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Crystallization and preliminary crystallographic study of 3 alpha, 20 beta-hydroxysteroid dehydrogenase from Streptomyces hydrogenans.

3 alpha, 20 beta-Hydroxysteroid dehydrogenase, an NADH-dependent oxidoreductase isolated from Streptomyces hydrogenans , is a tetramer containing four subunits each of Mr 25,000. The enzyme has been crystallized by the vapor diffusion technique using either phosphate or borate buffered ammonium sulfate (pH between 6.0 and 8.7) as the precipitant. The crystals are hexagonal bipyramids ; they have the symmetry of space group P6(4)22 (or P6(2)22), with unit cell dimensions a = 127.3 A, c = 112.2 A. Volume and density considerations imply that the crystallographic asymmetric unit contains two monomers, and therefore that the tetramer possesses a 2-fold axis of symmetry that is coincident with a crystallographic 2-fold symmetry element.