TYR-LYS BASED CATALYSIS : THE FOUNDATION FOR STRUCTURAL HOMOLOGY AMONG SHORT-CHAIN DEHYDROGENASE/REDUCTASES. D. Ghosh*, W. L. Duax, V. Pletnev, B. M. Burkhart, N. Li, Hauptman-Woodward Institute, 73 High St., Buffalo, NY 14203 USA, *also Roswell Park Cancer Inst., Buffalo, NY.

Short chain dehydrogenase/reductases (SDR) constitute a new class of dinucleotide-linked oxidoreductases that use a wide variety of substrates. We have determined the crystal structures of various forms of two members of the SDR family: the bacterial 3[[alpha]],20ß-hydroxysteroid dehydrogenase and the human type 1 estrogenic 17ß-hydroxysteroid dehydrogenase. Structures of five other SDR's have been determined elsewhere. These are : rat liver dihydropteridine reductase, oilseed rape and bacterial enoyl acyl carrier protein reductase, mouse lung carbonyl reductase and bacterial 7[[alpha]]-hydroxysteroid dehydrogenase. Despite a low intra-family amino acid sequence identity (15-35%), these enzymes are characterized not only by strikingly similar subunit tertiary structures that include a dinucleotide-binding fold, but also an analogous quaternary association and a strictly conserved Tyr-Lys pair at the catalytic end of the active site. The conserved catalytic residues form a Tyr-Lys-Ser triad in many of the SDR's, owing to a semi-conserved Ser residue. A mechanism in which the Tyr-Lys pair acts as a general acid and an electrophile to the substrate carbonyl has been proposed.

All seven enzymes of the SDR family form homo-dimeric or -tetrameric crystal structures. While the C-terminal residues that provide substrate-specific interactions are situated at the outer periphery of the oligomer, the Tyr-Lys pair is located on the helix that forms the four-helix bundle dimer interface - the so-called Q-axis dimer, the most conserved interface in the family. The formation of the core of the oligomer, a consequence of tertiary structures of subunits and their quaternary association, is thus influenced by the presence of the Tyr-Lys pair. The dinucletide-binding fold may have little effect on the subunit association, since other well-known dinucleotide-binding enzymes, such as long-chain dehydrogenases, have different quaternary structures. In SDR's, unlike the long-chain dehydrogenases, the catalytic end of the active site borders the interfacial structure elements, such as a helices and ß sheets, near the inner core of the functional oligomers. This suggests that the evolution of the SDR functional oligomer may be driven by the requirements of a Tyr-Lys based catalytic mechanism. Supported by Grant No. DK26546.