Ficin- and papain-catalyzed reactions. Effect of temperature on reactivity of the essential sulfhydryl group of ficin in the presence and absence of competitive inhibitors

Abstract

Effect of temperature on the following reactions for ficin have been determined: (1) K m (app) for benzoyl- l-arginine ethyl ester, (2) K i for benzoyl- d-arginine ethyl ester, (3) rate of acylation by benzoyl- l-arginine ethyl ester, and (4) rates of alkylation of the essential sulfhydryl group with chloroacetamide in the absence and presence of benzoyl- d-arginine ethyl ester. In addition, the effect of temperature on the rate of alkylation of the sulfhydryl group of mercaptoethanol was determined. There is little or no effect of temperature on K m (app) and K i (ΔH∘, ΔF∘, and ΔS∘ = −0.36 kcal/mole, −1.80 kcal/mole, and −7.27 eu, respectively for both). The acylation and alkylation activation parameters for the sulfhydryl group of ficin are similar (ΔH∗, ΔF∗, and ΔS∗ for acylation (alkylation) were 6.61 (3.72) kcal/mole, 16.6 (15.8) kcal/ mole, and −33.4 (−40.6) eu, respectively). There were marked differences in ΔH∗ and ΔS∗ in the absence and presence of benzoyl- d-arginine ethyl ester (ΔH∗, ΔF∗, and ΔS∗ in absence (presence) were 3.72 (8.62) kcal/mole, 15.8 (14.9) kcal/mole and −40.6 (−21.0) eu, respectively). ΔS∗ in the presence of benzoyl- d-arginine ethyl ester was similar to that for alkylation of the sulfhydryl group of mercaptoethanol (ΔH∗, ΔF∗, and ΔS∗ for alkylation of mercaptoethanol (ficin in presence of benzoyl- d-arginine ethyl ester) were 13.8 (8.62) kcal/mole, 18.7 (14.9) kcal/mole and −16.5 (−21.0) eu, respectively). These results are discussed in relation to a possible conformational change in the active site of ficin on binding substrate or competitive inhibitor.