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In enzymology, a steroid Delta-isomerase is an enzyme that catalyzes the chemical reaction


 * a 3-oxo-Delta5-steroid $$\rightleftharpoons$$ a 3-oxo-Delta4-steroid

Hence, this enzyme has one substrate, 3-oxo-Delta5-steroid, and one product, 3-oxo-Delta4-steroid.

Introduction
This enzyme belongs to the family of isomerases, specifically those intramolecular oxidoreductases transposing C=C bonds. The systematic name of this enzyme class is 3-oxosteroid Delta5-Delta4-isomerase. Other names in common use include ketosteroid isomerase (KSI), hydroxysteroid isomerase, steroid isomerase, Delta5-ketosteroid isomerase, Delta5(or Delta4)-3-keto steroid isomerase, Delta5-steroid isomerase, 3-oxosteroid isomerase, Delta5-3-keto steroid isomerase, and Delta5-3-oxosteroid isomerase.

KSI has been studied extensively from the bacteria Comamonas testosteroni (TI), formerly referred to as Pseudomonas testosteroni, and Pseudomonas putida (PI). Mammalian KSI has been studied from bovine adrenal cortex and rat liver. This enzyme participates in c21-steroid hormone metabolism and androgen and estrogen metabolism. An example substrate is delta-5-androstene-3,17-dione, which KSI converts to delta-4-androstene-3,17-dione. The above reaction in the absence of enzyme takes 7 weeks to complete in aqueous solution. KSI performs this reaction on an order of 1011 times faster, ranking it among the most proficient enzymes known. Bacterial KSI also serves as a model protein for studying enzyme catalysis and protein folding.

Structural Studies
KSI exists as a homodimer with two identical halves. The interface between the two monomers is narrow and well defined, consisting of neutral or apolar amino acids, suggesting the hydrophobic interaction is important for dimerization. Results show that the dimerization is essential to function. The active site is highly apolar and folds around the substrate in a manner similar to other enzymes with hydrophobic substrates, suggesting this fold is characteristic for binding hydrophobic substrates.

As of late 2007, 25 structures have been solved for this class of enzymes, with PDB accession codes, , , , , , , , , , , , , , , , , , , , , , , , and.

Mechanism
The conversion of a Delta-5 steroid to a conjugated system Delta-4 steroid begins with Asp-38 abstracting a hydrogen at the 4 position to form an enolate. Asp-38 then transfers the proton proton to the 6 position to give the product. There have been conflicting results on the ionization state of the intermediate, whether it exists as the enolate or enol. Pollack uses a thermodynamic argument to suggest the intermediate exists as the enolate. Also of contention is the nature of the hydrogen bonding network stabilizing the reaction intermediate, whether Tyr-14 and Asp-99 both form hydrogen bonds directly to the O-3 or Asp-99 hydrogen bonds to Tyr-14 which hydrogen bonds to O-3.

Biological Function
KSI occurs in animal tissues concerned with steroid hormone biosynthesis, such as the adrenal, testis, and ovary. KSI in Comamomas testosteroni is used in the degradation pathway of steroids, allowing this bacteria to utilize steroids containing a double bond at delta-5, such as testosterone, as its sole source of carbon.

Model Enzyme
KSI has been used as a model system to test different theories to explain how enzymes achieve their catalytic efficiency. Low-barrier hydrogen bonds and unusual pKa values for the catalytic residues have been proposed as the basis for the fast action of KSI. Another proposal explaining enzyme catalysis tested through KSI is the geometrical complementarity of the active site to the transition state, which proposes the active site electrostatics is complementary to the substrate transition state. KSI has also been a model system for studying protein folding. Kim et al. studied the effect of folding and tertiary structure on the function of KSI.