Synthetic substrates and inhibitors of beta-poly(L-malate)-hydrolase (polymalatase)

Polymalatase from Physarum polycephalum calalysed the hydrolysis of beta-poly[L-malate] and of the synthetic compounds beta-di(L-malate), beta-tetra(L-malate), beta-tetra(L-malate) beta-propylester, and L-malate beta-methylester. Cyclic beta-tri(L-malate), cyclic beta-tetra(L-malate), and D-malate beta-methylester were not cleaved, but were competitive inhibitors. The O-terminal acetate of beta-tetra(L-malate) was neither a substrate nor an inhibitor. L-Malate was liberated; the Km, Ki and Vmax values were measured. The appearance of comparable amounts of beta-tri(L-malate), and beta-di(L-malate) during the cleavage of beta-tetra(L-malate) indicated a distributive mechanism for small substrates. The accumulation of a series of oligomers, peaking with the 11-mer and 12-mer in the absence of higher intermediates, indicated that the depolymerization of beta-poly(L-malate) was processive. The results indicate that beta-poly(L-malate) is anchored at its OH-terminus by the highly specific binding of the penultimate malyl residue. The malyl moieties beyond 12 residues downstream from the OH-terminus extend into a diffuse second, electrostatic binding site. The catalytic site joins the first binding site, accounting for the cleavage of the polymer into malate residues. It is proposed that the enzyme does not dissociate from beta-poly(L-malate) during hydrolysis, when both sites are filled with the polymer. When only the first binding site is filled, the reaction partitions at each oligomer between hydrolysis and dissociation.