Fig. 3.

The biogenesis of cobalamin.

(a) The biosynthesis of cobalamin from the common tetrapyrrole primogenitor uroporphyrinogen III involves in excess of twenty steps. Although there are two pathways for the synthesis of cobalamin, referred to as the aerobic (or cobalt-late) and anaerobic (cobalt early) routes, the series of methylation, amidation and rearrangement reactions are broadly similar and these steps are highlighted. The cobalt-insertion steps are also shown, with the cobalt-early insertion shown on the left hand side and the cobalt-late insertion stage shown on the right hand side. (b) The modifications that are associated with the cobalt-late pathway are highlighted and cross-referenced with (a). The Figure highlights the extensive modification that take place during the biogenesis of cobalamin.