Fig. 2.

Cobalt coordination and chemistry associated with cobamides.

(a) Co(III) corrinoids such as adenosylcobalamin (where X = deoxyadenosyl) can undergo homolytic cleavage to generate a Co(II) species and an adenosyl radical (X•), whereas methylcobalamin undergoes heterolytic cleavage to generate a Co(I) species and a methylated product. (b) The corrin ring naturally adopts a helical arrangement and this fits well with Co(I). More increasingly planar forms of the corrin ring are generated with Co(II) and Co(I) respectively. The ability to transition between planar and helical conformations means that the corrin ring acts as an entatic state module.