On the mechanism of action of methyl chymotrypsin

Breaking the low barrier hydrogen bond in a serine protease

The serine protease subtilisin BPN' is a useful catalyst for peptide synthesis when dissolved in high concentrations of a water-miscible organic co-solvent such as N,N-dimethylformamide (DMF). However, in 50% DMF, the k(cat) for amide hydrolysis is two orders of magnitude lower than in aqueous solution. Surprisingly, the k(cat) for ester hydrolysis is unchanged in 50% DMF. To explain this alteration in activity, the structure of subtilisin 8397+1 was determined in 20, 35, and 50% (v/v) DMF to 1.8 A resolution. In 50% DMF, the imidazole ring of His64, the central residue of the catalytic triad, has rotated approximately 180 degrees around the Cbeta-Cgamma bond. Two new water molecules in the active site stabilize the rotated conformation. This rotation places His64 in an unfavorable geometry to interact with the other members of the catalytic triad, Ser221 and Asp32. NMR experiments confirm that the characteristic resonance due to the low barrier hydrogen bond between the His64 and Asp32 is absent in 50% DMF. These experiments provide a clear structural basis for the change in activity of serine proteases in organic co-solvents.

Formation of pi, tau-dimethylhistidine on alkylation of trypsin with active-site-directed sulfonic acid methyl esters

The possibility of synthesizing stable alkyl analogues of acyl trypsins by introducing the alkyl residue by means of active-site-directed sulfonic acid esters was studied. Nine amidino- or guanidino-substituted sulfonic acids of different geometries and their methyl esters were prepared. The time-dependent inhibition of bovine trypsin by these esters, indicating modification at the active site of the enzyme, was followed. With the exception of p-guanidinobenzenesulfonic acid methyl ester, all the esters acted as irreversible inhibitors. The site of methylation, Ser-195 or His-57 (chymotrypsinogen numbering), was determine by analyzing for O-methylserine and methylhistidines. With four of the esters indications of a possible formation of, at most, 0.1 residue of O-methylserine per inactivated trypsin molecule were obtained. tau-Methylhistidine (but no pi-methylhistidine) was, however, always observed as the main product of the modification reaction. A further product, hitherto not yet described in active site methylations of serine proteinases, was pi, tau-dimethylhistidine (1,3-dimethylhistidine). The failure of an attempted synthesis of the N-acetyl-ethanolamine ester of p-toluene-sulfonic acid reported in the literature is shown to be due to the high instability of this ester.

New developments in enzymatic peptide synthesis

This review article describes new enzymatic methods developed for the efficient and irreversible synthesis of peptides based on native and modified proteases, and for the synthesis of polypeptides containing D- and/or unnatural amino acids. Potential opportunities for future developments in the field based on new enzymes, tailor-made catalytic antibodies, and on the technique of in vitro mutagenesis are also described.

Mechanism of action of cutinase: chemical modification of the catalytic triad characteristic for serine hydrolases

Cutinase from Fusarium solani f. sp. pisi was inhibited by diisopropyl fluorophosphate and phenylboronic acid, indicating the involvement of an active serine residue in enzyme catalysis. Quantitation of the number of phosphorylated serines showed that modification of one residue resulted in complete loss of enzyme activity. One essential histidine residue was modified with diethyl pyrocarbonate. This residue was buried in native cutinase and became accessible to chemical modification only after unfolding of the enzyme by sodium dodecyl sulfate. The modification of carboxyl groups with 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide in the absence of sodium dodecyl sulfate did not result in inactivation of the enzyme; however, such modifications in the presence of sodium dodecyl sulfate resulted in complete loss of enzyme activity. The number of residues modified was determined by incorporation of [14C]glycine ethyl ester. Modification of cutinase in the absence of sodium dodecyl sulfate and subsequent unfolding of the enzyme with detergent in the presence of radioactive glycine ester showed that one buried carboxyl group per molecule of cutinase resulted in complete inactivation of the enzyme. Three additional peripheral carboxyl groups were modified in the presence of sodium dodecyl sulfate. Carbethoxylation of the essential histidine and subsequent incubation with the esterase substrate p-nitrophenyl [1-14C]acetate revealed that carbethoxycutinase was about 10(5) times less active than the untreated enzyme. The acyl-enzyme intermediate was stabilized under these conditions and was isolated by gel permeation chromatography. The results of the present chemical modification study indicate that catalysis by cutinase involves the catalytic triad and an acyl-enzyme intermediate, both characteristic for serine proteases.