Unusual arrangement of catalytic domains in head-to-tail associated homodimer of 6-hydroxymellein synthase, a multifunctional polyketide biosyntheticenzyme

6-Hydroxymellein synthase, a multifunctional polyketide synthetic enzyme in carrot, is organized as a homodimer, and the activity of the synthase was appreciably inhibited upon the specific alkylation of cysteine- and cysteam ine-SHs at the reaction center with iodoacetoamide and chloroacetyl-CoA, re spectively. Dissociation and stoichiometric recombination of the unmodified and the SH-modified enzyme subunits yielded a combination of unmodified-un modified, unmodified-modified and modified-modified hybrid dimers that toge ther exhibit 50% activity. In contrast, hybrid dimers obtained by reconstru ction of the two modified enzymes showed essentially no catalytic activity. These results suggest that the two subunits of 6-hydroxymellein synthase a re aligned in head-to-tail orientation to organize two reaction centers whi ch are comprised of a cysteine and a complementary cysteamine SH group, bel onging to and contributed from the same subunit in the homodimer structure. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.