Structure-function relationships of SDR hydroxysteroid dehydrogenases.

Abstract

Since its discovery and subsequent establishment as a superfamily, rapid progress on the knowledge of short chain dehydrogenases/reductases (SDR) has been achieved (Jornvall et al., 1981; Persson et al., 1991; Jornvall et al., 1995). Based on conserved sequence characteristics, over 100 different enzymes in the databases now belong to the SDR family. The conserved residues are restricted to certain sequence segments and include the coenzyme binding site and the catalytic center (Krook et al., 1990; Persson et al., 1991; Krozowski, 1994; Ghosh et al., 1994; Jornvall et al., 1995). Despite a residue identity level of 20–30% between different SDR members, the 3D structures thus far analyzed (Ghosh et al., 1991, 1994; Varughese et al., 1992; Ghosh et al., 1995; Tanaka et al., 1996a,b; Benach et al., 1996) reveal a highly similar architecture with a one-domain α/β folding pattern. In particular, most of the conserved residues are found at positions 10–40 (comprising strands βA and βB, helix αB and the joining turns), 80–90 (strand βD), 110 (in helix αE), 130–180 (strands βE and βF, and helix αF) and 183–184 (at the end of strand βF) in the 3β/17β-hydroxysteroid dehydrogenase numbering system. Table 1 lists these segments with residues conserved in more than 80% of all SDR structures and relates them to the secondary structure elements. Sequence comparisons, chemical modifications, site-directed mutageneses and crystallographic analyses reveal that most of these parts form the coenzyme binding and catalytic sites of SDR proteins, thus establishing the secondary structure elements βA to αD as parts of the coenzyme binding site with a typical “Rossmann fold” and a highly conserved Y-X-X-X-K segment (residues 150–154, helix αF) as part of the catalytic center (Persson et al., 1991; Ghosh et al., 1994; Jornvall et al., 1995).