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Mechanism of action of coenzyme B 12 . Hydrogen transfer in the isomerization of -methylaspartate to glutamate.

Use of a mixture of unlabeled and tetradeuterio-,Bmethylaspartate 
with coenzyme B_(12) dependent β-methylaspartate-glutamate mutase has shown that the hydrogen that 
migrates becomes one of three equivalent hydrogens during 
the isomerization. Kinetic isotope effects suggest that cleavage 
of the bond in the substrate from carbon to that hydrogen which migrates is an important component of the rate-determining 
step. The evidence also supports the existence of an 
intermediate which can partition with similar probabilities 
to β-methylaspartate or to glutamate. Mechanistic implications 
of these findings are discussed.