During the biosynthesis of vitamin B12, the
aerobic bacterium Pseudomonas denitrificans uses 2
enzymes, CobG and CobJ, to convert precorrin-3 to the
ring-contracted intermediate, precorrin-4.
CobG is a monooxygenase that adds an OH
group, derived from O2, to C-20, whereas CobJ is bifunctional, inserting a Me
group at C-17 of the macrocycle and catalyzing ring
contraction.
O2 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, Co-precorrin-4,
isolated as Co-factor IV. 13C labeling studies
have proved that Co-precorrin-4 is incorporated into
cobyrinic acid, thereby confirming that Co-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 O2 serves as the cofactor.