Free CoA as activator of 6-hydroxymellein synthase, a multifunctional polyketide biosynthetic enzyme in carrot

The activity of 6-hydroxymellein (6HM) synthase, a multifunctional polyketi de biosynthetic enzyme in carrot, was appreciably inhibited in the presence of ATP: citrate lyase, a CoA-scavenging enzyme. Catalytic rate of the synt hase markedly decreased when deuterium-labeled NADPH was employed in the en zyme reaction, however, the isotope effect was appreciably reduced by the a ddition of free CoA. These observations suggest that CoA is the activator o f 6HM synthase both in low and high concentrations, and the compound functi ons as the acyl acceptor for proper entry of the substrates and enhances th e ketoreducing reaction, the rate limiting step of 6HM biosynthesis. (C) 19 98 Elsevier Science Ltd. All rights reserved.