EFFECT OF NADPH-ASSOCIATED KETO-REDUCING DOMAIN ON SUBSTRATE ENTRY INTO 6-HYDROXYMELLEIN SYNTHASE, A MULTIFUNCTIONAL POLYKETIDE SYNTHETIC ENZYME INVOLVED IN PHYTOALEXIN BIOSYNTHESIS IN CARROT

6-Hydroxymellein synthase is a polyketide biosynthetic enzyme induced in carrot cells which is organized as a homodimer composed of multifun ctional subunits. The synthase liberates triacetic acid lactone, inste ad of 6-hydroxymellein, as a derailment product when the keto-reducing reaction at the triketide intermediate stage is interrupted, However, the efficiency of the triacetic acid lactone-forming reactions is mar kedly lower than that of the normal reaction, and the kinetic analyses have revealed that the affinity of the enzyme protein for acetyl-CoA is appreciably reduced in the abnormal reactions, It is assumed that t he interaction of the NADPH-associated keto-reducing domain with a put ative primary binding site(s) of the acyl-CoA in the enzyme structure affects the entry of the starter unit into the protein. The present fi nding should provide an example of the novel class of ''subunit commun ication'' of multimer enzymes, (C) 1996 Academic Press, Inc.