STUDIES OF THE MECHANISM OF ACTION OF COBAMIDE COENZYMES

Aerobacter aerogenes converts propanediol to propionaldehyde, and ethylene glycol to acetaldehyde. ' Dioldehydrase, the enzyme that cata-lyzes this reaction, has been purified 200-fold and a requirement for any one of three cobamide coenzymes and for a monovalent metal ion has been established.2 The present work seeks to extend the limited informa-tion3 now available on the mechanism of action of cobamide coenzymes. We have previously reported the following properties of the diolde-hydrase system:4r5 ( 1) The dioldehydrase reaction involves the replacement of a hydroxyl group at C-2 of the diol by a hydrogen atom. It was demonstrated that the hydrogen did not arise from the solvent but originated from C-1 of the diol. (2) When the substrate was propanediol-l-Dz, a large isotope effect was observed; the deuterated substrate reacting at one-tenth the rate of the nonisotopic compound. ( 3) Formation of a fully active enzyme-coenzyme complex appears to be a relatively slow process, since a lag period is observed prior to attain-ment of maximal velocity when the enzyme, coenzyme and substrate are mixed simultaneously. Incubation of the enzyme and coenzyme prior to substrate addition eliminates the lag period. After complex formation certain properties of the apoenzyme and the coenzyme are markedly altered: no inactivation by p-hydroxy-mercuri-benzoate occurs and the coenzyme is no longer light sensitive. ( 4) In the absence of substrate the enzyme-coenzyme complex becomes inactive; both coenzyme and enzyme activity are lost. This inactivation is accompanied by a spectral change which results in a spectrum very sim-ilar to hydroxo-cobalamin. These results suggested that the enzyme is capable of modifying the coenzyme, possibly by breaking the carbon-cobalt bond, and this process may reflect an important aspect of the actual catalytic process. Since during the catalytic process the coenzyme activity is not lost, a reversible modification needs to be postulated. Based upon these considerations, we wish to propose the following tentative scheme for the reaction