Protein of the Week: methylmalonyl-CoA mutase
Methylmalony-CoA mutase (PDB code 4REQ, structure generated with PyMOL) converts L-methylmalonyl-CoA, the break down-product of odd-chain fatty acids, into a metabolic intermediate useful to cells, succinyl-CoA. The enzyme exchanges the carbonyl-S-CoA on the 2-carbon for a hydrogen on the neighboring carbon:
This reaction is made possible by vitamin B12 (cobalamin), a cofactor with a trivalent cobalt amidst a corrin ring:
On one side the cobalt is bound to the 5' carbon of an adenosine ring. In the course of the reaction, this bond splits by homolytic cleavage (one electron to each atom in the bond, forming a radical), sending one electron to the deoxyadenosine and the other to the cobalt. The radical generated in the deoxyadenosine plucks the hydrogen from the substrate, and the ensuing carbon radical attacks the carbonyl-S-CoA. Once this is completed, the substrate grabs its hydrogen, and the cofactor resets itself.