Crystal structure of cholesterol oxidase from Brevibacterium sterolicum refined at 1.8 A resolution

Cholesterol oxidase (3 beta-hydroxysteroid oxidase, EC 1.1.3.6) is an FAD-dependent enzyme that carries out the oxidation and isomerization of steroids with a trans A : B ring junction. The crystal structure of the enzyme from Brevibacterium sterolicum has been determined using the method of isomorphous replacement and refined to 1.8 A resolution. The refined model includes 492 amino acid residues, the FAD prosthetic group and 453 solvent molecules. The crystallographic R-factor is 15.3% for all reflections between 10.0 A and 1.8 A resolution. The structure is made up of two domains: an FAD-binding domain and a steroid-binding domain. The FAD-binding domain consists of three non-continuous segments of sequence, including both the N terminus and the C terminus, and is made up of a six-stranded beta-sheet sandwiched between a four-stranded beta-sheet and three alpha-helices. The overall topology of this domain is very similar to other FAD-binding proteins. The steroid-binding domain consists of two non-continuous segments of sequence and contains a six-stranded antiparallel beta-sheet forming the "roof" of the active-site cavity. This large beta-sheet structure and the connections between the strands are topologically similar to the substrate-binding domain of the FAD-binding protein para-hydroxybenzoate hydroxylase. The active site lies at the interface of the two domains, in a large cavity filled with a well-ordered lattice of 13 solvent molecules. The flavin ring system of FAD lies on the "floor" of the cavity with N-5 of the ring system exposed. The ring system is twisted from a planar conformation by an angle of approximately 17 degrees, allowing hydrogen-bond interactions between the protein and the pyrimidine ring of FAD. The amino acid residues that line the active site are predominantly hydrophobic along the side of the cavity nearest the benzene ring of the flavin ring system, and are more hydrophilic on the opposite side near the pyrimidine ring. The cavity is buried inside the protein molecule, but three hydrophobic loops at the surface of the molecule show relatively high temperature factors, suggesting a flexible region that may form a possible path by which the substrate could enter the cavity. The active-site cavity contains one charged residue, Glu361, for which the side-chain electron density suggests a high degree of mobility for the side-chain. This residue is appropriately positioned to act as the proton acceptor in the proposed mechanism for the isomerization step.