Structural Modeling and Site-Directed Mutagenesis of the Actinorhodin β-Ketoacyl-Acyl Carrier Protein Synthase

A three-dimensional model of the Streptomyces coelicolor actinorhodin β-ketoacyl synthase (Act KS) was constructed based on the X-ray crystal structure of the related Escherichia coli fatty acid synthase condensing enzyme β-ketoacyl synthase II, revealing a similar catalytic active site organization in these two enzymes. The model was assessed by site-directed mutagenesis of five conserved amino acid residues in Act KS that are in close proximity to the Cys169 active site. Three substitutions completely abrogated polyketide biosynthesis, while two replacements resulted in significant reduction in polyketide production. (3)H-cerulenin labeling of the various Act KS mutant proteins demonstrated that none of the amino acid replacements affected the formation of the active site nucleophile.