Crystal structure of the xanthine dehydrogenase dimer divided into the three major domains and two connecting loops. The two monomers have symmetry related domains in the same colors, in lighter shades for the monomer on the left and in darker shades for the monomer on the right. From the N to the C terminus, the domains are the iron/sulfur-center domain (residues 3–165; red), the FAD domain (residues 226–531; green), and the molybdopterin center (Mo-pt) domain (residues 590–1331; blue). The loop connecting the iron/sulfur domain with the FAD domain (residues 192–225) is shown in yellow, the one connecting the FAD domain with the Mo-pt domain (residues 537–589) is in brown, and the N and C termini are labeled. The FAD cofactor, the two iron/sulfur centers, the molybdopterin cofactor, and the salicylate also are included (Enroth et al., 2000). Copyright © 2000 National Academy of Sciences (Washington, DC). Reproduced with permission.