The biosynthesis of gram-negative endotoxin. Formation of lipid A disaccharides from monosaccharide precursors in extracts of Escherichia coli.

Bryan,Ray,G. Painter,Christian R. H. Raetzg

Published 1984 in Journal of Biological Chemistry

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.

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