Structural basis for activation of cobalt‐carbon bond and control of adenosyl radical in coenzyme B12 catalysis

Adenosylcobalamin (AdoCbl) or coenzyme B12 is a naturally occurring organometallic compound that serves as a cofactor for enzymes that catalyze intramolecular group-transfer reactions and ribonucleotide reduction in a wide variety of organisms from bacteria to animals. AdoCbl-dependent enzymes are radical enzymes and generate an adenosyl radical by homolysis of the coenzyme’s cobalt-carbon (Co-C) bond for catalysis. How do the enzymes activate and cleave the Co-C bond to form the adenosyl radical? How do the enzymes utilize the high reactivity of adenosyl radical for catalysis by suppressing undesirable side reactions? Our recent structural studies aimed to solve these problems with diol dehydratase and ethanolamine ammonia-lyase established the crucial importance of steric strain of the Co-C bond and conformational stabilization of adenosyl radical for coenzyme B12 catalysis. We outline here our results obtained with these eliminating isomerases and compare them with those obtained with other radical B12 enzymes.