The substrate-binding domain found in Cholesterol oxidase is composed of an eight-stranded mixed beta-pleated sheet and six alpha-helices. This domain is positioned over the isoalloxazine ring system of the FAD cofactor bound by FAD_binding_4 (PF:PF01565) and forms the roof of the active site cavity, allowing for catalysis of oxidation and isomerisation of cholesterol to cholest-4-en-3-one [1].

1: Oxygen access to the active site of cholesterol oxidase through a narrow channel is gated by an Arg-Glu pair. Coulombe R, Yue KQ, Ghisla S, Vrielink A; J Biol Chem. 2001;276:30435-30441. PMID:11397813