Electron Spin Resonance Studies with Dioldehydrase

The addition of the substrate analog chloroacetaldehyde to the dioldehydrase-coenzyme B12 complex results in the appearance of an electron spin resonance signal at g = 2.2 and multiple signals at g = 2.0, amounting to a total of ∼2 unpaired electrons per molecule of enzyme-bound coenzyme. Substitution of deuterium for protium at the C-2 positions of chloroacetaldehyde results in a ∼5-G narrowing of the transition at g = 2.0 and in an additional transition in the g = 2.0 region, at 3285 G. Substitution of chloroacetaldehyde, uniformly enriched to 90% with 13C, results in a ∼10-G broadening of the signals at g = 2.0. No effect of such isotopic substitution is detected in the signal at g = 2.2, and little residual effect is noted at g = 2.0 after a 5-min preincubation. These results establish the transient appearance of spin density on carbon and possibly on hydrogen atoms derived from chloroacetaldehyde and partially assign the signals at g = 2.0 to radicals derived from chloroacetaldehyde. The presence of unpaired spin density on the methylene hydrogens of the substrate analog, the unusually wide ESR signals at g = 2.0 even at liquid nitrogen temperature, and the large kinetic isotope effects previously found with dioldehydrase, all suggest the participation of hydrogen atom transfer reactions in catalysis by coenzyme B12 enzymes.