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 Δ5-Δ4-isomerase. Other names in common use include ketosteroid isomerase (KSI), hydroxysteroid isomerase, steroid isomerase, Δ5-ketosteroid isomerase, Δ5(or Δ4)-3-keto steroid isomerase, Δ5-steroid isomerase, 3-oxosteroid isomerase, Δ5-3-keto steroid isomerase, and Δ5-3-oxosteroid isomerase.
KSI has been studied extensively from the bacteria Comamonas testosteroni (TI), formerly referred to as Pseudomonas testosteroni, and Pseudomonas putida (PI). The enzymes from these two sources are 34% homologous, and structural studies have shown that the placement of the catalytic groups in the active sites is virtually identical. 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 Δ5-androstene-3,17-dione, which KSI converts to Δ4-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.
Read more about this topic: Steroid Delta-isomerase
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