Elucidation of the Catalytic Function of Phospholipase A_2 and C

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1. Effects of Ca^<2+> on the bindings of Group I and II PLA_2s to a genuine substrate, an amide-type substrate analog, or an oxazolidinone-type analog were studied. The Ca^<2+> dependency in the binding of the gunuine substrate to the both types of PLA_2s were found to be very similar to that for the oxazolidinone-type substrate analog, but differed greatly from that for the amide-type substrate analog. This finding suggests that the binding mode of oxazolidinone-type substrate analog is very similar to that of the genuine substrate.2. Chemical modification and inactivation of bovine pancreatic PLA_2 by a manoalide (MLD) -analog were investigated. It was found that Lys-56 of the enzyme was modified and this modification was responsible for the enzyme inactivation.3. pH dependences of the kinetic parameters for the hydrolysis of Lyso-PC,catalyzed by PC-PLC,were studied. One and three transitions were observed on the pH dependence curves of 1/K_m and k_<cat>, respectively. Although similar results were obtained also by using PC as a substrate, the extents pK sifts of an ionizable group accompanying the bindings of Lyso-PC and PC were different from each other.4. pH dependences of the kinetic parameters for the hydrolysis of HNP,catalyzed by Bacillus cereus sphingomyelinase (SMase) and their mutants (D126G and D156G), were studied. The results suggested that the Asp-126 participated in the substrate binding and catalysis, and the Asp-156 reduced the binding ability of HNP and catalytic efficiency.