosphate Isomerase Triosephosphate isomerase (TIM) catalyzes the inter-conversion of D-glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). The equilibrium lies far to the side of DHAP, hence the longer arrow pointing to that compound. The enzyme operates with a turnover number of ~107 s-1, which is nearly as fast as the diffusion-controlled limit. TIM is therefore […]


The source of the stereospecificity of enzyme-catalyzed reactions is clearly revealed by the fit of the substrate to the enzyme’s active site that spatially then directs the stereochemical course of the chemical events. The speed of these reactions has been attributed to the lowering of the activation energy for the process by the greater affinity […]