Coupling between Cbz-Arg-OH and Leu-X catalyzed by trypsin and papain

Trypsin [EC 3.4.21.4] and papain [EC 3.4.22.2] catalyzed the coupling between Cbz-Arg-OH and Leu-X (X=-NH2, -OEt, -OBut, or -NH) in solution. The optimum pH for synthesis by trypsin and by papain was 6.5-7.0 and 5.5, respectively. Papain possessed reactivity several times higher than that of trypsin. To shift the reaction toward synthesis, the concentration of amine components must be at least several times higher than that of the carboxyl component (Cbz-Arg-OH). The effect of the concentration of Leu-X depended upon the nature of X; higher concentrations were required in the following order: -NH2 > -OEt, -OBut > -NH. High concentrations of organic cosolvents were also significant for promoting the synthesis.