This model identifies the reductase component (HpaC) of 4-hydroxyphenylacetate 3-monooxygenase. This enzyme catalyzes the first step (hydroxylation at the 3-position) in the degradation of 4-hydroxyphenylacetate to succinate and pyruvate. 4-hydroxyphenylacetate arises from the degradation of tyrosine. These reductases catalyze the reduction of free flavins by NADPH. The flavin is then utilized by the large subunit of the monooxygenase.