Secondary fatty acyl residue

Once the secondary fatty acyl residue(s), VLCFA, has been added to the Kd02lipid-IVA, a number of the core glycosyl residues are added and then, presumably, the resulting COS-LA molecule is flipped to the periplasmic side of the inner membrane after which the OPS is ligated to the COS-LA which is then translocated to the outer leaflet of the outer membrane. The enzymes which modify the... 

In summary, the biosynthesis of rhizobial LA consists of conserved steps found in numerous Gram-negative bacteria that involve the synthesis of Kd02lipid-IVA from UDP-GlcNAc. This is followed by the addition of a unique VLCFA as a secondary fatty acyl residue which is found in all members of the Rhizobiales (except for A. caulinodans) as well as in certain intracellular pathogens that form chronic disease, some of which are phylogenetically related to rhizobia. After Kd02lipid-IVA synthesis, there are unique enzymes in different rhizobial species that processes this precursor into the unique rhizobial LA. 

Lipid A contains primary fatty acids directly linked to hydroxyl and amino groups of the backbone, and secondary fatty acids bound to hydroxyl groups provided by the primary acyl residues. The number of carbon atoms of primary and secondary fatty acids is, in the majority of lipid A studied, in the range of 10 to 18. They are, in general, saturated, even-numbered, and straight-chain fatty acids and in only few cases, are unsaturated, odd-numbered, and iso- and ante-iso branched derivatives present in molar amounts. 

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. 

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