. ▁M amm al ian ▁K SI ▁has ▁been ▁studied ▁from ▁b ov ine ▁ad ren al ▁cor tex ▁and ▁rat ▁li ver . ▁This ▁en zym e ▁particip ates ▁in ▁c 2 1 - st ero id ▁h orm one ▁met abol ism ▁and ▁and ro gen ▁and ▁est ro gen ▁met abol ism . ▁An ▁example ▁substr ate ▁is ▁ Δ 5 - andro st ene - 3 , 1 7 - d ione , ▁which ▁K SI ▁converts ▁to ▁ Δ 4 - andro st ene - 3 , 1 7 - d ione . ▁The ▁above ▁reaction ▁in ▁the ▁absence ▁of ▁en zym e ▁takes ▁ 7 ▁weeks ▁to ▁complete ▁in ▁a que ous ▁solution . ▁K SI ▁performs ▁this ▁reaction ▁on ▁an ▁order ▁of ▁ 1 0 1 1 ▁times ▁faster , ▁ranking ▁it ▁among ▁the ▁most ▁prof ic ient ▁en zym es ▁known . ▁B acter ial ▁K SI ▁also ▁serves ▁as ▁a ▁model ▁protein ▁for ▁studying ▁en zym e ▁catal ysis ▁and ▁protein ▁fol ding . ▁ ▁Struct ural ▁studies ▁ ▁K SI ▁exists ▁as ▁a ▁hom od imer ▁with ▁two ▁identical ▁hal ves . ▁The ▁interface ▁between ▁the ▁two ▁mon om ers ▁is ▁narrow ▁and ▁well ▁defined , ▁consisting ▁of ▁neutral ▁or ▁ap olar ▁am ino ▁ac ids , ▁suggesting ▁the ▁hydro ph ob ic ▁interaction ▁is ▁important ▁for ▁dim er ization . ▁Results ▁show ▁that ▁the ▁dim er ization ▁is ▁essential ▁to ▁function . ▁The ▁active ▁site ▁is ▁highly ▁ap olar ▁and ▁fol ds