Conserved amino acid residues correlating with ketoreductase stereospecificity in modular polyketide synthases.

Table 1 classifies the KR domains in the PKSs reviewed in [4] and the amphotericin PKS. Fig. 4 shows the ClustalW-generated alignments of A and B-type KR domains. A-type KRs are above the gap, B-type below. In fatty acid and polyketide biosynthesis, dehydration of a β-hydroxyacyl chain normally forms a trans double bond. Fig. 5 shows alignments of the 88-103 region and the 134-149 region in KR domains for which stereospecificity is masked by subsequent dehydration, or dehydration and enoyl reduction. Apart from two unusual cases, all of these domains are predicted to be B-type. The two exceptions are the EPOKR3 and RIFKR10 domains which are predicted to give A-type 3-hydroxyacyl chains. Module 3 of the epothilone PKS does not contain a DH domain. The dehydration that masks the EPOKR3 stereospecificity occurs as a late modification and forms an unusual cis double bond. RIF KR 10 is another special case. This is from the final module of the rifamycin PKS which forms a double bond that is cis in rifamycins but is trans in the linear intermediate that is produced by mutants lacking the RifF cyclase.