ABSTRACT
The use of biological affinity to label amino acid residues at enzyme active sites, allosteric binding sites, substrate binding sites, and other types of binding sites on proteins has proved to be a powerful tool in the study of the relationship between structure and function. There are at least three major considerations which must be satisfied in order to justify identification of a compound as a true affinity label. It should be stressed that affinity labeling is conceptually a different process than "fortuitous" modification of a uniquely reactive residue at, for instance, the active site of an enzyme. The reaction of peptide chloromethyl ketones with sulfhydryl proteases has been investigated. Despite the presence of a histidine residue at the active site of these enzymes, reaction occurs at the active site sulfhydryl group. The development of peptide chloromethyl ketones serves as a good example of the logical development of an active-site directed inhibitor.
