CRYSTAL-STRUCTURE OF THE TERNARY COMPLEX OF 1,3,8-TRIHYDROXYNAPHTHALENE REDUCTASE FROM MAGNAPORTHE-GRISEA WITH NADPH AND AN ACTIVE-SITE INHIBITOR

Background: The enzyme 1,3,8-tribydroxynaphthalene reductase (THNR) ca talyzes an essential reaction in the biosynthesis of melanin, a black pigment crucial for the pathogenesis of the rice blast fungus, Magnapo rthe grisea. The enzyme is the biochemical target of several commercia lly important fungicides which are used to prevent blast disease in ri ce plants. We have determined the structure of the ternary complex of THNR with bound NADPH and a fungicide, tricyclazole. Results: Crystall ographic analysis showed four identical subunits of THNR to form a tet ramer with 222 symmetry. The enzyme subunit consists of a single domai n comprising a seven-stranded beta sheet flanked by eight alpha helice s; the subunit contains a dinucleotide-binding fold which binds the co enzyme, NADPH. Tricyclazole, an inhibitor of the enzyme, binds at the active site in the vicinity of the NADPH nicotinamide ring. The active site contains a Ser-Tyr-Lys triad which is proposed to participate in catalysis. Coenzyme specificity is partly conferred by the interactio n of a single basic residue, Arg39, with the 2' phosphate group of NAD PH. Conclusions: The structural model reveals THNR to belong to the fa mily of short chain dehydrogenases. Despite the diversity of the chemi cal reactions catalyzed by this family of enzymes, their tertiary stru ctures are very similar. In particular THNR has many amino acid sequen ce identities, and thus most probably high structural similarities, to enzymes involved in fungal aflatoxin synthesis. The structure of THNR in complex with NADPH and tricyclazole provides new insights into the structural basis of inhibitor binding. This new information may aid i n the design of new inhibitors for rice crop protection.