Specific modification of methionine-192 of -chymotrypsin by an affinity label exploiting the orienting properties of the linear acetylenic group

Inactivation of alpha-chymotrypsin by new bifunctional reagents: halomethylated derivatives of dihydrocoumarins

alpha-Chymotrypsin is rapidly and completely inactivated by a series of halomethylated derivatives of dihydrocoumarins at pH 7 and 25 degrees. The inactivation is pH-dependent and optimal at neutral pH; it is also more or less complete depending on the excess of inhibitor with respect to the enzyme. These compounds are substrates for alpha-chymotrypsin and, during the catalytic process, a latent alkylating function of the reagent is activated at the active site of the enzyme and reacts with some vicinal nucleophilic amino acid residues. The stoichiometry of the reaction of the corresponding radioactive reagents with the enzyme is slightly superior to one, but one histidine residue of alpha-chymotropsin is mainly modified. This histidine is identified as histidine-57 by the diagonal peptide mapping method. In comparison with other reagents, the efficiency of these suicide substrates" and particularly that one of a derivative (compound 2) carrying a substrate-like side chain is found to be very high.

Studies on the role of methionine in porcine pancreatic phospholipase A2

The unique methionine-15 residue located at the N-terminal site of iso- or beta-phospholipase A2 from porcine pancrease has been specifically carboxymethylated with iodoacetic acid. The modification results in a complete inactivation of the enzymatic activity toward micellar and monomeric substrates. Spectroscopic measurements reveled that the carboxymethylated protein still binds Ca2+ and monomeric substrates with comparable affinities as the native enzyeme. The active site histidine-54 residue in the modified enzyme shows a reactivity toward the active site-directed irreversible inhibitor p-bromophenacylbromide which is identical to that of the native enzyme. The alkylated protein, however, has lost its ability to bind to lipid-water interfaces. Although circular dichroic spectra of the carboxymethylated enzyme display some changes in the tertiary structure as compared with the native enzyme, the alpha-helix content remains rather constant. It is concluded that carboxymethylation of methionine-15 destroys the interface recognition site but has only limited influence on the active site of the molecule. Therefore, it seems that methionine-15 is not involved in the catalytic events but that this residue is part of the interface recognition site which embraces the N-terminal hydrophobic part of the enzyme: Ala-Leu-Trp-Gln-Phe-Arg-Ser-Met.

Reinvestigation of the phenacyl bromide modification of alpha-chymotrypsin

The modification of alpha-chymotrysin with phenacyl bromide has been reinvestigated over a wide pH range. Evidence is presented that indicates that the nature of the phenacyl-modified enzymes prepared by this reaction is dependent upon the pH of the reaction medium. The phenacyl alpha-chymotrypsin produced at low pH is most probably the Met-192 phenacylsulfonium salt, as proposed earlier, since it readily undergoes dealkylation using 2-mercaptoethanol. However, the phenacyl-enzyme prepared at neutral pH possesses a much reduced enzymatic activity and does not react with 2-mercaptoethanol to regenerate native alpha-chymotrypsin. In addition, incubation of the Met-192 phenacyl sulfonium enzyme at neutral pH causes a smooth irreversible change to the new phenacyl-enzyme as monitored by changes in enzymatic activity, susceptibility to dealkylation using 2-mercaptoethanol, and ultraviolet difference absorption spectral properties. The stoichiometries of both the low and neutral pH modification reactions have been determined, using [carbonyl-14C]phyenacyl bromide, to be 1 phenacyl group/enzyme molecule. In efforts to obtain information about the nature and mechanism of formation of the phenacyl alpha-chymotrypsin produced at neutral pH, alkylation reactions of modified alpha-chymotrypsins produced by His-57 functionalization with tosylphenylalanine chloromethyl ketone and by Met-192 oxidation to the sulfoxide have been investigated. The combined results of these studies have been initially interpreted in terms of a neutral pH, phenacyl bromide modification resulting in formation of a new modified enzyme via the Met-192 sulfonium salt.