Amide-linked fatty acyl residues

A pentaacyl lipid A is also present in P. aeruginosa (77) (Fig. 11 A). Here, the main lipid A species contains a total of five fatty acids, and a minor hexaacyl species (Fig. 1 IB) corresponds structurally to lipid A of C. viola-ceum (Fig. 10). The prominent pentaacyl component, which makes up approximately 75% (w/w) of P. aeruginosa lipid A, encompasses three structural forms that all possess the same /H1 — 6)-linked GlcpN backbone, but with only three (primary) 3-hydroxy fatty acids attached to positions 3, 2, 3, and 2 (Fig. 11A). These structural forms differ from each other by the 3-0-acylation of each of the two amide-linked 12 0(3-OH) residues by the secondary acyl groups 12 0 or 12 0(2-OH), as indicated by the dashed lines. Of the four conceivable structural types, the one bearing two 12 0(2-OH) residues is not present. 

A similar distribution of fatty acids has also been detected in lipid A of other bacteria (Fig. 5). Thus, in Fusobacterium nucleatum, 2 moles of (R)-3-OH-14 0 are ester-bound, one of which is 3-O-acylated by 14 0. In amide linkage, (R)-3-0(14 0)-16 0 is present. In Vibrio cholerae, a dimer of (R)-3-OH-12 0 is bound as an ester while (R)-3-0-(14 0)-14 0 and (R)-3-0-(16 0)-14 0 are amide-linked. The lipid A component of Chromobacterium violaceum possesses 2 moles of (R)-3-OH-10 0 in ester linkage. The amide-bound acyl groups are represented by (R)-3-0H-12 0 residues which are 3-0-acylated by 12 0 and (S)-2-OH-12 0. In P. mirabilis, 3-0H-14 0 is, like in Salmonella, ester-and amide-bound. In this case, however, exclusively 14 0 substitutes the 3-hydroxyl groups of both 0- and N-linked 3-OH-14 0. 

The chemical structure of lipid A of lipopolysaccharide isolated from Comamonas testosteroni was recently determined by lida et al. (1996) by means of methylation analysis, mass spectrometry and NMR. The lipid A backbone was found to consist of 6-0-(2-deoxy-2-amino-P-D-glucopyrano-syl)-2-deoxy-2-amino-alpha-D-glucose which was phosphorylated in positions 1 and 4. Hydroxyl groups at positions 4 and 6 were unsubstituted, and position 6 of the reducing terminal residue was identified as the attachment site of the polysaccharide group. Fatty acid distribution analysis and ES/MS of lipid A showed that positions 2,2, 3 and 3 of the sugar backbone were N-acylated or O-acylated by R-3-hydroxydecanoic acid and that the hydroxyl groups of the amide-linked residues attached to positions 2 and 2 were further O-acylated by tetradecanoic and dodecanoic acids, respectively. 

Amide bonds are found in many proteins. One is the acyl carrier protein of Escherichia coli (see 90), which contains the peptide backbone, and a 4 -phosphopantetheine unit (in violet in the illustration) is attached to a serine residue. Note the amine bonds in the pantothenic acid unit and also the 0-P=0 unit, which is a phosphate ester (an ester of phosphoric acid). An acyl carrier protein is involved in fatty acid synthesis, linking acetyl and malonyl groups from acetyl coenzyme A and malonyl coenzyme A to form P-keto acid acyl carrier protein (abbreviated as ACP). The widely utilized acetyl CoA is an ester (91) attached to coenzyme A. Acetyl CoA is a key intermediate in aerobic intermediary metabolism of carbohydrates, lipids, and some amino acids. 

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