Protein and is connected to the remainder of the protein by a long proline plus alanine-rich linker region (9). The modified subunits form noncovalent interactions with other members of a protein complex of the three or four protein species that constitute the active enzyme. The cofactor-modified domains then shuttle intermediates between the multiple active sites of the enzyme complex (9). The mobility of the domains is due to the proline-alanine linkers and the domains constitute the distal ends (the “hands”) of the swinging arms long ago postulated for these enzyme complexes. These arrangements can be 11-Deoxojervine supplier considered as providing substrate channeling via covalent attachment (9). Finally both biotin and lipoic acid are needed in only trace quantities. In E. coli only a few AcadesineMedChemExpress AICA Riboside hundred molecules of biotin per cell are sufficient for growth (10) and the requirement for lipoic acid is similar. Therefore, the enzymes of these pathways are expressed at very low levels (< 350 molecules/cell [11]) and the enzymes have generally low turnover numbers.Author Manuscript Author Manuscript Author Manuscript Author ManuscriptSynthesis of BiotinThe early steps of biotin biosynthesis are not well understood in any organism, but clearly differ between E. coli and Bacillus subtilis. In both cases a seven carbon dicarboxylic acid, pimelic acid is assembled with one of its carboxyl groups in thioester linkage. PimeloylCoA has long been thought to be the thioester-activated form of pimelic acid, but recent evidence indicates a role for the acyl carrier protein (ACP) of fatty acid synthesis as the thiol moiety (12, 13). In contrast the steps that follow formation of the pimeloyl-thioester are well conserved throughout biology even in organisms (e.g., Saccharomyces cerevisiae) that lackEcoSal Plus. Author manuscript; available in PMC 2015 January 06.CronanPagethe ability to perform any early biosynthetic steps. In E. coli the atoms of biotin are derived from rather disparate sources, acetate, alanine, CO2, S-adenosylmethionine (SAM) and sulfide. Two groups have traced the origins of the biotin and dethiobiotin carbon atoms by 13C labeling followed by analysis by 13C NMR (14, 15). Using the numbering system of Figure 1, the C-3, C-5, and C-7 carbon atoms of biotin are derived from C-1 of acetate whereas the C-2 of acetate contributes the biotin C-2, C4, and C-6 carbon atoms. Acetate labeled in both carbon atoms is incorporated intact as shown by 13C coupling. Biotin carbon atoms C-9 and C10 are contributed by L-alanine. The C-1 and ureido (C-2) carbon atoms are derived from CO2 (14). The nitrogen atom adjacent to C7 is from SAM whereas the other nitrogen atom is from alanine. The labeling pattern is consistent with formation of a pimelic acid moiety by head to tail incorporation of three intact acetate units as is the case in fatty acid (or polyketide) synthesis (14, 15) and the labeling pattern eliminates other plausible pathways from tryptophan, lysine, diaminopimelic acid or elongation of 2-oxoglutarate (15). Moreover, the 13C labeling results eliminate free pimelic acid as an intermediate in biotin biosynthesis. Pimelic acid is a symmetrical dicarboxylic acid whose carboxyl groups cannot be stereochemically distinguished and if free pimelic acid is an intermediate, then biotin carbon atoms C-1 and C-7 would have the same labeling pattern. This is not the case (14, 15) and thus, the pimelate moiety must be assembled with one of the carboxyl groups covalently link.Protein and is connected to the remainder of the protein by a long proline plus alanine-rich linker region (9). The modified subunits form noncovalent interactions with other members of a protein complex of the three or four protein species that constitute the active enzyme. The cofactor-modified domains then shuttle intermediates between the multiple active sites of the enzyme complex (9). The mobility of the domains is due to the proline-alanine linkers and the domains constitute the distal ends (the "hands") of the swinging arms long ago postulated for these enzyme complexes. These arrangements can be considered as providing substrate channeling via covalent attachment (9). Finally both biotin and lipoic acid are needed in only trace quantities. In E. coli only a few hundred molecules of biotin per cell are sufficient for growth (10) and the requirement for lipoic acid is similar. Therefore, the enzymes of these pathways are expressed at very low levels (< 350 molecules/cell [11]) and the enzymes have generally low turnover numbers.Author Manuscript Author Manuscript Author Manuscript Author ManuscriptSynthesis of BiotinThe early steps of biotin biosynthesis are not well understood in any organism, but clearly differ between E. coli and Bacillus subtilis. In both cases a seven carbon dicarboxylic acid, pimelic acid is assembled with one of its carboxyl groups in thioester linkage. PimeloylCoA has long been thought to be the thioester-activated form of pimelic acid, but recent evidence indicates a role for the acyl carrier protein (ACP) of fatty acid synthesis as the thiol moiety (12, 13). In contrast the steps that follow formation of the pimeloyl-thioester are well conserved throughout biology even in organisms (e.g., Saccharomyces cerevisiae) that lackEcoSal Plus. Author manuscript; available in PMC 2015 January 06.CronanPagethe ability to perform any early biosynthetic steps. In E. coli the atoms of biotin are derived from rather disparate sources, acetate, alanine, CO2, S-adenosylmethionine (SAM) and sulfide. Two groups have traced the origins of the biotin and dethiobiotin carbon atoms by 13C labeling followed by analysis by 13C NMR (14, 15). Using the numbering system of Figure 1, the C-3, C-5, and C-7 carbon atoms of biotin are derived from C-1 of acetate whereas the C-2 of acetate contributes the biotin C-2, C4, and C-6 carbon atoms. Acetate labeled in both carbon atoms is incorporated intact as shown by 13C coupling. Biotin carbon atoms C-9 and C10 are contributed by L-alanine. The C-1 and ureido (C-2) carbon atoms are derived from CO2 (14). The nitrogen atom adjacent to C7 is from SAM whereas the other nitrogen atom is from alanine. The labeling pattern is consistent with formation of a pimelic acid moiety by head to tail incorporation of three intact acetate units as is the case in fatty acid (or polyketide) synthesis (14, 15) and the labeling pattern eliminates other plausible pathways from tryptophan, lysine, diaminopimelic acid or elongation of 2-oxoglutarate (15). Moreover, the 13C labeling results eliminate free pimelic acid as an intermediate in biotin biosynthesis. Pimelic acid is a symmetrical dicarboxylic acid whose carboxyl groups cannot be stereochemically distinguished and if free pimelic acid is an intermediate, then biotin carbon atoms C-1 and C-7 would have the same labeling pattern. This is not the case (14, 15) and thus, the pimelate moiety must be assembled with one of the carboxyl groups covalently link.
