Biosynthesis of guanidinoacetic acid. II. Mechanism of amidine group transfer

Purified transamidinase has been shown to catalyze the transfer of the amidine group from the donors arginine, guanidinoacetic acid, and canavanine to the acceptors glycine, ornithine, and canaline. All possible donor-acceptor pair combinations can interact at appreciable rates. The results of isotope incorporation studies and of kinetic studies of product inhibition are consistent with a mechanism of amidine group transfer by direct interaction of donor and acceptor on the enzyme surface without the intermediate formation of an enzyme-amidine compound. The hypothesis proposed requires that the substrates be attached to the enzyme at adjoining sites in the catalytic area, that an acceptor molecule be capable of occupying either an acceptor site or the site occupied by the acceptor moiety of the donor molecule, and, conversely, that a donor molecule be capable of occupying both a donor and acceptor site. Competitions for the enzyme have been demonstrated between two acceptors, between an acceptor and a donor, and between two donors. It is suggested that where direct proof for the existence of an enzyme-compound or complex has not or cannot be obtained, inhibition analysis may serve to distinguish between a direct interaction and a successive-step mechanism.