Electron Spin Resonance Studies with Dioldehydrase

When 1,2-propanediol is added to dioldehydrase-coenzyme B12, electron spin resonance signals appear which indicate the presence of at least two paramagnetic species. One signal, at g = 2.2, is attributed to Co(II) of B12(r). Other signals, at g ≈ 2.0, which show different power saturation behavior, are tentatively assigned to organic radicals, possibly derived from the substrate and the adenosyl moiety of the coenzyme. The occurrence of such species is in accord with a previously proposed mechanism (Finlay, T. H., Valinsky, J., Sato, K., and Abeles, R. H. (1972) J. Biol. Chem. 247, 4197–4207). Integration of the signal indicates that approximately 0.75 unpaired electrons are present per molecule of enzyme-coenzyme complex. Signals with similar but not identical g values were obtained with the substrate analogues chloroacetaldehyde and glycolaldehyde. With chloroacetaldehyde the signal at g = 2.0 shows pronounced fine structure and non-uniform saturation behavior, confirming the multiplicity and substrate dependence of this signal. With both substrate analogues nearly two unpaired electrons are detected per molecule of enzyme-coenzyme complex. The paramagnetic species have only a small effect on the longitudinal relaxation rate of the water protons, suggesting that they are shielded from the solvent. The distance of closest approach of the water protons to the paramagnetic center was estimated to be 10.2 A. These experiments established that dioldehydrase can bring about the homolytic cleavage of the carbon-cobalt bond of coenzyme B12 and suggest the possibility that radical species may be intermediates in the catalytic reaction. A kinetic analysis will be required to establish conclusively that the radicals play a significant role in the catalytic process.