Steroid delta-isomerase (KSI)

Steroid delta-isomerase (KSI) is an enzyme (EC 5.3.3.1) that catalyzes the isomerization of 3-oxo-Δ5-steroids to 3-oxo-Δ4-steroids, a key step in steroid metabolism[1][2]. In bacteria such as *Pseudomonas putida* and *Comamonas testosteroni*, it enables the breakdown of steroid molecules that can be used as carbon sources[2][4]. In mammals, a functionally similar activity is carried out as part of the steroidogenic pathway, but typically by a bifunctional enzyme, 3-beta-hydroxysteroid dehydrogenase/delta-5,4-isomerase. The enzyme works at a diffusion-limited rate and involves a mechanistic base (Asp-99) and multiple hydrophobic residues to position the steroid substrate for catalysis[2][3]. There is ongoing scientific interest in KSI as a model enzyme for studying protein dynamics, catalysis, and structural evolution, rather than as a direct therapeutic target. Caveats and limitations: - The term "Steroid delta-isomerase" most specifically refers to the bacterial enzyme (KSI), not the mammalian bifunctional 3β-HSD/Δ5,4-isomerase[2][6][7]. - There are no well-established clinical drugs specifically targeting this enzyme in humans; interacting drugs listed are experimental or have unknown physiological relevance[4]. - Direct safety concerns or biomarker use in humans have not been established.

Other names

5.3.3.1Delta(5)-3-ketosteroid isomeraseketosteroid isomerase (KSI)3-oxo-Δ5-steroid isomerase

Catalyzes the isomerization of steroid substrates by facilitating the shift of a double bond from Δ5 to Δ4 position in the steroid nucleus, enabling conversion from 3-oxo-Δ5-steroids to 3-oxo-Δ4-steroids[1][2][5]