Alanine scanning mutagenesis of the testosterone binding site of rat 3 alpha-hydroxysteroid dehydrogenase demonstrates contact residues influence the rate-determining step.

Aldo-keto reductase (AKR1C) isoforms can regulate ligand access to nuclear receptors by acting as hydroxysteroid dehydrogenases. The principles that govern steroid hormone binding and steroid turnover by these enzymes were analyzed using rat 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD, AKR1C9) as the protein model. Systematic alanine scanning mutagenesis was performed on the substrate-binding pocket as defined by the crystal structure of the 3alpha-HSD.NADP(+).testosterone ternary complex. T24, L54, F118, F129, T226, W227, N306, and Y310 were individually mutated to alanine, while catalytic residues Y55 and H117 were unaltered. The effects of these mutations on the ordered bi-bi mechanism were examined. No mutations changed the affinity for NADPH by more than 2-3-fold. Fluorescence titrations of the energy transfer band of the E.NADPH complex with competitive inhibitors testosterone and progesterone showed that the largest effect was a 23-fold decrease in the affinity for progesterone in the W227A mutant. By contrast, changes in the K(d) for testosterone were negligible. Examination of the k(cat)/K(m) data for these mutants indicated that, irrespective of steroid substrate, the bimolecular rate constant was more adversely affected when alanine replaced an aromatic hydrophobic residue. By far, the greatest effects were on k(cat) (decreases of more than 2 log units), suggesting that the rate-determining step was either altered or slowed significantly. Single- and multiple-turnover experiments for androsterone oxidation showed that while the wild-type enzyme demonstrated a k(lim) and burst kinetics consistent with slow product release, the W227A and F118A mutants eliminated this kinetic profile. Instead, single- and multiple-turnover experiments gave k(lim) and k(max) values identical with k(cat) values, respectively, indicating that chemistry was now rate-limiting overall. Thus, conserved residues within the steroid-binding pocket affect k(cat) more than K(d) by influencing the rate-determining step of steroid oxidation. These findings support the concept of enzyme catalysis in which the correct positioning of reactants is essential; otherwise, k(cat) will be limited by the chemical event.