Edyr Rogana

Thrombin-like enzyme from Lachesis muta muta venom: isolation and topographical analysis of its active site structure by means of the binding of amidines and guanidines as competitive inhibitors

A serine protease enzyme was purified from Lachesis muta muta venom, with 40% yield, by gel filtration on Sephadex G-100 and affinity chromatography on Sepharose-agmatin. Homogeneity of the enzyme preparation was demonstrated by sodium dodecyl sulfate polyacrylamide gel electrophoresis and the enzyme had a relative mol. wt of 45,000. The molar extinction coefficient at 280 nm was 62,127 (M x cm)-1. The enzyme hydrolysed Bz-Arg-Nan with Ks = 0.233 +/- 0.08 mM and kcat = 2.80 +/- 0.07 sec-1. All the amidines and guanidines tested for their inhibitory effect on thrombin-like enzyme behaved as competitive inhibitors of the enzyme with Ki values in the range 6.2 microM to 42.3 mM for amidines and 0.19 mM to 9.31 mM for guanidines. Dissociation constant values were analyzed in terms of the binding of the inhibitors with the subsite S1, the specificity pocket of the enzyme, Ki values were discussed in accordance with those for trypsin inhibition. beta-Naphthamidine was the strongest inhibitor, while guanidine was the weakest. The differences among the Ki values were interpreted in terms of the shape of the enzyme active site. For meta- and para-substituted benzamidinium ions a good correlation was found between log l/Ki and sigma Hammett values of the substituents. The substituent effects in the pi-electrons of the benzamidine ring were considered in the frame of Hückel molecular orbital theory. A model for the binding of p-benzamidine derivatives with the primary specificity S1 subsite of the enzyme active site was proposed.

Kinetic parameters for the activation of α- and β-trypsins by the methyl ester of tosyl-l-arginine (Tos-l-Arg-OMe)

Abstract The kinetic parameters describing the activation of α- and β-trypsins by the methyl ester of tosyl- l -arginine were determined at 37 °C, pH 8.0, according to the model proposed by C. G. Trowbridge, A. Krehbiel, and M. Laskowski, Jr. (1963 , Biochemistry2, 843-50). The parameters, calculated through a computer program called ATIV3, were: Km(2) = 19.2 mM and k′cat = 653 s−1, for α-trypsin; Km(2) = 50.3 mPm and k′cat = 1192 s−1 for β-trypsin. Thus, α-trypsin has a higher affinity for the activator, but β-trypsin has a higher value of the enhanced catalytic constant, k′cat. The ratio k′ cat K m(2) indicates that α-trypsin is more susceptible to activation than the β-form.