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The refined three-dimensional structure of 3 alpha,20 beta-hydroxysteroid dehydrogenase and possible roles of the residues conserved in short-chain dehydrogenases.

BACKGROUND
Bacterial 3 alpha,20 beta-hydroxysteroid dehydrogenase reversibly oxidizes the 3 alpha and 20 beta hydroxyl groups of steroids derived from androstanes and pregnanes. It was the first short-chain dehydrogenase to be studied by X-ray crystallography. The previous description of the structure of this enzyme, at 2.6 A resolution, did not permit unambiguous assignment of several important groups. We have further refined the structure of the complex of the enzyme with its cofactor, nicotinamide adenine dinucleotide (NAD), and solvent molecules, at the same resolution.


RESULTS
The asymmetric unit of the crystal contains four monomers, each with 253 amino acid residues, 38 water molecules, and 176 cofactor atoms belonging to four NAD molecules--one for each subunit. The positioning of the cofactor molecule has been modified from our previous model and is deeper in the catalytic cavity as observed for other members of both the long-chain and short-chain dehydrogenase families. The nicotinamide-ribose end of the cofactor has several possible conformations or is dynamically disordered.


CONCLUSIONS
The catalytic site contains residues Tyr152 and Lys156. These two amino acids are strictly conserved in the short-chain dehydrogenase superfamily. Modeling studies with a cortisone molecule in the catalytic site suggest that the Tyr152, Lys156 and Ser139 side chains promote electrophilic attack on the (C20-O) carbonyl oxygen atom, thus enabling the carbon atom to accept a hydride from the reduced cofactor.