Free coenzyme A-induced activation of a multifunctional polyketide biosynthetic enzyme 6-hydroxymellein synthase in carrot cells

6-Hydroxymellein (6HM) synthase, a multifunctional polyketide biosynthetic enzyme in carrot, catalyzes the condensation of acyl-CoAs and the reduction of the ketomethylene intermediate, and the enzyme activity markedly increa ses in the presence of submillimolar concentrations of CoA. However, the re action rate was appreciably decreased by the addition of CoA when the ketor educing process was omitted from the series of partial reactions of 6HM bio synthesis. The velocity of the catalytic reaction of the CoA-associated syn thase showed a marked increase in 6HM formation, while the value of the con densation reaction did not show the significant change even in the presence of CoA. The optimal pH of the 6HM synthase-catalyzing reactions shifted fr om 7.5-8.0 to 6.0 by the addition of CoA only when the ketoreducing reactio n was involved. These results suggest that the association of free CoA with 6HM synthase protein evokes the alternation of microstructure at the react ion center of the enzyme, which results in the enhancement of the reaction rate of ketoreduction involved in 6HM biosynthesis. (C) 1999 Elsevier Scien ce Ireland Ltd. All rights reserved.