Dinuclear site binding constant

An additional crucial piece of information emerges from the alloxan-thine study (24). Thus, it was shown that one alloxanthine binds to the enzyme per active molybdenum site. This result clearly implies that the molybdenum site is mononuclear. If a dinuclear site were involved, then it would be unlikely to require two alloxanthine molecules for inhibition and would be expected to be at least partially inhibited with one alloxan-thine/two molybdenum. Also, a difference in binding constant would be expected for the second compared with the first bound alloxanthine, but none is found. This result, coupled with the lack of evidence for Mo(V)-Mo(V) spin-spin interactions in the EPR spectra, clearly implicates a mononuclear site, and it would seem that xanthine oxidase possesses two full catalytic units, each containing one molybdenum, one flavin, and two Fe2S2 units (20). Other molybdenum oxidases also contain paired prosthetic groups and subunits, and it is likely that they each have two catalytic units per molecule. 

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