How corrinoids are synthesized without oxygen: nature's first pathway to vitamin B 12

Abstract

Background: During the biosynthesis of vitamin B 12, 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 B 12-producing anaerobic bacteria and members of the ancient Archaea, so the question arises of how these microbes accomplish the key ring-contraction process. Results: 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 B 12. 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 B 12 biosynthesis. Conclusions: 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 B 12. 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 B 12 have now been clearly distinguished by their differing mechanisms for ring contraction.