17Beta-hydroxysteroid dehydrogenase from Cochliobolus lunatus: model structure and substrate specificity.

A homology-built structural model of 17beta-hydroxysteroid dehydrogenase from the fungus Cochliobolus lunatus, a member of the short-chain dehydrogenase/reductase family, was worked out using the known three-dimensional structure of trihydroxynaphthalene reductase (EC 1.3.1.50) from Magnaporthe grisea as a template. Due to 61% sequence identity, the model also revealed a similar backbone trace. On the basis of qualitative thin-layer chromatography and comparative kinetic tests of the activity toward various potential steroid substrates, we conclude that androgens are more efficiently converted than estrogens. Their specific oxidoreduction predominantly occurs at the C17 position while no significant conversion at C3 and C20 was determined. Additionally, a thousand times effective inhibition by 5-methyl-(1,2,4)-triazolo[3,4-b]benzothiazole and no activity toward 2,3-dihydro-2,5-dihydroxy-4H-benzopyran-4-one indicate distinct specificies of 17beta-hydroxysteroid dehydrogenase from the fungus C. lunatus and trihydroxynaphthalene reductase. The results of the analysis of progress curve measurements for the forward and backward reactions are consistent with the Theorell-Chance reaction mechanism also predicted from the structural model. In accordance with these results, 4-androstene-3,17-dione was docked into the enzyme active site using molecular modeling and dynamics calculations.