Structure and enzymology of Δ5-3-ketosteroid isomerase

The three-dimensional structures of Delta (5)-3-ketosteroid isomerases from two different bacterial species have been determined. The structures reveal an unusually apolar active site, in which each of several competitive inhibitors of the enzyme are held by two hydrogen bonds with the general acids Tyr14 and Asp99, and by hydrophobic interactions. The hydrogen bond between the Tyr14 hydroxyl and the C3 oxyanion of a transition-state analog is a low-barrier hydrogen bond, as indicated by a highly deshielded nuclear magnetic resonance. Structural and other biochemical studies have enabled the proposal of a detailed catalytic mechanism for Delta (5)-3-ketosteroid isomerase and provided a major thrust towards understanding the mechanism not only in chemical terms but also in energetics terms.