Part of a two-component P450 system that efficiently O-demethylates diverse aromatic substrates such as guaiacol and a wide variety of lignin-derived monomers. Is likely involved in lignin degradation, allowing Amycolatopsis sp. ATCC 39116 to catabolize plant biomass. GcoB transfers electrons from NADH to the cytochrome P450 subunit GcoA. Highly prefers NADH over NADPH as the electron donor. NADH + 2 oxidized [cytochrome P450] = H(+) + NAD(+) + 2 reduced [cytochrome P450] Binds 1 FAD per subunit. Binds 1 [2Fe-2S] cluster per subunit. kcat is 44 sec(-1) for NADH-mediated reduction of cytochrome c. Aromatic compound metabolism. Monomer. Forms a heterodimer with GcoA. GcoB has an unusual three-domain structure, with an N-terminal 2Fe-2S domain and a C-terminal region that consists of an FAD-binding domain followed by an NADH-binding domain.