Vitamin B12 synthesis exposed

Scientists have worked out the final steps of vitamin B12 biosynthesis, which involve the unusual transformation of one vitamin -- a flavin -- into another, according to a report in this week's Nature. The results provide the finishing touches to scientists' understanding of the synthesis of this complex molecule. Vitamin B12 "has played a role in fascinating chemists and biochemists for 50 years," A.I. Scott, of Texas A&M University, who peer-reviewed the paper for Nature, told The Scientist. "Nature put this beautiful cofactor in front of us and it's fascinating to realize step by step how the whole thing was built. It's like detective work," added Scott, who was not involved in the research.Vitamin B12, the only vitamin synthesized exclusively by microorganisms, is approximately three times larger than all other vitamins, requiring about 30 enzyme steps for its synthesis. Until now, the final stages of how it is made have eluded scientists, primarily due to the difficulty of isolating each of the many complicated steps. Study author Graham Walker, at the Massachusetts Institute of Technology, told The Scientist that he did not intend to study vitamin B12, and instead focused his work on the bacterium Sinorhizobium meliloti, and how it managed to live inside plant cells and fix nitrogen. In experiments using bacteria with a mutant gene called bluB, his group noticed that the bacteria were not behaving as expected. It turns out, the bacteria were not synthesizing B12; the mutation had impaired their ability to form the last ligand of the vitamin, known as 5,6-dimethylbenzimidazole (DMB).Michiko Taga, first author on the paper (also based at MIT), began searching for the genes that might be playing a role in the final stages of B12 synthesis, assuming that BluB was one of several proteins working to create DMB. "You can't tell by genetics that a protein can function alone or in a pathway,"Taga told The Scientist. "We thought that BluB would be on the pathway because the reaction seemed too complex [for it] to be working alone."But Taga didn't find any other proteins in the pathway. So she joined forces with an old friend from her undergraduate days, Nicholas Larson of Harvard Medical School, to crystallize B12. The structure revealed an unusual mechanism: The BluB enzyme formed DMB by using oxygen to dismantle another vitamin co-factor -- flavin mononucleotide, sometimes used by other enzymes as an electron shuttle. Specifically, the researchers showed that BluB has evolved into a closed "box" shape that traps the flavin, then uses oxygen to destroy and remold it into DMB. "One still has to marvel at how such a small enzyme is able to perform this catalysis which involves the breaking and making of so many carbon-carbon bonds," Martin Warren at the University of Kent, and who was not involved in the research, told The Scientist in an Email. It is particularly remarkable, he added, considering this action includes "the surgical excision of a carbon from a six-membered ring to generate a five-membered ring."The researchers report a similar "cannibalism" action in the synthesis of vitamin H, but note that the unique structure and action of BluB suggest it belongs to a new class of enzymes called the "flavin destructase" family.Walker said his lab continues to examine B12 in rhizobia and what evolutionary forces have favored production of B12, considering it's such a costly compound, energy-wise. He added that he suspects that the oxidative stress placed on bacterial organisms living inside eukaryotic cells may have induced the unusual mechanism described in the paper. Scott noted that many anaerobic bacteria do not have the BluB protein, suggesting that there is still much to be discovered.Andrea Gawrylewski mail@the-scientist.comLinks within this articleM Taga et al, "BluB cannibalizes flavin to form the lower ligand of vitamin B12," Nature 446:449-453, March 22, 2007. https://www.Nature.com/index.htmlA.I. Scott https://www.chem.tamu.edu/faculty/faculty_detail.php?ID=56ML Phillips, "Vitamin A receptor found," The Scientist, January 25, 2007. https://www.the-scientist.com/news/display/43512/Graham Walker https://web.mit.edu/biology/www/facultyareas/facresearch/walker.htmlD. Bruce, "The genome of Sinorhizobium meliloti," The Scientist, July 31, 2001. https://www.the-scientist.com/article/display/19808/GRO Campbell, et all, "Sinorhizobium meliloti blub is necessary for production of 5,6-dimethylbenzimidazole, the lower ligand of B12," Proc Natl Acad Sci, 103:4634-4639, 2006. https://www.the-scientist.com/pubmed/16537439Martin Warren https://www.kent.ac.uk/bio/warrenS. Blackman, "Rooting out the cheats," The Scientist, September 4, 2003. https://www.the-scientist.com/article/display/21566

Vitamin B12 synthesis exposed

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