Spin–spin interaction between molybdenum and one of the iron–sulphur systems of xanthine oxidase and its relevance to the enzymic mechanism

Abstract

1. Electron-paramagnetic-resonance (e.p.r.) studies at 9 and 35GHz at helium temperatures have given new information relating to the structure and mechanism of action of xanthine oxidase. 2. As reported by others, the enzyme gives two types of e.p.r. signal attributed to iron–sulphur systems. The first has gav.=1.95. Parameters of the second are determined as g1 2.12, g2 2.007 and g3 1.91, with gav.=2.01. This species seems to have a slightly higher redox potential than the former one. 3. Temperature-dependent changes in the form of Mov e.p.r. signals from the enzyme, observed under certain conditions, are shown to be due to weak spin–spin interaction between Mov and gav.=1.95 Fe/S. The phenomenon has been studied most fully for the Slow Mov signal. Here, the spectral change takes the form of an additional approximately isotropic 11G splitting, detected below about 45°K only. Samples without Fe/S reduced showed no such changes of spectrum. 4. Similar spectral changes were observed in the Rapid Mov signals, obtained in rapid-freezing experiments, but only in samples corresponding to relatively long reaction times with the substrate. It is suggested therefore that the phenomenon may provide a means of distinguishing enzyme centres with Mo only reduced from those in which both Mo and Fe/S are reduced. 5. Additional rapid-freezing data tending to support a two- rather than a one-electron transfer of reducing equivalents from substrates to xanthine oxidase are reported.