Origin of carbon atoms of biotin. 13C-NMR studies on biotin biosynthesis in Escherichia coli.
The origin of the carbon atoms of pimeloyl-CoA, the earliest known precursor in the pathway of de novo biotin biosynthesis in Escherichia coli, was investigated by 13C-NMR spectroscopy. In fermentation of the biotin-overproducing DRK332/pXBA312 strain of Escherichia coli (a repressor mutant carrying a biotin operon fragment in the plasmid), a high dose of L-alanine (8 g/l) stimulated dethiobiotin and biotin accumulation. Although L-alanine is a known precursor of 7-keto-8-aminopelargonic acid in biotin biosynthesis, the 13C-NMR spectrum of dethiobiotin showed that the C-3 of L-[3-13C]alanine was incorporated into not only the methyl carbon (C-9) but also alternate carbons (C-2, C-4, C-6) of the side chain, and these latter positions are the same as those labeled with D-[1-13C]glucose. These data indicate that L-alanine can act as an alternative carbon source, suggesting that acetyl-CoA is a possible precursor for pimeloyl-CoA synthesis. In accordance with this hypothesis, the C-1 of sodium (1-13C)acetate and the C-2 of sodium (2-13C)acetate were incorporated into alternate carbons in the side chain of dethiobiotin, i.e., (C-1, C-3, C-5, C-7) and (C-1, C-2, C-4, C-6), respectively. These results suggested firstly that in E. coli pimeloyl-CoA is biosynthesized from L-alanine and/or acetate via acetyl-CoA, but not via pimelic acid, which has been suggested as a biotin precursor in other species, and secondly that the carboxyl group of biotin originates from carbon dioxide produced through the tricarboxylic acid cycle.