Xanthine oxidase electron transfer

Molybdenum Milk, milk products dried legumes or pulses liver and kidney grains Prosthetic group of enzymes aldehyde oxidase Xanthine oxidase Electron transfer chain enzymes... 

Molybdenum. Molybdenum is a component of the metaHoen2ymes xanthine oxidase, aldehyde oxidase, and sulfite oxidase in mammals (130). Two other molybdenum metaHoen2ymes present in nitrifying bacteria have been characteri2ed nitrogenase and nitrate reductase (131). The molybdenum in the oxidases, is involved in redox reactions. The heme iron in sulfite oxidase also is involved in electron transfer (132). 

In the original rapid-freezing work on xanthine oxidase (53) it was found that in experiments employing about 1 mole of xanthine per mole of enzyme and an excess of oxygen, the time sequence of appearance of the various EPR signals was molybdenum (V), followed by flavin semi-quinone radical (FADH), followed by iron. This suggested that the electron transfer sequence might be ... 

Palmer, G., Bray, R.C., and Beinert, H. 1964. Direct studies on the electron transfer sequence in xanthine oxidase by electron paramagentic resonance spectroscopy. The Journal of Biological Chemistry 239 2657-2666. 

R. Hille and R.F. Anderson, Coupled electron/proton transfer in complex flavoproteins — solvent kinetic isotope effect studies of electron transfer in xanthine oxidase and trimethylamine dehydrogenase. J. Biol. Chem. 276, 31193-31201 (2001). 

The possibility that there might be long-range electron transfer between redox-active centers in enzymes was first suspected by biochemists working on the mechanism of action of metalloenzymes such as xanthine oxidase which contain more than one metal-based redox center. In these enzymes electron transfer frequently proceeds rapidly but early spectroscopic measurements, notably those by electron paramagnetic resonance, failed to provide any indication that these centers were close to one another. 

The rationale for studies on flavin semiquinone metal interactions stems from the presence of flavin coenzymes which participate in electron transfer in a number of metalloflavoproteins. Iron-containing redox centers such as the heme and nonheme iron sulfur prosthetic groups (Fe2/S2, Fe+ZS, or the rubredoxin-type of iron center) constitute the more common type of metal donor-acceptor found in metalloflavoproteins, although molybdenum is encountered in the molybdenum hydroxylases (e.g. xanthine oxidase, aldehyde dehydrogenase). 

Shown in Figures 5-7 are the redox pathways for xanthine oxidase, sulfite oxidase, and nitrate reductase (assimilatory and respiratory), respectively. These schemes address the electron and proton (hydron) flows. The action of the molyb-doenzymes is conceptually similar to that of electrochemical cells in which half reactions occur at different electrodes. In the enzymes, the half reactions occur at different prosthetic groups and intraprotein (internal) electron transfer allows the reactions to be coupled (i.e., the circuit to be completed). In essence, this is the modus operandi of these enzymes, which must be determined before intimate mechanistic considerations are seriously addressed. 

The substrate half-reactions are displayed in Tables I and II. In each case, a two-electron process seems to be involved. Only in nitro-genase are greater numbers of electrons transferred, and the discussion earlier in this paper summarizes the evidence that these processes occur in two-electron steps. The two-electron reaction of the molybdenum site never appears to be simply an electron transfer reaction. In the case of nitrogenase, each substrate takes up an equal (or greater) number of protons to form the product. In the other molybdenum enzymes, proton transfer and addition or removal of H20 are also required. In each case, however, there is at least one proton transferred in the same direction as the pair of electrons. These data, taken in conjunction with the EPR evidence for proton transfer from the substrate to the active site in xanthine oxidase, suggest that the molybdenum site in all the enzymes... 

Mechanistic speculations about the molybdoenzymes must be considered to be in their infancy with the possible exception of those for xanthine oxidase. Although the detailed structural nature of the molybdenum site is unknown, there is sufficient information from biochemical and coordination chemistry studies to allow informed arguments to be drawn. Here we first discuss evidence for the nuclearity of the molybdenum site and then discuss both oxo-transfer and proton-electron transfer mechanisms for molybdenum enzymes. A final discussion considers the unique aspects of nitrogenase and the possible reasons for the use of molybdenum in enzymes. 

Figure 5. Proposed coupled proton-electron transfer scheme for xanthine oxidase activity (66, 67, 68)...

Many experimental observations on xanthine oxidase activity are correlated by this scheme, and at present, there appear to be no major inconsistencies. The coupled proton-electron transfer scheme (66, 67, 68) has been successfully incorporated into an overall mechanistic scheme (69) which explains, with great economy, a large amount of rather demanding data, from both kinetic and electron uptake experiments. 

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