Influence of Leaving-Group Electronic Effect on alpha-Chymotrypsin: Catalytic Constants of Specific Substrates

The specificity of chymotrypsin. A statistical analysis of hydrolysis data

From the literature we collected all available quantitative data on the chymotrypsin-catalyzed hydrolysis of series of amino acid and peptide substrates. Utilizing this data base, we performed calculations on their quantitative structure/activity relationship (QSAR). The substrates were considered to be composed of fragments; log(kcat/Km) values for the substrates resulted from additive contributions of their fragments. Despite the fact that the kinetic constants in the data base were determined by different authors under various reaction conditions, the data are well described by the simple additivity model. Obviously, the intrinsic specificity of chymotrypsin dominates the influence of varying reaction conditions.

Kinetics of subtilisin and thiolsubtilisin

Subtilisin is a bacterial serine protease with a broad specificity in the S1 subsite. It has been very extensively studied using a variety of kinetic and physical techniques. A chemical derivative, thiolsubtilisin, has been subjected to similar studies in order to analyze the effects of the OH to SH conversion on enzyme activity. The native structure of thiolsubtilisin is indicated by a variety of physical techniques. Oligopeptides bind nearly equally well to both enzymes, and a peptide chloromethylketone is much more reactive to thiolsubtilisin than to subtilisin. Both enzymes have a similar level of activity towards activated nonspecific amides and esters. However, thiolsubtilisin is inactive towards highly specific peptide amides and esters. Thiolsubtilisin also does not show good binding to boronic and arsonic acids. The observation that these transition state analog inhibitors bind poorly to thiolsubtilisin while other compounds bind nearly equally well to both enzymes suggests that thiolsubtilisin may not be able to stabilize the transition state during acylation by specific substrates.

Do cleavages of amides by serine proteases occur through a stepwise pathway involving tetrahedral intermediates?

The mechanism of the serine protease-catalyzed cleavage of amides (acylation) was examined in terms of the basicity of the functional groups participating in the catalysis. It is proposed that the reaction does not proceed through a stepwise pathway, as opposed to the cleavage of esters and anilides, which start with general base-catalyzed formation of the tetrahedral intermediate followed by its general acid-catalyzed breakdown. Instead, the proton abstracted from the hydroxyl group of the serine by the imidazolyl group of the histidine is donated to the nitrogen atom of the leaving group of the amide before the bond between the carbonyl carbon atom of the amide and the attacking serine oxygen atom is completed. Reactions proceed by a SN2-like reaction through the cooperation of acid catalysis by the imidazolyl cation and nucleophilic attack by the serine. The mechanisms of the enzymatic hydrolyses of anilides and esters proceed through a discrete tetrahedral intermediate, but the enzymatic hydrolyses of amides probably do not.

Binding rates, O--S substitution effects, and the pH dependence of chymotrypsin reactions

The pH dependence for acylation of alpha-chymotrypsin by N-acetyltryptophan p-nitrophenyl-, p-nitrothiophenyl-, ethyl-, and thiolethyl esters has been studied by the stopped-flow technique. Values for the acylation rate constant, k2, and the binding constant, KS, were obtained by using measurements of phenolate release, for the p-nitrophenyl esters, and proflavin displacement, for the ethyl esters. The oxygen esters tested have slightly higher k2 values, and substantially higher KS values relative to the analogous thiol esters. Whereas k2/KS for the thiolethyl ester is higher than that for the analogous oxygen ester, the k2/KS values for oxy- and thio-p-nitrophenyl esters are nearly identical. These data are interpreted to indicate rate-determining formation of a tetrahedral intermediate in acylation of alpha-chymotrypsin by p-nitrophenyl esters, and rate-determining breakdown of such an intermediate in the case of the ethyl esters. It is also concluded that the oxygen to sulfur substitution causes a substantial increase in the proportion of nonproductive binding in these substrates. pH dependent k2 and KS values were used to calculate values for k1 and k-1, the binding and debinding rate constants for the two p-nitrophenyl compounds. This is the first such calculation based on experimentally determined acylation rate constants.

Structure-activity relationship in the urokinase hydrolysis of alpha-N-acetyl-L-lysine anilides

The absence of both nonproductive binding and substrate activation and also the good solubility of the substrates make the urokinase-catalysed hydrolysis of specific anilides a very suitable reaction for substrate structure-enzyme activity studies. Derivatives of alpha-N-acetyl-L-lysine anilide with high sigma minus-value substituents in the aniline ring were synthesized. Rate constants kappa-cat. and apparent Michaelis-Menten constants K-m (app.) are presented. From the substituent dependence of kappa-cat. and from the fact that kappa-cat. is 13 to 37 times smaller than the deacylation rate constant it is concluded that the rate-limiting step proceeds prior to deacylation. The catalytic rate constant kappa-cat. obeys a linear free-energy relationship of the Hammett type with Q equals +0.72. Two different mechanisms implied by the results obtained from the model reaction (specific base and general acid-base catalysed hydrolysis of N-acetylglycine anilides under extreme conditions) are proposed in order to account for this positive and low Q-value. In the first mechanism the breakdown of an enzyme tetrahedral intermediate is rate-limiting, while in the second one its formation controls the overall rate. The discrimination between the two mechanisms, however, could not be found.

Acylation of alpha-chymotrypsin by oxygen and sulfur esters of specific substrates: kinetic evidence for a tetrahedral intermediate

The acylation step of the alpha-chymotrypsincatalyzed hydrolysis of N-acetyl-L (or DL)-tryptophan p-nitrophenyl, p-nitrothiophenyl, ethyl, and thiolethyl esters has been studied by the stopped-flow technique at 25 degrees . The acylation rate constant, k(2), and the enzyme substrate dissociation constant, K(s), were directly determined at pH 4, 5, and 8. Steady-state kinetics were studied at pH 7. The k(2) values are nearly identical for oxygen esters and their sulfur counterparts, whereas the K(s) value of the ethyl ester is larger by an order of magnitude than those of the other three. The results strongly suggest that oxygen and thiol esters of these specific substrates are hydrolyzed via the same pathway, and furthermore that acylation consists of more than one step, the formation and breakdown of a tetrahedral intermediate, the former being rate-determining. Effects of leaving-group hydrophobicity on k(2) and K(s) are also discussed.