In enzymology, a 4-hydroxymandelate synthase () is an enzyme that catalyzes the chemical reaction 4-hydroxyphenylpyruvate + O2 4-hydroxymandelate + CO2 Thus, the two substrates of this enzyme are 4-hydroxyphenylpyruvate and oxygen, whereas its two products are 4-hydroxymandelate and carbon dioxide. This enzyme belongs to the family of oxidoreductases, specifically those acting on single donors with O2 as oxidant and incorporation of two atoms of oxygen into the substrate (oxygenases). The oxygen incorporated need not be derived from O2. The systematic name of this enzyme class is 4-hydroxyphenylpyruvate:oxygen oxidoreductase (decarboxylating). This enzyme is also called 4-hydroxyphenylpyruvate dioxygenase II. References EC 1.13.11 Enzymes of unknown structure