The X-ray structure of Brassica napus beta-keto acyl carrier protein reductase and its implications for substrate binding and catalysis

Background: beta-Keto acyl carrier protein reductase (BKR) catalyzes the py ridine-nucleotide-dependent reduction of a 3-oxoacyl form of acyl carrier p rotein (ACP), the first reductive step in de novo fatty acid biosynthesis a nd a reaction often performed in polyketide biosynthesis, The Brassica napu s BKR enzyme is NADPH-dependent and forms part of a dissociable type II fat ty acid synthetase (FAS). Significant sequence similarity is observed with enoyl acyl carrier protein reductase (ENR), the other reductase of FAS, and the short-chain alcohol dehydrogenase (SDR) family. Results: The first crystal structure of BKR has been determined at 2.3 Angs trom resolution in a binary complex with an NADP(+) cofactor, The structure reveals a homotetramer in which each subunit has a classical dinucleotide- binding fold, A triad of Ser154, Tyr167 and Lys171 residues is found at the active site, characteristic of the SDR family, Overall BKR has a very simi lar structure to ENR with good superimposition of catalytically important g roups. Modelling of the substrate into the active site of BKR indicates the need for conformational changes in the enzyme. Conclusions: A catalytic mechanism can be proposed involving the conserved triad, Helix alpha 6 must shift its position to permit substrate binding to BKR and might act as a flexible lid on the active site. The similarities i n fold, mechanism and substrate binding between BKR, which catalyzes a carb on-oxygen double-bond reduction, and ENR, the carbon-carbon double-bond oxi doreductase in FAS, suggest a close evolutionary link during the developmen t of the fatty acid biosynthetic pathway.