(A) Molecular structure of the XDH 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 N to C terminus, the domains are: iron/sulfur-center domain
(residues 3–165; red), FAD domain (residues 226–531; green), and
Mo-pt domain (residues 590–1,331; 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. The positions of
residues discussed in the text are indicated. (B) For
clarity, the arrangement of the cofactors and salicylate in one subunit
of XDH are presented. The Mo ion is in green, the iron ions are in
light blue, and the sulfur atoms in yellow.