Initial characterization of a type I fatty acid synthase and polyketide synthase multienzyme complex NorS in the biosynthesis of aflatoxin B1

The biosynthesis of the potent environmental carcinogen aflatoxin B-1 is initiated by norsolorinic acid synthase (NorS), a complex of an iterative type 1 polyketide synthase and a specialized yeast-like pair of fatty acid synthases. NorS has been partially purified from Aspergillus parasiticus, has been found to have a mass of similar to1.4 x 10(6) Da, and carries out the synthesis of norsolorinic acid in the presence of acetylCoA, malonylCoA, and NADPH where hexanoyICoA is not a free intermediate. The N-acetylcysteamine thioester of hexanoic acid can substitute for the catalytic functions of HexA/B to initiate norsolorinic acid synthesis by the complex in the presence of only malonylCoA. An alpha(2)beta(2)gamma(2) stoichiometry is proposed for NorS in keeping with its estimated mass and the observed dimeric or higher-order quarternary structures of PKS and FAS enzymes.