FIGURE 2.

The active site of alloxanthine-inhibited xanthine dehydrogenase from R. capsulatus (Ref. 12; Protein Data Bank code 1JRP). Shown is the numbering convention for the bacterial enzyme, and bovine numbering is shown in parentheses. B is a 90° clockwise rotation about the vertical axis from A. The planar sulfur and apical oxygen were switched given recent evidence that it is the oxygen rather than the sulfur that occupies the apical position in the square pyramidal coordination sphere of the molybdenum (30).