For several decades the specificity of proteases has been presented as an active site divided into subsites, using the nomenclature of Schechter & Berger from 1967 (S1, S2...for subsites of the active site; P1, P2...for residues of the substrate occupying the corresponding subsites). At early stages of the research (1960s) it was realized that the size of the active site was larger than expected and important interactions occur in regions remote from the catalytic site. Since the active site was found to be large it was divided into subsites, and a procedure to map it up was developed. The map provides information on the size of the active site (number of subsites), the properties of each subsite (free energy of ligand binding, nature of binding forces, etc.), and it enables rational design of new substrates and inhibitors. Already in 1968 inhibitors with binding constants ten thousand fold higher than available inhibitors, were prepared. The model of a large active site was initially met with strong opposition. Before long, however, predictions of the model (size of the active site, interactions in subsites remote from the catalytic site) were confirmed by X-ray crystallography (1970). During the 1990s proteolytic enzymes received renewed attention in biology and medicine, they became therapeutic targets, and protease inhibitors were successfully applied in the treatment of AIDS and hypertension. The model of large active site divided into subsites, proposed 38 years ago, stood the test of time. This model is still in use in basic research to evaluate enzyme activity, and in pharmaceutical research for the development of inhibitors/drugs.
Protease active site, model of active site, map of active site, subsites of active site, ligand subsite interaction, rational design of inhibitors, nomenclature of active site, antibody binding site
Department of Immunology,The Weizmann Institute of Science, Rehovot 76100, Israel.