Also known as "5'-adenosylcobalamine" for obvious reasons :-). It is essential for making the amino acid methionine
, among other things. It is a peculiar compound that was one of the first complex organic structures crystallised
. The crystallographer was the famous Dorothy Crowfoot Hodgkin
who also produced many protein structures, as well as her more famous role in the determination of DNA
An unusual cobalt-porphyrin compound, it is used quite a lot by methanogenic bacteria (or, rather, analagous cofactors are used) to do tricky carbon bond chemistry. Said to be the only C-M bond in biology, it is also useful for isomerisation.