The affinity of phospholipase A2 for the interface of the substrate and analogs

In the intravesicle scooting mode of interfacial catalysis, the interfacial complex E*S is formed by the interaction of the membrane bound phospholipase A2 (E*) with the substrate monomer (S) in the interface. In the presence of nonhydrolyzable substrate analogs (I) the kinetics of interfacial catalysis is modified. If phospholipase A2 is added to a mixture of the vesicles of L-DMPMe ester and of DTPMe ether or D-DMPMe ester, the extent of hydrolysis, A, decreases and the interfacial scooting rate constant, ki, remains unchanged. On the other hand, when the enzyme is added to the vesicles prepared from premixed L-DMPMe ester with D-DMPMe ester or L-DTPMe ether, ki decreases but A remains constant. Qualitatively, these results are in excellent accord with the Scheme I for interfacial catalysis. However, a quantitative departure has been noted, which suggests that the interfacial dissociation constant for E*S is larger than that for E*I. These results are interpreted to suggest that the catalytic rate constant for decomposition of E*S to E* + P is larger than the rate constant for decomposition of E*S to E* + S. Broader implications of the scooting mode of interfacial catalysis are discussed.