KINETICS OF THE HYDROLYSIS OF MICELLAR SUBSTRATES CATALYZED BY SNAKE-VENOM PHOSPHOLIPASES-A2

Effects of Ca2+ on the kinetic parameters for the hydrolysis of mixed micelles of 1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine (diC16PC) with Triton X-100, catalyzed by a cobra (Naja naja atra) (Group I) and a Habu (Trimeresurus flavoviridis) (Group II) PLA2s, were studied and compared with the results reported for other Group I and II enzymes. The substrate bindings to Group I enzymes were independent of the Ca2+ binding, whereas the substrate bindings to Group II enzymes were facilitated more than 10 times by the Ca2+ binding to the enzymes. The result for Group II enzymes, but not Group I enzymes, seemed compatible with the hypothesis for interpreting the catalytic mechanism that an intermediate complex should be stabilized by the coordination of the bound Ca2+ with the phosphoryl group and the carbonyl oxygen atom of the ester bond at the sn-2 position of the bound substrate molecule [Verheij et al. (1980) Biochemistry 19, 743-750 and (1981) Rev. Physiol. Biochem. Pharmacol. 91, 91-203]. The pH dependence of the kinetic parameters for the hydrolysis of the mixed micellar diC16PC, catalyzed by the cobra (N. naja atra) (Group I) and Habu (T. flavoviridis) (Group II) PLA2S, was also studied. The pK values of the catalytic group, His 48, and Tyr 52 for N. naja atra PLA2, shifted from 7.25 to 7.70 and from 10.30 to 10.85, respectively, and the corresponding values for T. flavoviridis PLA, shifted from 5.80 to 6.95 and from 10.10 to 10.76, respectively, on binding of the micellar substrates to the enzymes. On the other hand, no participation of these ionizable groups was observed for the findings of monodispersed substrates [Teshima et al. (1985) J. Biochem. 98,1509-1517 and Teshima et al. (1989) J. Biochem. 105, 1044-1051]. Consequently, it was concluded that the increases in the pK values of His 48 and Tyr 52 on binding of micellar substrates to PLA2S were a property common to both types of enzymes (Groups I and II).