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The Absolute Configuration of Methylmalonyl Coenzyme A and Stereochemistry of the Methylmalonyl Coenzyme A Mutase Reaction

From the ¹ From the Department of Biochemistry, College of Physicians and Surgeons, Columbia University, New York, New York 10032

Propionyl coenzyme A was carboxylated with epimerase-free carboxylase, and the resulting methylmalonyl coenzyme A (a) was reduced with Raney nickel. Isolation of (-)-(R)-ß-hydroxyisobutyric acid N-phenylcarbamate (11) from the reaction mixture showed that methylmalonyl coenzyme A (a) has the (S) configuration.

Propionyl coenzyme A was also converted to deuteriosuccinate with a D₂O extract of a beef liver mitochondrial acetone powder. The deuteriosuccinate had a plain positive optical rotatory dispersion curve corresponding to the (S) configuration. It was concluded that (S)-methylmalonyl coenzyme A, formed by carboxylation of propionyl coenzyme A, was epimerized in D₂O to (RS)-methylmalonyl(coenzyme A)-2-D, and that (R)-methylmalonyl(coenzyme A)-2-D was isomerized to succinyl coenzyme A with net retention of configuration of C-2.

Since, in contrast, the glutamate mutase reaction proceeds with inversion, the lack of stereochemical uniformity in these two otherwise closely related rearrangements suggests that they may involve at least two steps, one or more of which are analogous, but at least one taking place with different stereochemistry. It is suggested that the cobamide coenzyme releases an electrophilic 5'-deoxyadenosyl group (perhaps reversibly to a methionyl residue on the enzyme), and that the resulting Co:ⁱ complex participates in the rearrangement by removal and transfer of a proton or other cationic fragment.

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