TRANSACYLASE-LIKE STRUCTURE AND ITS ROLE IN SUBSTRATE CHANNELING OF 6-HYDROXYMELLEIN SYNTHASE, A MULTIFUNCTIONAL POLYKETIDE BIOSYNTHETIC ENZYME IN CARROT CELL-EXTRACTS

6-Hydroxymellein synthase, a multifunctional polyketide biosynthetic e nzyme of carrot, lost the binding ability toward its co-substrates, ac etyl- and malonyl-CoAs, by the treatment with the blocking reagents fo r serine-OH. In contrast, the enzyme retained the binding ability even when the two SH groups at the reaction center (cysteine-SH of the con densation enzyme and cysteamine-SH of acyl carrier protein) were block ed, and one substrate bound to the SH-blocked enzyme was readily repla ced by the other, It appeared that the cysteine-SH accepted only acety l moiety while cysteamine-SH was preferentially malonylated in the pre sence of both of the substrates. These results suggest that transacyla se-like domain is involved in the structure of 6-hydroxymellein syntha se as a common primary binding site of its co-substrates, and acetyl a nd malonyl moieties are properly channeled from their CoA esters to cy steine-SH and acyl carrier protein-SH via this domain, respectively.