Structural and stereochemical analysis of a modular polyketide synthase ketoreductase domain required for the generation of a cis-alkene

The formation of an activated cis-3-cyclohexylpropenoic acid by Plm1, the first extension module of the phoslactomycin polyketide synthase, is proposed to occur through an L-3-hydroxyacyl-intermediate as a result of ketoreduction by an A-type ketoreductase (KR). Here, we demonstrate that the KR domain of Plm1 (PlmKR1) catalyzes the formation of an L-3-hydroxyacyl product. The crystal structure of PlmKR1 revealed a well-ordered active site with a nearby Trp residue characteristic of A-type KRs. Structural comparison of PlmKR1 with B-type KRs that produce D-3-hydroxyacyl intermediates revealed significant differences. The active site of cofactor-bound A-type KRs is in a catalysis-ready state, whereas cofactor-bound B-type KRs are in a precatalytic state. Furthermore, the closed lid loop in substrate-bound A-type KRs restricts active site access from all but one direction, which is proposed to control the stereochemistry of ketoreduction.

Functional characterization of an NADPH dependent 2-alkyl-3-ketoalkanoic acid reductase involved in olefin biosynthesis in Stenotrophomonas maltophilia

OleD is shown to play a key reductive role in the generation of alkenes (olefins) from acyl thioesters in Stenotrophomonas maltophilia. The gene coding for OleD clusters with three other genes, oleABC, and all appear to be transcribed in the same direction as an operon in various olefin producing bacteria. In this study, a series of substrates varying in chain length and stereochemistry were synthesized and used to elucidate the functional role and substrate specificity of OleD. We demonstrated that OleD, which is an NADP(H) dependent reductase, is a homodimer which catalyzes the reversible stereospecific reduction of 2-alkyl-3-ketoalkanoic acids. Maximal catalytic efficiency was observed with syn-2-decyl-3-hydroxytetradecanoic acid, with a k(cat)/K(m) 5- and 8-fold higher than for syn-2-octyl-3-hydroxydodecanoic acid and syn-2-hexyl-3-hydroxydecanoic acid, respectively. OleD activity was not observed with syn-2-butyl-3-hydroxyoctanoic acid and compounds lacking a 2-alkyl group such as 3-ketodecanoic and 3-hydroxydecanoic acids, suggesting the necessity of the 2-alkyl chain for enzyme recognition and catalysis. Using diastereomeric pairs of substrates and 4 enantiopure isomers of 2-hexyl-3-hydroxydecanoic acid of known stereochemistry, OleD was shown to have a marked stereochemical preference for the (2R,3S)-isomer. Finally, experiments involving OleA and OleD demonstrate the first 3 steps and stereochemical course in olefin formation from acyl thioesters; condensation to form a 2-alkyl-3-ketoacyl thioester, subsequent thioester hydrolysis, and ketone reduction.

Antimalarial activity of natural and synthetic prodiginines

Prodiginines are a family of linear and cyclic oligopyrrole red-pigmented compounds. Herein we describe the in vitro antimalarial activity of four natural (IC(50) = 1.7-8.0 nM) and three sets of synthetic prodiginines against Plasmodium falciparum. Set 1 compounds replaced the terminal nonalkylated pyrrole ring of natural prodiginines and had diminished activity (IC(50) > 2920 nM). Set 2 and set 3 prodiginines were monosubstituted or disubstituted at either the 3 or 5 position of the right-hand terminal pyrrole, respectively. Potent in vitro activity (IC(50) = 0.9-16.0 nM) was observed using alkyl or aryl substituents. Metacycloprodiginine and more potent synthetic analogues were evaluated in a P. yoelii murine patent infection using oral administration. Each analogue reduced parasitemia by more than 90% after 25 (mg/kg)/day dosing and in some cases provided a cure. The most favorable profile was 92% parasite reduction at 5 (mg/kg)/day, and 100% reduction at 25 (mg/kg)/day without any evident weight loses or clinical overt toxicity.