This two-component enzyme from Pseudomonas belongs to the family of xanthine dehydrogenases, but differs from most other members of this family. While most members contain an FAD cofactor, the large subunit of this enzyme contains three c-type cytochromes, enabling it to interact with the electron transfer chain, probably by delivering the electrons to a cytochrome oxidase. The small subunit contains a typical molybdopterin cytosine dinucleotide(MCD) cofactor and two [2Fe-2S] clusters .
small subunit NicA harbors the 41-C-X4-C-G-X-C-Xn-C-59 and 100-C-G-X-C-X31-C-X-C-137 conserved motifs likely involved in binding of the two [2Fe-2S] clusters; the N-terminal region of large subunit NicB contains the active site and the molybdopterin cytosine dinucleotide binding sites in an arrangement MPT2-MPT1-MPT3. The C-terminal region of NicB, residues750-1187, contains three conserved cytochrome c hemebinding motifs 817-CAVCH-823, 963-CTACH-969, and 1087-CLGCH-1093