Vitamin B12 is an indispensable dietary supplement richly available inliver and other organ meats. Trace amounts of B12 are needed for thenormal maturation of red blood cells; pernicious anemia develops whenpeople lack the vitamin.B12 cannot be made either by plants or by animals. Onlymicroorganisms manufacture this essential vitamin. Until recently,defining the biosynthetic pathway has challenged, but eluded, theingenuity and perseverance of many a chemist. The work began in1926 when a liver factor was discovered that cured anemia but thefactor was not isolated until 1948. In the June 10, 1994 issue ofScience, the perplexing pathways are summarized in an article by AlanBattersby: "How Nature Builds the Pigments of Life: The Conquest ofVitamin B12." Battersby, of Cambridge, England, and hiscollaborators, J. Crouzet and F. Blanche, of Rhone-Poulenc Rorer inFrance, used genetic, biochemical and molecular biological techniquesto define the complete pathway. The practical consequence ofunderstanding each step in the biosynthetic pathway is that the vitaminis potentially more available for animal feedstuff and human health.Battersby calls Vitamin B12 and its cousin-molecules, heme (theoxygen transport molecule in red blood cells) and chlorophyll (thecentral player in photosynthesis) the brightly colored "pigments oflife." All three are derived from one starting point _ theuroporphyrinogen III molecule "the parent of the pigments of life."This patriarch molecule is essentially "channeled toward B12 synthesisby methylation at C-2 to produce the first intermediate, percorrin-1."Once committed to this pathway the synthesis culminates in B12 ratherthan its kissing-cousins.Although the structure of the molecule was solved almost 40 years ago,the actual synthesis was hampered by technical problems. The initialbreakthrough occurred in the 1950s and 1960s when radioactive carbonand hydrogen came into common use as labeling compounds. Progresswas further accelerated when nuclear magnetic resonance wasintroduced to study labeled sites in the complex molecule. Then in themid-1980s a bacterium, Propionbactonium sharemanii, yielded newpigments (percorrin 1, 2, 3).Further structural work on these intermediate pigments showed that thefirst three synthetic steps in making Vitamin B12 involved methylation.Methylation essentially distinguishes the B12 pathway from thesynthetic route followed by its kindred molecules, heme andchlorophyll.The most recent progress in describing B12 synthesis relied ongenetically engineering strains of P. deruitrificans to produce newintermediates such as percorrin 6A. Describing the percorrin-6Aintermediate was a joint effort by the British and French team whowere surprised to find that the methylation site of this moleculeoccurred at an unexpected site.Researchers have now identified genes expressing enzymes thatcatalyze the conversion of intermediates. The encoded enzymes drivethe methylation, oxidation, ring contraction, reduction andrearrangements of the various percorrin molecules. A cocktail of six ormore enzymes are routinely employed in the synthesis to reducepreparation times of percorrin-6A. Enzymatic synthesis shortens thetime to a few hours or days from the years of effort required by non-enzymatic methods.Biosynthesis entered a new era when, as Battersby said, it becamepossible to "jump from one intermediate to the next by testing one afteranother of the palette of enzymes until the right one was found." Thisapproach yielded rapid progress in B12 synthesis.Finally the pathway beyond percorrin-6 was attacked. The mostfrustrating task faced by the Paris-Cambridge collaboration was thework on the percorrin-8x intermediate because the molecule wasunstable and difficult to study. Although the team ultimately diddetermine the main features of the molecules, the stereochemicaldetails are still needed.At this point, all the synthetic steps are understood and the puzzlingand demanding task of defining vitamin B12 synthesis are completeafter decades of work.061394B12
-- Bonnie Sedlak Special To BioWorld Today
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