   1.3.99.5 | to 1.6 A resolution, structure reveals three conserved residues, Y319, Y466, and S468 in a pocket near the isoalloxazine ring system of the FAD co-factor. Crystal structure with bound product 4-androstene-3,17-dione shows that residue S468 is in a position in which it can serve as the base abstracting the 4beta-proton from the C4 atom of the substrate. S468 is assisted by Y319, which possibly is involved in shuttling the proton to the solvent. Y466 is at hydrogen bonding distance to the C3 oxygen atom of the substrate and can stabilize the keto-enol intermediate occurring during the reaction. The FAD N5 atom is in a position to be able to abstract the 5alpha-hydrogen of the substrate as a hydride ion |
