Xanthine oxidase Very Rapid signal

The decrease in rate of reaction of xanthine oxidase with the size of purine substrate is also consistent with a size-selective active site pocket and possible metal binding of substrate [242,243], A strongly coupled nitrogen is not observed in the very rapid signal, which is thought to include bound product, so it would appear that the urate is not N bound to the molybdenum center [152-158],... 

Figure 3.8 Orientation dependent Q-band CW ENDOR spectra of C-labeled sample of HMP-gene rated very rapid signal of xanthine oxidase across the EPR envelope. (A) Experimental spectra. Conditions v w = 35.165 GHz, temperature 2 K. (B) Corresponding simulated ENDOR spectra. Only the largest nuclear frequency v+ is given for clarity. The parameters used for the simulation are given in Table 3.4 Reprinted with permission from ref. 43. Copyright (2001) American Chemical Society.

Scheme 2. Relationship between the Rapid and Very Rapid signals in reduction of xanthine oxidase by xanthine and other substrates. The pathway via III is favored at high pH for xanthine, whereas that via IV is favored under most other conditions for most other substrates. Species IV bears the proton transferred from the substrate in the site strongly coupled to molybdenum in VI this proton has exchanged with water protons. Modified from Bray et al (1979).

Scheme 3. Suggested chemical nature of intermediates in the reduction of xanthine oxidase by xanthine, based on information in Schemes 1 and 2. The numbering of the species corresponds with that in Scheme 2. The sulfur atom corresponds to Y of Scheme 1. Bold face type denotes atoms of or derived from the 7, 8, and 9 positions of the substrate molecule. Xanthine or H2O occupies the anion site, A of Scheme 1. Species I and II are not detected by EPR III gives Very Rapid and IV and VI give Rapid V is the product, uric acid. Though Mo(IV) is shown in the signal-giving species, internal oxidation-valuation equilibria with iron-sulfur and flavin will yield some Mo(V). Modified from Bray et al. (1979).

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