Studies on the mechanism of hydrogen transfer in the cobamide coenzyme-dependent dioldehydrase reaction.

When dl-1 ,2-propanediol-l-3H is converted to propionaldehyde in the presence of dioldehydrase and cobamide coenzyme, tritium is transferred to the coenzyme. The tritiated coenzyme so obtained transfers tritium to the reaction product when reacted with dl-1,2-propanediol and apoenzyme. The coenzyme is tritiated exclusively at the C-5’ position of the adenosyl moiety. The location of tritium was established by chemical degradation a.nd further confirmed by showing that chemically synthesized cobamide coenzyme, containing tritium at the C-5’ position, transferred tritium to the product when added to enzyme and unlabeled substrate. The conversion of d&l, 2-propanediol-1-3H to propionaldehyde proceeds with interand intramolecular tritium transfer. Approximately 1% of the tritiated substrate reacts by intmmolecular transfer, i.e. the hydrogen abstracted from C-l of a substrate molecule is found in the a! position of the aldehyde derived from that molecule. A partial reaction occurs, as evidenced by tritium exchange between tritiated coenzyme and propionaldehyde or acetaldehyde under conditions in which no net reaction occurs. The results obtained have led to the following tentative reaction sequence. Hydrogen is abstracted from C-l of dI-1,2propanediol and transferred to the coenzyme, where it becomes equivalent with at least one, but probably both, hydrogens of the C-5’ position. This results in the formation of a reduced form of the coenzyme and a molecule derived through the oxidation of the substrate. In a subsequent step the hydrated form of propionaldehyde is formed by a transfer of hydrogen from the reduced coenzyme to the intermediate derived from the substrate.