Three-dimensional structure of holo 3α,20β-hydroxy steroid dehydrogenase: A member of a short-chain dehydrogenase family

The x-ray structure of a short-chain dehydrogenase, the bacterial holo 3α,20β-hydroxysteroid dehydrogenase (EC 1.1.1.53), is described at 2.6 Å resolution. This enzyme is active as a tetramer and crystallizes with four identical subunits in the asymmetric unit. It has the α/β fold characteristic of the dinucleotide binding region. The fold of the rest of the subunit, the quarternary structure, and the nature of the cofactor-enzyme interactions are, however, significantly different from those observed in the long-chain dehydrogenases. The architecture of the postulated active site is consistent with the observed stereospecificity of the enzyme and the fact that the tetramer is the active form. There is only one cofactor and one substrate-binding site per subunit; the specificity for both 3α- and 20β-ends of the steroid results from the binding of the steroid in two orientations near the same cofactor at the same catalytic site. (.