Delta5-3 ketosteroid isomerase catalyses the isomerisation of delta5-3-ketosteroid to delta4-3-ketosteroid by intramolecular transfer of the C4beta proton to the C6beta position via a dienolic intermediate. The mechanism proceeds at a diffusion controlled limit within an environment shielded from solvent.
In mammals, this reaction is an essential step in the biosynthesis of steroid hormones and is catalysed by a membrane-bound complex. In bacteria, the enzyme appears to play a metabolic role in the degrading steroid substrates.
Defined by 4 residues: TYR:A-16ASP:A-40ASP:A-100ASP:A-103