Abstract
Alpha-chymotrypsin, methylated specifically at the N δ2-position of His-57, crystallizes isomorphously with the native enzyme. This finding enabled us to determine the structure of methyl-chymotrypsin utilizing the phase information of tosyl-chymotrypsin. The resulting difference Fourier and Fourier maps indicate small movements of the active site histidine (57) and serine (195) among other less significant features. The covalently bound methyl group displaces a water molecule which is hydrogen-bonded to His-57 (N δ2) in both native and acyl-chymotrypsin and which is thought to take the place of the leaving group in deacylation. The serine side chain is flexible. Its γ-oxygen may reside either in a position coincident with that found in acyl-enzymes, or in a nearby position from which it can hydrogen-bond to an ordered water molecule. The structure of crystalline methyl-chymotrypsin does not seem to be consistent with the properties of the modified enzyme observed in solution (Henderson, 1971). A tentative explanation for these, based on model-building, is given.
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