Efficient synthesis of aromatic polyketides in vitro by the actinorhodin polyketide synthase

Polyketide synthase (PKSs) are a large family of enzymes that catalyze the biosynthesis of numerous structurally diverse and medicinally important natural products known as polyketides. These enzymes direct the polymerization of activated carboxylic acids in a manner analogous to fatty acid biosynthesis. Unlike fatty acid biosynthesis, the beta-carbonyl of a growing polyketide chain may be left unreduced or converted to hydroxy, enoyl, or methylene functionalities. The level of ketoreduction, as well as the initial cyclization pattern of the full-length polyketide chain, is determined by the PKS. In conclusion, several aspects of the act minimal PKS have been reconstituted in cell-free systems. First, efficient conversion of malonyl CoA into 1 and 2 demonstrates the catalytic potency of the preparation. Second, enzymatic synthesis of equimolar amounts of SEK4 (1) and SEK4b (2) is in excellent quantitative agreement with the in vivo behavior of the act minimal PKS. Finally, the ability of the act ketoreductase, aromatase, and cyclase to assist in the conversion of the nascent 16-carbon polyketide produced by the minimal PKS into DMAC (3) shows that the entire metabolic pathway consisting of at least six different proteins can be reconstituted in vitro. Together, these results provide an attractive starting point for kinetic, protein chemical, and structural studies of the act PKS as a paradigm for this interesting class of enzymes.