The interaction of inhibitors of proteolytic enzymes with 3-methylhistidine-57-chymotrypsin.

Abstract

The inhibition of proteolytic enzymes by protein inhibitors is accompanied by the formation of unusually stable complexes. The recognition of a specific substrate-like amino acid on the inhibitor is believed to be the initial event of the inhibitory process. In addition to the interactions involved in the binding of a good substrate, a variety of other noncovalent interactions are known to stabilize the complex. The formation of stable complexes between several inactive derivatives of proteolytic enzymes and a variety of protein inhibitors suggests strongly that the formation of any species resembling catalytic intermediates is unnecessary for inhibition. We have examined the interaction between several avian ovomucoids and alpha-chymotrypsin in which histidine-57 has been converted to 3-methylhistidine-57. This derivative is easily prepared and can be isolated by affinity chromatography. Methylchymotrypsin retains unaltered its ability to bind specific substrates, but is essentially inactive. In spite of this loss of enzymatic activity, methylchymotrypsin forms strong complexes with several inhibitors. In addition, methylchymotrypsin which has been covalently linked to Sepharose is particularly useful for the isolation of protein inhibitors without the complications due to isolation of a mixture of partially cleaved forms of the inhibitor.