HOW CORRINOIDS ARE SYNTHESIZED WITHOUT OXYGEN - NATURES FIRST PATHWAYTO VITAMIN-B-12

Background: During the biosynthesis of vitamin B-12, the aerobic bacte rium Pseudomonas denitrificans uses two enzymes, CobG and CobJ, to con vert precorrin-3 to the ring-contracted intermediate, precorrin-4. Cob G is a monooxygenase that adds a hydroxyl group, derived from molecula r oxygen, to C-20, whereas CobJ is bifunctional, inserting a methyl gr oup at C-17 of the macrocycle and catalyzing ring contraction. Molecul ar oxygen is not available to vitamin B-12-producing anaerobic bacteri a and members of the ancient Archaea, so the question arises of how th ese microbes accomplish the key ring-contraction process. Results: Clo ning and overexpression of Salmonella typhimurium genes has led to the discovery that a single enzyme, CbiH, is responsible for ring contrac tion 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 c ontraction and lactonization to yield the intermediate, cobalt-precorr in-4, isolated as cobalt-factor IV. C-13 labeling studies have proved that cobalt-precorrin-4 is incorporated into cobyrinic acid, thereby c onfirming that cobalt-precorrin-4 is an intermediate in vitamin B-12 b iosynthesis. Conclusions: Two distinct mechanisms exist in nature for the ring contraction of porphyrinoids to corrinoids - an ancient anaer obic pathway that requires cobalt complexation prior to nonoxidative r earrangement, and a more recent aerobic route in which molecular oxyge n serves as the cofactor. The present results offer a rationale for th e main differences between aerobic and anaerobic biosynthesis of vitam in B-12. Thus, in anaerobes there is exchange of oxygen at the C-27 ac etate site, extrusion of acetaldehyde and early insertion of cobalt, w hereas the aerobes show no exchange of oxygen at C-27, extrude acetic acid and insert cobalt very late in the biosynthetic pathway, after ri ng contraction has occurred. These parallel routes to vitamin B-12 hav e now been clearly distinguished by their differing mechanisms for rin g contraction.