Serine 124 completes the Tyr, Lys and Ser triad responsible for the catalysis of human type 1 3β-hydroxysteroid dehydrogenase

Human 3β-hydroxysteroid dehydrogenase/isomerase (3β-HSD) is a key steroidogenic enzyme that catalyzes the first step in the conversion of circulating dehydroepiandrosterone (DHEA), pregnenolone or 17α-hydroxypregenolone to produce the appropriate, active steroid hormone(s): estradiol, testosterone, progesterone, aldosterone or cortisol respectively. Our mutagenesis studies have identified Tyr154 and Lys158 as catalytic residues for the 3β-HSD reaction. Our three-dimensional homology model of 3β-HSD shows that Tyr154 and Lys158 are oriented near the 3β-hydroxyl group of the bound substrate steroid, and predicts that Ser123 or Ser124 completes a Tyr-Lys-Ser catalytic triad that operates in many other dehydrogenases. The S123A and S124A mutants of human type 1 3β-hydroxysteroid dehydrogenase/isomerase (3β-HSD1) were created by PCR-based mutagenesis, expressed in insect cells using baculovirus and purified to homogeneity. The S124A mutant exhibits no 3β-HSD activity and has a K_m value (83·6 μM) for the isomerase substrate that is threefold greater than that of wild-type 1 isomerase. In contrast, S123A has substantial 3β-HSD activity (DHEA K_m=11·2 μM; k_cat=0·8 min^-1) and utilizes isomerase substrate, 5-androstene-3,17-dione, with a K_m value (27·6 μM) that is almost identical to wild-type. The K_m value (4· 3 μM) of S124A for NADH as an allosteric activator of isomerase is similar to that of the wild-type 1 enzyme, indicating that Ser124 is not involved in cofactor binding. S123A utilizes NAD as a cofactor for 3β-HSD and NADH as the activator for isomerase with K_m values that are similar to wild-type. The 3β-HSD activities of S123A and wild-type 3β-HSD increase by 2·7-fold when the pH is raised from 7·4 to the optimal pH 9·7, but S124A exhibits very low residual 3β-HSD activity that is pH-independent. These kinetic analyses strongly suggest that the Ser124 residue completes the catalytic triad for the 3β-HSD activity. Since there are 29 Ser residues in the primary structure of human 3β-HSD1, our homology model of the catalytic domain has been validated by this accurate prediction. A role for Ser124 in the binding of the isomerase substrate, which is the 3β-HSD product-steroid of the bifunctional enzyme protein, is also suggested. These observations further characterize the structure/function relationships of human 3β-HSD and bring us closer to the goal of selectively inhibiting the type 1 enzyme in placenta to control the timing of labor or in hormone-sensitive breast tumors to slow their growth.