An electron-nuclear double-resonance study of methanol dehydrogenase and its coenzyme radical.

Proton as well as deuteron ENDOR (electron-nuclear double resonance) spectroscopy were performed of methanol dehydrogenase and pyrrolo-quinoline semiquinone (PQQH). Samples were examined in H2O- and 2H2O-containing buffers at 4.2 degrees K with Ka-band (33.5 GHz) frequency. Measurements of the enzyme in 2H2O revealed that the signals observed around the proton free-precession frequency belong to exchangeable protons. Therefore, our earlier assumption (R. de Beer et al. (1979) J. Chem. Phys. 70, 4491-4495) that these signals originate from protons in the aromatic ring of PQQH is incorrect. The proton matrix signal of the enzyme in H2O and 2H2O are nearly similar, while a deuteron matrix signal is not observed in the latter case. It is concluded, therefore, that the coenzyme is situated in a hydrophobic site of the enzyme.