Substrate

A substrate is the substance upon which an enzyme acts in an enzymatic reaction. Enzymes are biological catalysts that increase the rate of chemical reactions by decreasing the activation energy required for that reaction. An enzyme catalyzes a chemical reaction converting a substrate reactant to a product. An individual enzyme generally has more than one substrate and may be specific to several reaction intermediates that are part of an over-all reaction.

The three-dimensional structure of an enzyme determines its substrate binding specificity. A simple hypothesis proposed by German chemist Emil Fischer in 1894 suggested that the specificity of an enzymatic reaction could be likened to a lock and key. In the lock and key hypothesis, the geometric complementarity of the structures of the enzyme (the lock) and the substrate (the key) accounts for the specificity of the reaction. Although scientists were not able to determine the actual three-dimensional structures of enzymes and substrates until many years later, the basic idea of the lock-and-key hypothesis has held. A more refined hypothesis, known as the induced fit hypothesis, proposes that the binding of the substrate by the enzyme changes the structure of the enzyme, resulting in an even greater affinity of the enzyme for the substrate. The site on an enzyme that binds the substrate (known simply as the substrate binding site) is most often a pocket or cleft in the approximately globular structure of the enzyme.

The term substrate has another meaning in chemistry. Some chemical syntheses are carried out in mixed phases; for example, the reactants exist in solution but the reaction itself occurs at the surface of a solid. The identity of the solid, specified in experimental protocols, influences the synthesis reactions, and the solid is referred to as the substrate.

SEE ALSO Fischer, Emil Hermann .

Robert Noiva

Bibliography

Berg, Jeremy M.; Tymoczko, John L.; and Stryer, Lubert (2002). Biochemistry, 5th edition. New York: W. H. Freeman.

Voet, Donald; Voet, Judith G.; and Pratt, Charlotte W. (2002). Fundamentals of Biochemistry, updated edition. New York: Wiley.