In enzymology, a steroid 9alpha-monooxygenase () is an enzyme that catalyzes the chemical reaction
The 3 substrates of this enzyme are pregna-4,9(11)-diene-3,20-dione, an electron acceptor AH<sub>2</sub>, and O<sub>2</sub>, whereas its 3 products are 9,11alpha-epoxypregn-4-ene-3,20-dione, the reduction product A, and H<sub>2</sub>O.
This enzyme belongs to the family of oxidoreductases, specifically those acting on paired donors, with O2 as oxidant and incorporation or reduction of oxygen. The oxygen incorporated need not be derive from O miscellaneous. The systematic name of this enzyme class is steroid,hydrogen-donor:oxygen oxidoreductase (9-epoxidizing). This enzyme is also called steroid 9alpha-hydroxylase. It has 2 cofactors: FMN, and Iron-sulfur.