Identification of determinants of substrate and reaction specificity of 1-aminocyclopropane-1-carboxylate synthase and 7,8-diaminopelargonic acid synthase

1-Aminocyclopropane-1-carboxylate (ACC) synthase and 7,8-diaminopelargonic acid (DAPA) synthase are pyridoxal 5'-phosphate (PLP)-dependent enzymes that utilize the same substrate, S-adenosyl- L-methionine (SAM), but yield different products. The former produces ACC by alpha,gamma-elimination, while the latter makes S-adenosyl-4-methylthio-2-oxobutanoate by transamination. The mechanisms of these two reactions are the same up to the formation of a quinonoid intermediate, from which they diverge. The active-site topology of the enzyme-intermediate complexes decides this pathway bifurcation. ACC synthase-catalyzed transamination is prevented by exclusion of water from the active site during turnover, while the elimination reaction is favored by stabilization of the reactive quinonoid intermediate and the transition state leading from it to product.;The substrate specificities of these two enzymes are also explored. Both exhibit high pKa values for the enzyme-PLP aldimine, which are necessary to complement the low pKa's of the substrates. This high enzyme pKa contributes to the low reactivity towards proteinogenic amino acids at physiological pH and contrasts with that of the closely related aspartate aminotransferase. Active site residues have been identified which act to modulate the p Ka of ACC synthase.;DAPA synthase further possesses dual substrate specificity, a requirement for catalysis of transamination. This enzyme, unlike other aminotransferases, binds each of its substrates in distinct binding sites, and it is thereby able to accommodate two remarkably different structures.