How corrinoids are synthesized without oxygen: nature's first pathway to vitamin B12

During the biosynthesis of vitamin B12, the aerobic bacterium Pseudomonas denitrificans uses two enzymes, CobG and CobJ, to convert precorrin-3 to the ring-contracted intermediate, precorrin-4. CobG is a monooxygenase that adds a hydroxyl group, derived from molecular oxygen, to C-20, whereas CobJ is bifunctional, inserting a methyl group at C-17 of the macrocycle and catalyzing ring contraction. Molecular oxygen is not available to vitamin B12-producing anaerobic bacteria and members of the ancient Archaea, so the question arises of how these microbes accomplish the key ring-contraction process.

Cloning and overexpression of Salmonella typhimurium genes has led to the discovery that a single enzyme, CbiH, is responsible for ring contraction during anaerobic biosynthesis of vitamin B12. The process occurs when CbiH is incubated with precorrin-3, but only in the presence of cobalt. CbiH functions as a C-17 methyltransferase and mediates ring contraction and lactonization to yield the intermediate, cobalt-precorrin-4, isolated as cobalt-factor IV. 13C labeling studies have proved that cobalt-precorrin-4 is incorporated into cobyrinic acid, thereby confirming that cobalt-precorrin-4 is an intermediate in vitamin B12 biosynthesis.

Two distinct mechanisms exist in nature for the ring contraction of porphyrinoids to corrinoids-an ancient anaerobic pathway that requires cobalt complexation prior to nonoxidative rearrangement, and a more recent aerobic route in which molecular oxygen serves as the cofactor. The present results offer a rationale for the main differences between aerobic and anaerobic biosynthesis of vitamin B12. Thus, in anaerobes there is exchange of oxygen at the C-27 acetate site, extrusion of acetaldehyde and early insertion of cobalt, whereas the aerobes show no exchange of oxygen at C-27, extrude acetic acid and insert cobalt very late in the biosynthetic pathway, after ring contraction has occurred. These parallel routes to vitamin B12 have now been clearly distinguished by their differing mechanisms for ring contraction.