During assembly of complex polyketide antibiotics like erythromycin A,
molecular recognition by the multienzyme polyketide synthase controls
the stereochemical outcome as each successive methylmalonyl-coenzyme
A (CoA) extender unit is added. Acylation of the purified erythromycin
-producing polyketide synthase has shown that all six acyltransferase
domains have identical stereospecificity for their normal substrate (2
S)-methylmalonyl-CoA. In contrast, the configuration of the methyl-bra
nched centers in the products, that are derived from (2S)-methylmalony
l-CoA, is different. Stereoselection during the chain building process
must, therefore, involve additional epimerization steps.