title: The biosynthesis of gram-negative endotoxin. Formation of lipid A disaccharides from monosaccharide precursors in extracts of Escherichia coli.
abstract: We have discovered an enzyme in the cytosol of Escherichia coli that generates lipid A disaccharides from monosaccharide precursors by the following route: 2,3-diacyl-GlcN-1-P + UDP-2,3-diacyl-GlcN---- 2,3-diacyl-GlcN (beta, 1----6) 2,3-diacyl-GlcN-1-P + UDP. Previous studies from our laboratory have documented the presence in vivo of the precursors 2,3-diacylglucosamine 1-phosphate (2,3-diacyl-GlcN-1-P) (lipid X of E. coli) and UDP-2,3-diacylglucosamine (UDP-2,3-diacyl-GlcN) (Bulawa, C.E., and Raetz, C.R.H.J. Biol. Chem. 259, 4846-4851). Both substrates are novel glucosamine-derived phospholipids, acylated with beta-hydroxymyristoyl moieties, and they accumulate in E. coli mutants defective in the pgsB gene. Synthetic ADP-, GDP-, and CDP-2,3-diacylglucosamines are inefficient substrates compared to the naturally occurring UDP derivative. The free-acid form of the tetraacyldisaccharide 1-phosphate product (C68H129N2O20P) that is generated in vitro has Mr = 1325.74 as judged by fast atom bombardment mass spectrometry. Mild acid hydrolysis (0.1 M HCl for 30 min at 100 degrees C) liberates greater than 95% of the phosphate moiety as Pi. Detailed analysis by 1H and 13C NMR spectroscopy confirms the presence of a phosphate residue at position 1 of the disaccharide, an alpha-anomeric configuration at the reducing end, and a beta, 1----6 linkage between the two glucosamines. Importantly the disaccharide 1-phosphate synthase is missing in extracts of E. coli strains harboring the pgsB1 mutation, consistent with the massive accumulation of 2,3-diacyl-GlcN-1-P and UDP-2,3-diacyl-GlcN in vivo. The enzymatic reaction reported here represents a major biosynthetic route for the formation of lipid A disaccharides in E. coli and other Gram-negative bacteria. An in vitro system for the biosynthesis of lipid A disaccharides has not been described previously.
mesh_terms: D004187:Disaccharides / Q000096:biosynthesis*; D004731:Endotoxins / Q000096:biosynthesis*; D004926:Escherichia coli / Q000378:metabolism*; D005958:Glucosephosphates / Q000378:metabolism; D006017:Glycolipids / Q000378:metabolism; D007700:Kinetics; D008050:Lipid A / Q000096:biosynthesis*; D009682:Magnetic Resonance Spectroscopy; D013058:Mass Spectrometry; D008958:Models, Molecular; D008968:Molecular Conformation; D010761:Phosphorus Radioisotopes; D014537:Uridine Diphosphate N-Acetylglucosamine / Q000031:analogs & derivatives / Q000378:metabolism
chemical_list: D004187:Disaccharides; D004731:Endotoxins; D005958:Glucosephosphates; D006017:Glycolipids; D008050:Lipid A; D010761:Phosphorus Radioisotopes; D014537:Uridine Diphosphate N-Acetylglucosamine; C041030:UDP-N(2),3-di(3-hydroxymyristoyl)glucosamine