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Anthranoyl-CoA monooxygenase/reductase (ACMR) participates in an unusual pathway for the degradation of aromatic compounds in Azoarcus evansii. It catalyzes the monooxygenation of anthranoyl-CoA to 5-hydroxyl-2-aminobenzoyl-CoA and the subsequent reduction to the dearomatized product 2-amino-5-oxo-cyclohex-1-ene-1-carbonyl-CoA. The two reactions occur in separate domains, termed the monooxygenase and reductase domain. Both domains were reported to utilize FAD as a cofactor for hydroxylation and reduction, respectively. We have heterologously expressed ACMR in Escherichia coli BL21 and found that the monooxygenase domain contains FAD. However, the reductase domain utilizes FMN and not FAD for the reduction of the intermediate 5-hydroxyl-2-aminobenzoyl-CoA. A homology model for the reductase domain predicted a topology similar to the Old Yellow Enzyme family, which exclusively bind FMN, in accordance with our results. Binding studies with 2-aminobenzoyl-CoA (AbCoA) and p-hydroxybenzaldehyde (pHB) as probes for the monooxygenase and reductase domain, respectively, indicated that two functionally distinct and independent active sites exist. Given the homodimeric quartenary structure of ACMR and the compact shape of the dimer as determined by small-angle X-ray scattering experiments we propose that the monooxygenase and reductase domain of opposite peptide chains are involved in the transformation of anthranoyl-CoA to 2-amino-5-oxo-cyclohex-1-ene-1-carbonyl-CoA., (Copyright © 2015 Elsevier B.V. All rights reserved.)