Processive interfacial catalytic turnover by Bacillus cereus sphingomyelinase on sphingomyelin vesicles

Sphingomyelinase (SMase), a water-soluble enzyme from Bacillus cereus, is shown to bind with high affinity to vesicles of sphingomyelin (SM) but not to vesicles of phosphatidylcholine (PC). The reaction progress by SMase bound to SM vesicles occurs in the scooting mode with virtually infinite processivity of the successive interfacial turnover cycles. Three conditions for the microscopic steady state during the reaction progress at the interface are satisfied: the bound SMase does not leave the interface even after all the SM in the outer layer is converted to ceramide; the SMase-treated vesicles remain intact; and the ceramide product does not exchange with SM present in excess vesicles or in the inner layer of the hydrolyzed vesicle. Within these constraints, on accessibility and replenishment of the substrate, the extent of hydrolysis in the scooting mode reaction progress is a measure of the number of vesicles containing enzyme. The slope of the Poisson distribution plot, for the enzyme per vesicle versus the logarithm of the fraction of the total accessible substrate remaining unhydrolyzed in excess vesicles, shows that a single 32 kDa subunit of SMase is fully catalytically active. The maximum initial rate of hydrolysis, at the limit of the maximum possible substrate mol fraction, X(S)*=1, is 400 s(-1) in H(2)O and 220 s(-1) in D(2)O, which is consistent with the rate-limiting chemical step. The integrated reaction progress suggests that the ceramide product does not codisperse ideally on the hydrolyzed vesicles. Furthermore, complex reaction progress seen with covesicles of SM+PC are attributed to slow secondary changes in the partially hydrolyzed SM vesicles.