Engineered biosynthesis of novel polyketides: evidence for temporal, but not regiospecific, control of cyclization of an aromatic polyketide precursor

Aromatic polyketide synthases (PKSs) catalyze the formation and cyclization of polyketide chains of variable lengths, generating a family of compounds of proven medical significance. Initial control over the regiospecificity of cyclization is believed to be exercised by the minimal PKS, composed of the three essential components for polyketide biosynthesis, which catalyzes an intramolecular aldol condensation towards the middle of the chain. Subsequent cyclization reactions are either catalyzed by additional components of the PKS, or occur in the absence of specific catalysts.

Structural and biosynthetic studies on SEK4b, a novel octaketide product of a minimal PKS, revealed an unusual cyclization pattern. The first cyclization (an aldol condensation) occurs at the methyl end of the unreduced polyketide backbone precursor. This is followed by hemiketal formation and lactonization. The overall structure of SEK4b is similar to that of SEK4, a previously-identified product of the same genetically-engineered strain, differing only in the positions of a methyl and a pyrone group around a common fused-ring system. The biosynthetic pathways of the two molecules are quite different, however. The yield of SEK4b relative to SEK4 is much higher in the absence of PKS components (aromatases and cyclases) acting later in the pathway.

In this cyclization pathway, the regiospecificity of cyclization is not directly controlled by the minimal PKS. Instead, we propose that the enzyme influences cyclization by controlling the timing of chain release. Chain release and cyclization may be concurrent with synthesis. Other PKS subunits appear to stabilize the complex of the PKS with the nascent chain, preventing premature release.