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 Δ5, such as testosterone, as its sole source of carbon. In mammals, transfer of a double bond at Δ5 to Δ4 is catalyzed by 3-β-hydroxy-Δ5-steroid dehydrogenase at the same time as the dehydroxylation of 3-β-hydroxyl group to ketone group, while in C. testosteroni and P. putida, Δ5,3-ketosteroid isomerase just transfers a double bond at Δ5 of 3-ketosteroid to Δ4.
A Δ5-3-ketosteroid isomerase-disrupted mutant of strain TA441 can grow on dehydroepiandrosterone, which has a double bond at Δ5, but cannot grow on epiandrosterone, which lacks a double bond at Δ5, indicating that C. testosteroni KSI is responsible for transfer of the double bond from Δ5 to Δ4 and transfer of the double bond by hydrogenation at Δ5 and following dehydrogenation at Δ4 is not possible.
Read more about this topic: Steroid Delta-isomerase
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