Information on EC 2.3.1.292 - (phenol)carboxyphthiodiolenone synthase

The enzyme, which is a complex of five polyketide synthase proteins, is involved in the synthesis of the lipid core common to phthiocerols and phenolphthiocerols. The first protein, PpsA, can accept either a C₁₈ or C₂₀ long-chain fatty acyl, or a (4-hydroxyphenyl)-C₁₇ or C₁₉ fatty acyl. The substrates must first be adenylated by EC 6.2.1.59, long-chain fatty acid adenylase/transferase FadD26, which also loads them onto PpsA. PpsA then extends them using a malonyl-CoA extender unit. The PpsB protein adds the next malonyl-CoA extender unit. The absence of a dehydratase and an enoyl reductase domains in the PpsA and PpsB modules results in the formation of the diol portion of the phthiocerol moiety. PpsC adds a third malonyl unit (releasing a water molecule due to its dehydratase domain), PpsD adds an (R)-methylmalonyl unit, releasing a water molecule, and PpsE adds a second (R)-methylmalonyl unit, without releasing a water molecule. The incorporation of the methylmalonyl units results in formation of two branched methyl groups in the elongated product. The enzyme does not contain a thioesterase domain , and release of the products requires the tesA-encoded type II thioesterase .