Structure of 3(17)α-hydroxysteroid dehydrogenase (AKR1C21) holoenzyme from an orthorhombic crystal form: an insight into the bifunctionality of the enzyme

Mouse 3(17)α-hydroxysteroid dehydrogenase (AKR1C21) is a bifunctional enzyme that catalyses the oxidoreduction of the 3- and 17-hydroxy/keto groups of steroid substrates such as oestrogens, androgens and neurosteroids. The structure of the AKR1C21–NADPH binary complex was determined from an orthorhombic crystal belonging to space group P2₁2₁2₁ at a resolution of 1.8 Å. In order to identify the factors responsible for the bifunctionality of AKR1C21, three steroid substrates including a 17-keto steroid, a 3-keto steroid and a 3α-hydroxysteroid were docked into the substrate-binding cavity. Models of the enzyme–coenzyme–substrate complexes suggest that Lys31, Gly225 and Gly226 are important for ligand recognition and orientation in the active site.