Vitamin B12, Cyanocobalamine, is a cobalt containing protein which is involved in a number of synthetic reactions.
The formation of a cobalt-carbon bond (an organometallic bond) is unusual in nature. With the help of cobalamines, carbon-carbon bonds can be split homolytically. For this reason, the enzyme is common in the liver.
Vitamine B12, among other things, takes part in the biosynthesis of choline cysteine and the formation of red blood cells.