|
This is a ribbon diagram of the enzyme BluB, which
MIT researchers have shown catalyzes the formation of DMB, a
fragment of vitamin B12. The stick drawings (in yellow, red and
blue) represent cofactors that the enzyme cannibalizes to form
DMB.
Image courtesy / Graham Walker laboratory
Professor Graham Walker and postdoctoral fellow
Michiko Taga have discovered the last unknown step in the
production of vitamin B12. The vitamin is synthesized by soil
microbes that form symbiotic relationships with plant roots.
Photo / Donna Coveney
|
The researchers report that a single enzyme
synthesizes the fragment, and they outline a novel reaction mechanism
that requires cannibalization of another vitamin.
The work, which has roots in an MIT undergraduate
teaching laboratory, "completes a piece of our understanding of a
process very fundamental to life," said Graham Walker, MIT professor
of biology and senior author of a paper on the work that will appear
in the March 22 online edition of Nature.
Vitamin B12 is produced by soil microbes that live
in symbiotic relationships with plant roots. During the 1980s, an
undergraduate research course taught by Walker resulted in a novel
method for identifying mutant strains of a soil microbe that could not
form a symbiotic relationship with a plant.
Walker's team has now found that one such mutant
has a defective form of an enzyme known as BluB that leaves it unable
to synthesize B12.
BluB catalyzes the formation of the B12 fragment
known as DMB, which joins with another fragment, produced by a
separate pathway, to form the vitamin. One of several possible reasons
why it took so long to identify BluB is that some bacteria lacking the
enzyme can form DMB through an alternate pathway, Walker said.
One of the most unusual aspects of BluB-catalyzed
synthesis is its cannibalization of a cofactor derived from another
vitamin, B2. During the reaction, the B2 cofactor is split into more
than two fragments, one of which becomes DMB.
Normally, the B2-derived cofactor would assist in a
reaction by temporarily holding electrons and then giving them away.
Such cofactors are not consumed in the reaction.
Cannibalization of a cofactor has very rarely been
observed before in vitamin synthesis or any type of biosynthetic
pathway, says Michiko Taga, an MIT postdoctoral fellow in Walker's lab
and lead co-author of the Nature paper.
"There are almost no other examples where the
cofactor is used as a substrate," she said.
One early clue to BluB's function was that a gene
related to it is located near several other genes involved in B12
synthesis in a different bacterium. Still, the researchers were not
convinced that one enzyme could perform all of the complicated
chemistry needed to produce DMB.
"It looked like a number of things had to happen in
order to make the DMB," said Walker. "We originally thought that BluB
might be just one of several enzymes involved in DMB synthesis."
Therefore, it came as a surprise when Taga isolated
the BluB protein and showed that it could make DMB all by itself.
Nicholas Larsen, lead co-author and a former
college classmate of Taga's now at Harvard Medical School, did a
crystallographic analysis of the protein after Taga told him about her
research over coffee one day. The protein structure he developed
clearly shows the "pocket" of BluB where the DMB synthesis reaction
takes place.
Still to be explored is the question of why soil
bacteria synthesize B12 at all, Walker said. Soil microorganisms don't
require B12 to survive, and the plants they attach themselves to don't
need it either, so he speculates that synthesizing B12 may enable the
bacteria to withstand "challenges" made by the plants during the
formation of the symbiotic relationship.
More than 30 genes are involved in vitamin B12
synthesis, and "that's a lot to carry around if you don't need to make
it," Walker said.
The full implications of the new research will
probably not be known for some years, which is often the case with
basic research, Walker said. "I've been in many other situations in
research where we did something very basic and did not immediately
realize the importance of it, and subsequently the implications were
found to be much more broad-reaching," he said.
Other authors on the paper are Annaleise
Howard-Jones, a postdoctoral fellow at Harvard Medical School, and
Christopher Walsh, professor of biological chemistry and molecular
pharmacology at Harvard Medical School. |
