In enzymology, a taxadiene 5alpha-hydroxylase () is an enzyme that catalyzes the chemical reaction
The 3 substrates of this enzyme are taxa-4,11-diene, an electron acceptor AH<sub>2</sub>, and O<sub>2</sub>, whereas its 3 products are taxa-4(20),11-dien-5alpha-ol, 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 taxa-4,11-diene,hydrogen-donor:oxygen oxidoreductase (5alpha-hydroxylating). This enzyme participates in diterpenoid biosynthesis.