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Catalytic mechanism of KSI. KSI catalyzes the isomerization reaction; C4 proton of the substrate 5-androstene-3,17-dione is transferred to the enzyme catalytic base Asp38 to generate the dienolate intermediate, and the same proton is then transferred to C6 to generate 4-androstene-3,17-dione. Both Tyr14 and Asp99 stabilize the intermediate by forming a hydrogen bond with the C3-O of the steroid.
