The proton spin-flip lines of Mo(V) EPR signals from sulfite oxidase and xanthine oxidase

Abstract The proton spin-flip transitions in Mo(V) EPR spectra of the different reduced forms of the enzymes xanthine oxidase and sulfite oxidase have been examined. The proton spin-flip transitions of xanthine oxidase originate from weakly coupled nonexchangeable nuclei, probably carbon-bound protons of amino acid ligands or of the molybdenum cofactor. The sulfite oxidase high-pH signal, on the other hand, in addition to proton spin-flip transitions similar to those of xanthine oxidase, shows transitions from an exchangeable, relatively strongly coupled proton. The hyperfine coupling of this proton is not resolved in the powder lineshape because of noncolinearity of A ( 1 H) and g , and because of the largely anisotropic nature of its coupling. The possible significance in relation to the catalytic mechanism of this latter finding is discussed.