title: Structural and functional analysis of C2-type ketoreductases from modular polyketide synthases.
abstract: The process by which α-stereocenters of polyketide intermediates are set by modular polyketide synthases (PKSs) when condensation is not immediately followed by reduction is mysterious. However, the reductase-incompetent ketoreductase (KR) from the third module of 6-deoxyerythronolide B synthase has been proposed to operate as a racemase, aiding in the epimerization process that reverses the orientation of the α-methyl group of the polyketide intermediate generated by the ketosynthase to the configuration observed in the 6-deoxyerythronolide B final product. To learn more about the epimerization process, the structure of the C2-type KR from the third module of the pikromycin synthase, analogous to the KR from the third module of 6-deoxyerythronolide B synthase, was determined to 1.88 Å resolution. This first structural analysis of this KR-type reveals differences from reductase-competent KRs such as that the site NADPH binds to reductase-competent KRs is occluded by side chains and the putative catalytic tyrosine possesses more degrees of freedom. The active-site geometry may enable C2-type KRs to align the thioester and β-keto groups of a polyketide intermediate to reduce the pK(a) of the α-proton and accelerate its abstraction. Results from in vivo assays of engineered PKSs support that C2-type KRs cooperate with epimer-specific ketosynthases to set the configurations of substituent-bearing α-carbons.
authors.affiliation: Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, TX 78712, USA.
authors.forename: Jianting
authors.initials: J
authors.lastname: Zheng
authors.forename: Adrian T
authors.initials: AT
authors.lastname: Keatinge-Clay
mesh_terms: D000429:Alcohol Oxidoreductases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D000595:Amino Acid Sequence; D020134:Catalytic Domain; D003460:Crystallization; D008958:Models, Molecular; D008969:Molecular Sequence Data; D048630:Polyketide Synthases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D011487:Protein Conformation; D017386:Sequence Homology, Amino Acid; D013237:Stereoisomerism; D013306:Streptomycetaceae / Q000201:enzymology
chemical_list: D048630:Polyketide Synthases; D000429:Alcohol Oxidoreductases