Factorizing the role of a critical leucine residue in the binding of substrate to human 20alpha-hydroxysteroid dehydrogenase (AKR1C1): molecular modeling and kinetic studies of the Leu308Val mutant enzyme.

A comparison of the structures and kinetic properties of human 20alpha-hydroxysteroid dehydrogenase (AKR1C1) and its mutant enzymes (Leu308Val and Leu308Ala) indicates that Leu308 is a selectivity determinant for substrate binding. While the Leu308Val mutation improved the catalytic efficiency (k(cat)/K(m)) of AKR1C1 towards the two substrates 5alpha-pregnane-3alpha,20alpha-diol (PregA) and 5beta-pregnan-3alpha-ol-20-one (PregB), the Leu308Ala mutation rendered the enzyme inactive. In the docked model of PregA the conformation of the steroid molecule was similar to that of 20alpha-hydroxyprogesterone in the crystal structure of the AKR1C1 complex where the steroid did not interact with the catalytic residues Tyr55 and His117. In the case of PregB the steroid interacted with the catalytic residue His117 and formed close contacts with Leu308, suggesting that the binding mechanism of 3alpha-hydroxysteroids in the active site of AKR1C1 is different from that of 20alpha-hydroxysteroids.