The dehydratase (DH) domain of module 4 of the 6-deoxyerythronolide B synthase (DEBS) has been shown to catalyze an exclusive syn elimination/syn addition of water. Incubation of recombinant DH4 with chemoenzymatically prepared anti-(2R,3R)-2-methyl-3-hydroxypentanoyl-ACP (2a-ACP) gave the dehydration product 3-ACP. Similarly, incubation of DH4 with synthetic 3-ACP resulted in the reverse enzyme-catalyzed hydration reaction, giving a ~3:1 equilbrium mixture of 2a-ACP and 3-ACP. Incubation of a mixture of propionyl-SNAC (4), methylmalonylCoA, and NADPH with the DEBS β-ketoacyl synthase -acyl transferase [KS6][AT6] didomain, DEBS ACP6, and the ketoreductase domain from tylactone synthase module 1 (TYLS KR1) generated in situ anti-2a-ACP that underwent DH4-catalyzed syn dehydration to give 3-ACP. DH4 did not dehydrate either syn-(2S,3R)-2b-ACP, syn-(2R,3S)-2c-ACP, or anti-(2S,3S)-2d-ACPgenerated in situ by DEBS KR1, DEBS KR6, or the rifamycin synthase KR7 (RIFS KR7), respectively. Similarly, incubation of a mixture of (2S,3R)-2-methyl-3-hydroxypentanoyl-Nacetylcysteamine thioester (2b-SNAC), and NADPH with DEBS [KS6] [AT6], DEBS ACP6, and TYLS KR1 gave anti-(2R,3R)-6-ACP that underwent syn dehydration catalyzed by DEBS DH4 to give (4R,5R)-(E)-2,4-dimethyl-5-hydroxy-hept-2-enoyl-ACP (7-ACP). The structure and stereochemistry of 7 were established by GC-MS and LC-MS comparison of the derived methyl ester 7-Me to a synthetic sample of 7-Me.Of the more than 2000 non-aromatic polyketides, the vast majority contain one or more disubstituted or trisubstituted double bonds, most of which have E (trans) geometry.1 Moreover, essentially all polyketides that do not themselves display a double bond are biosynthesized by way of one or more unsaturated polyketide chain elongation intermediates. Thus although 6-deoxyerythronolide B (1, 6-dEB), the parent aglycone of the erythromycin family of antibiotics, does not have any double bonds in the final Direct evidence for the intermediacy of an unsaturated polyketide in erythromycin biosynthesis first came from disruption of the NADPH-binding motif of the ER4 domain, resulting in accumulation of a derivative of the corresponding (E)-Δ 6,7 -anhydro-6-dEB by mutants of the erythromycin producer Saccharopolyspora erythraea.4 Although the stereochemistry of the substrate for the DEBS DH4 dehydratase is not known, the responsible ketoreductase, DEBS KR4, is predicted to generate the (3R)-diastereomer of the 2-methyl-3-hydroxyacyl-ACP pentaketide, as deduced from the presence of a Leu-Ala-Asp triad closely correlated with the formation of (3R)-3-hydroxyacyl-ACP polyketide intermediates.5 Indeed, the vast majority of KR domains that are paired with a DH domain appear to harbor a conserved "Leu-Asp-Asp" motif.5a,b DEBS KR4 is also predicted to belong to the class of non-epimerizing ketoreductases, which would give rise to a (2R)-methyl group in the reduced product.5cTo establish the substrate specificity and stereochemical course of the DEBS DH4-catalyzed dehydration we used a che...

The dehydratase domain FosDH1 from module 1 of the fostriecin polyketide synthase (PKS) catalyzed the stereospecific interconversion of (3R)-3-hydroxybutyryl-FosACP1 (5) and (E)-2-butenoyl-FosACP1 (11), as established by a combination of direct LC-MS/MS and chiral GC-MS. FosDH1 did not act on either (3S)-3-hydroxybutyryl-FosACP1 (6) or (Z)-2-butenoyl-FosACP1 (12). FosKR2, the ketoreductase from module 2 of the fostriecin PKS that normally provides the natural substrate for FosDH2, was shown to catalyze the NADPH-dependent stereospecific reduction of 3-ketobutyryl-FosACP2 (23) to (3S)-3-hydroxybutyryl-FosACP2 (8). Consistent with this finding, FosDH2 catalyzed the interconversion of the corresponding triketide substrates (3R,4E)-3-hydroxy-4-hexenoyl-FosACP2 (18) and (2Z,4E)-2,4-hexadienoyl-FosACP2 (21). FosDH2 also catalyzed the stereospecific hydration of (Z)-2-butenoyl-FosACP2 (14) to (3S)-3-hydroxybutyryl-FosACP2 (8). Although incubation of FosDH2 with (3S)-3-hydroxybutyryl-FosACP2 (8) did not result in detectable accumulation of (Z)-2-butenoyl-FosACP2 (14), FosDH2 catalyzed the slow exchange of the 3-hydroxy group of 8 with [18O]-water. FosDH2 unexpectedly could also support the stereospecific interconversion of (3R)-3-hydroxybutyryl-FosACP2 (7) and (E)-2-butenoyl-FosACP2 (13).