|Cholesterol oxidase from Streptomyces sp.|
Cholesterol oxidase is a bifunctional oxidoreductase that catalyzes the oxidation and isomerization of cholesterol to yield 4-cholesten-3-one. This enzyme is found in a variety of bacteria, including both the harmless soil bacteria that use cholesterol as a source of carbon and in pathogenic bacteria such as Mycobacterium tuberculosis and Rhodococcus equi where the enzyme plays a role in the lysis of macrophages and leukocytes resulting in characteristic lesions found in infected immune-compromised patients such as those suffering from AIDS. The role of cholesterol oxidase in pathogenesis appears to be based on its ability to alter the physical structure of the lipid membrane by converting cholesterol, an important membrane component, into cholest-4-en-3-one. However, it is not yet clear exactly where this alteration fits into the pathway of infection.
Reaction catalyzed by cholesterol oxidase.
The first structure of type I ("non-covalent") cholesterol oxidase from Brevibacterium sterolicum was determined to 1.8 Å resolution in 1991 revealing two domains: a co-factor binding domain and a steroid-binding domain. The latter contains a hydrophobic cavity large enough to accommodate a steroid ring system which, in the absence of the substrate, contains a highly ordered lattice of water molecules.
Type I cholesterol oxidase from Brevibacterium Sterolicum. Vrielink et al., J Mol Biol 1991
The complex of the enzyme with a substrate, dehydroepiandosterone (DHA) was determined in 1993 under anaerobic conditions. Under thse conditions the enzyme was able to carry out a oxidative turnover on the steroid substrate, however, in the absence of oxygen the reduced enzyme could not be reoxidized. The resulting structure represents therefore the complex of a reduced enzyme with a steroid substate and provided important details of steroid binding and interactions with the protein. These studies led to the proosal that a histidine residue (His447) was important for oxidation and a glutamate residue (Glu361) was needed for the isomerization reaction. A mechanism for the catalytic steps of the enzyme was proposed using the histidine side chain as a general base to deprotonate the 3-beta hydroxl group of the steroid substrate via a conserved active site water molecule (Wat541).
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