4-Hydroxyphenylacetate (4HPA) 3-monooxygenase (HpaB) is a reduced flavin adenine dinucleotide (FADH,) utilizing monooxygenase. Its cosubstrate, FADH(2), is supplied by HpaC, an NAD(P)H-flavin oxidoreductase. Because HpaB is the first enzyme for 4HPA metabolism, FADH(2) production and utili...