Mechanistic Insights from the Three-Dimensional Structure of 3-Oxo-Δ5-steroid Isomerase

3-Oxo-Δ5-steroid isomerase (KSI) catalyzes the isomerization of β,γ-unsaturated 3-oxosteroids to their conjugated isomers through the formation of an intermediate dienolate. The three-dimensional structure of the enzyme from Pseudomonas testosteroni was solved by multidimensional heteronuclear magnetic resonance spectroscopy. This protein, a 28-kDa symmetric dimer, exhibits a three-dimensional fold with the two independently folded monomers packed together via extensive hydrophobic and electrostatic interactions. The previously identified catalytically important residues Tyr-14 (general acid) and Asp-38 (general base) are located near the bottom of a deep hydrophobic cavity and are positioned in a manner consistent with previous mechanistic hypotheses. The structure also revealed the presence of an unexpected acid group (Asp-99) located in the active site adjacent to Tyr-14. Mutagenesis and kinetic studies show that Asp-99 has an anomalously high pKa (>9), which allows it to contribute to catalysis by donating a hydrogen bond to the intermediate and to the transition states. In support of this hypothesis, effects on the kinetic parameters of the mutations Y14F and D99A are additive in the Y14F/D99A mutant.