Aldo-keto reductases (AKR) are monomeric oxidoreductases that retain a conserved catalytic tetrad (Tyr, Lys, Asp, and His) at their active sites in which the Tyr acts as a general acid-base catalyst. In rat liver 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD, AKR1C9), a well-characterized AKR, the catalytic tyrosine is Tyr 55. This enzyme displays a high catalytic efficiency for a common AKR substrate 9,10-phenanthrenequinone (9,10-PQ). Surprisingly, Y55F and Y55S mutants of 3alpha-HSD reduced 9,10-PQ with high kcat values. This is the first report whereby the invariant catalytic tyrosine of an AKR has been mutated with retention of kcat values similar to wild-type enzyme. The Y55F and Y55S mutants displayed narrow substrate specificity and reduced select aromatic quinones and alpha-dicarbonyls. kcat versus pH profiles for steroid oxidoreduction catalyzed by wild-type 3alpha-HSD exhibited a single ionizable group with a pK= 7.0-7.5, which has been assigned to Tyr 55. This group was not evident in the kcat versus pH profiles for 9, 10-PQ reduction catalyzed by either wild-type or the Tyr 55 mutant enzymes, indicating that the protonation state of Tyr 55 is unimportant for 9,10-PQ turnover. Instead, wild-type and the active-site mutants Y55F, Y55S, H117A, D50N, K84R, and K84M showed the presence of a new titratable group with a pKb = 8.3-9.9. Thus, the group being titrated is not part of the tetrad. All the mutants decreased kcat/Km considerably more than they decreased kcat. Thus, the K84R mutant demonstrated a 30-fold decrease in the pH-independent value of kcat but 2200-fold decrease in the pH-independent value of kcat/Km. This suggests that all the tetrad residues influence quinone binding and that Lys 84 plays a dominant role in maintaining proper substrate orientation. Using wild-type enzyme, the energy of activation (Ea) for 9,10-PQ reduction was approximately 11 kcal/mol less than steroid oxidoreduction. The Ea for 9,10-PQ reduction was unchanged in the Tyr 55 mutants, suggesting that the reaction proceeds through the same low-energy barrier in the wild-type enzyme and these mutants. The retention of quinone reductase activity in this AKR in the absence of Tyr 55 with kcat versus pH rate profiles and activation energies identical to wild-type enzyme suggests that quinone reduction occurs via a mechanism that differs from 3-ketosteroid reduction. In this mechanism, the electron donor (NADPH) and acceptor (o-quinone) are bound in close proximity, which permits hydride transfer without formal protonation of the acceptor carbonyl by Tyr 55. This represents a rare example where one enzyme can catalyze the same chemical reaction (carbonyl reduction) by either acid catalysis or by a propinquity effect and where these two mechanisms can be discriminated by site-directed mutagenesis.