Pulsed electron nuclear double resonance (ENDOR) spectra of nonexchangeable protons in the vicinity of the Mo(V) center of the high pH (hpH) and low pH (IpH) forms of native chicken liver sulfite oxidase (SO) and recombinant human SO have been obtained and analyzed for the first time. The close similarity of the spectra for the chicken and human enzymes indicates that the structures of their molybdenum centers are essentially identical. For IpH SO, the closest nonexchangeable proton is found to be similar to2.8 Angstrom from the Mo atom. To more accurately determine the distance to this proton and facilitate its assignment, the C-band electron spin-echo envelope modulation (ESEEM) spectra of IpH SO were also analyzed. From the obtained distance and comparison with the X-ray structure, this closest nonexchangeable proton is assigned to the alpha-proton of the coordinated conserved cysteinyl residue (Cys185 in chicken, Cys207 in human). The closest Mo...H distance for the nonexchangeable protons of hpH SO is found to be similar to3.3 Angstrom. For the cysteinyl alpha-proton, such an increase in the Mo H distance only requires a very small change in torsional angles. This study demonstrates that details of the enzyme structural rearrangements with pH can be monitored by ENDOR spectroscopy and suggests that a similar approach may be routinely used to probe the orientation of the coordinated cysteinyl residue in mutant forms of SO that are catalytically compromised.