The crystal structure of glutamate mutase with bound coenzyme B(12) suggests a radical shuttling mechanism within the active site of the enzyme. Quantum chemical calculations of the rearrangement in combination with kinetic and mutational studies suggest the catalytic mechanism of this enzyme to proceed via a fragmentation/recombination sequence with intermediates stabilized by partial protonation/deprotonation. Crucial residues in the active site have been identified. Solution structure studies indicate the mechanism of B(12) binding to the apoenzyme.