COMMON THEMES IN REDOX CHEMISTRY EMERGE FROM THE X-RAY STRUCTURE OF OILSEED RAPE (BRASSICA-NAPUS) ENOYL ACYL CARRIER PROTEIN REDUCTASE

Background: Enoyl acyl carrier protein reductase (ENR) catalyzes the N AD(P)H-dependent reduction of trans-Delta 2-enoyl acyl carrier protein , an essential step in de novo fatty acid biosynthesis. Plants contain both NADH-dependent and separate NADPH-dependent ENR enzymes which fo rm part of the dissociable type II fatty acid synthetase. Highly eleva ted levels of the NADH-dependent enzyme are found during lipid deposit ion in maturing seeds of oilseed rape (Brassica napus). Results: The c rystal structure of an ENR-NAD binary complex has been determined at 1 .9 Angstrom resolution and consists of a homotetramer in which each su bunit forms a single domain comprising a seven-stranded parallel beta sheet flanked by seven alpha helices. The subunit has a topology highl y reminiscent of a dinucleotide-binding fold. The active site has been located by difference Fourier analysis of data from crystals equilibr ated in NADH. Conclusions: The structure of ENR shows a striking simil arity with the epimerases and short-chain alcohol dehydrogenases, in p articular, 3 alpha,20 beta-hydroxysteroid dehydrogenase (HSD). The sim ilarity with HSD extends to the conservation of a catalytically import ant lysine that stabilizes the transition state and to the use of a ty rosine as a base - with subtle modifications arising from differing re quirements of the reduction chemistry.