Photolytic Cleavage of Co-C Bond in Coenzyme B 12 -Dependent Glutamate Mutase.

Glutamate mutase (GLM) is a coenzyme B12-dependent enzyme which catalyzes the conversion of S-glutamate to (2S,3S)-3-methyl aspartate. The initial step in the catalytic process is the homolytic cleavage of the coenzyme's Co-C bond upon binding of a substrate. Alternatively, the Co-C bond can be cleaved using light. To investigate the photolytic cleavage of the Co-C bond in GLM, we applied a combined DFT/MM and TD-DFT/MM methods to scrutinize the ground and the low-lying excited states. Potential energy surfaces were generated as a function of axial bond lengths to describe the photodissociation mechanism. The S1 PES was characterized as the crossing of two electronic states, metal-to-ligand charge transfer (MLCT) and ligand field (LF). In GLM, radical pairs (RP) generate from the LF state. Two pathways, Path A and Path B, were identified as possible channels to connect the MLCT and LF electronic states. The S1 PES in GLM was compared with the S1 PES for coenzyme B12-dependent ethanolamine ammonia lyase (EAL) as well as the isolated AdoCbl cofactor. Finally, the theoretical insights related to the photodissociation mechanism were compared with transient absorption spectroscopy (TAS), electron paramagnetic resonance (EPR), resonance Raman (RR) spectroscopy.