Lipid A Biosynthesis in E. coli

Figure 2. The constitutive pathway for lipid A biosynthesis in E. coli. The genes encoding the proteins which catalyze each reaction are shown. Nine enzymes are required for the biosynthesis of Kdo2-iipid A, also known as Re endotoxin.

As mentioned inO Sect. 3.1.1, the lipid A and the core oligosaccharides can undergo structural modifications in response to environmental signals [7]. Modifications of lipid A usually entail removal or modification of the phosphates and fatty acyl chains (O Fig. 15). Modifications occur at both early and late stages of OM biosynthesis. In E. coli grown at low tem-... 

Brozek, K. A., Bulawa, C. E., and Raetz, C. R. H. (1987). Biosynthesis of lipid A precursors in Escherichia coli A membrane bound enzyme that transfers a palmitoyl residue from a glycero-phospholipid to lipid X. J. Biol. Chem. 262, 5170-5179. 

Table 1.1 Information on nine enzymes required for the biosynthesis of Kdo2-lipid A in E. coli...

Fig. 1.1 Structure and biosynthetic pathway of Kdo2-lipid A in E. coli. Each reaction is catalyzed by a single enzyme. The names of the enzyme and substrate are highlit. The carbon position and the carbon number of fatty acid chains in lipid A are labeled. The genes encoding the enzymes of Kdo2-lipid A biosynthesis are present in single copy and highly conserved among bacteria (Raetz et al., 2007 Raetz and Whitfield, 2002)...

Fig. 7. Biosynthesis of endotoxin in E. coli. The first step (1) in the pathway is catalyzed by UDP-A -acetylglucosamine (UDP-GlcNAc) acyltransferase (LpxA). (2) The committed step is catalyzed by the LpxC deacetylase, followed by (3) a second acyltransferase (LpxD). (4) Lipid X is generated by the removal of UMP from UDP-2,3-diacyl-GlcN by an unknown enzyme. (5) Lipid X and UDP-2,3-diacyl-GlcN are then condensed together by LpxD to form Lipid IV. (6) A 4 -kinase phos-phorylates the disaccharide to produce lipid IV. (7) Two consecutive additions of KDO by KdtA, and two 0-acylations by (8) HtrB and (9) MsbB yield KDOj-lipid A. Subsequent addition of core sugars and 0-antigen chains (not shown) yield the mature LPS.

A review of the isolation and separation of phosphatldes and glyco-lipids, by solvent fractionation, preparative column chromatography, and h.p.l.c., has appeared. Lipid X (16) and Lipid Y (17)> rel-ated to lipid A and found in E. coli mutants, have been synthesized. Biologically active monosaccharide Lipid A analogues (18) have been synthesized from the appropriate I -substituted butyl glucosaminide. Reference is made to tri- and tetrasaccharide pyrophosphate esters, intermediates in N-glycoprotein biosynthesis. 

U.9 Phospholipid biosynthesis (general).- A few only of the very many papers published in this area that appear to have particular mechanistic importance can be described here. Stable carbon isotope ratios ( C/ C) at natural abundance levels were determined for each of the major fatty acid components of the phospholipids of E. coli. The results were consistent with a model of lipid metabolism in which fatty acids were released from the fatty acid synthase in free form, and required re-activation to the acyl-acyl carrier protein prior to esterification. A close coupling of fatty acid and phospholipid synthesis was implied. The characteristics of fatty acid transfer from acyl-acyl carrier protein to sn-glyoerol- -phosphate in E. coli have been inves-... 

Figure 4 (A) Coordinated interaction of members of the condensing (KAS) enzyme family results in biosynthesis of fatty acids in E. coli. The genes encoding each KAS as well as major destinations of the fatty acyl products are shown. Lipoic acid is a precursor of the coenzyme lipoamide. Lipid A consists of p-hydroxymyristate linked to saccharides in the cell membrane. PL are the membrane phospholipids. (B) How do KASes interact with one another and the other members of the FAS complex ACP, acyl carrier protein KR, p-ketoacyl-ACP reductase DH, P-hydroxyacyl-ACP dehydrase ER, enoylacyl-ACP reductase MAL TR, malonyl-CoA ACP transacylase TE, thioesterase AC TR acetyl-CoA ACP transacylase whose contribution to fatty acid synthesis is uncertain since the discovery and characterization of KAS III [33,38].

The gene encoding the palmitoyl transferase has recently been identified in Salmonella as the PhoPQ-activated gene pagP [51, 54, 55]. Overexpression of the E. coli homolog, crcA (now refered to as IpxY), leads to massive overproduction of palmitoyl transferase activity [51]. Analysis of inner and outer membrane fractions for palmitoyl transferase activity shows that LpxY (Figure 3) is located in the outer membrane [51], This is in contrast to the inner membrane localization of all other enzymes involved in lipid A biosynthesis. 

Figure 3. Other regulated acylation reactions involved in E. coli lipid A biosynthesis under special conditions. The reactions catalyzed by LpxP (formerly known as Ddg) [59] and LpxY (also known as PagP or CrcA) [51] are shown. LpxP is a cold induced protein which incorporates an unsaturated 16 carbon fatty acyl chain in place of the laurate normally incorporated by HtrB. LpxY, the expression of which is activated by the PhoP/PhoQ system [51], is an outer membrane protein that incorporates palmitate to make hepta-acylated lipid A. LpxY is capable of acylating lipid X, lipid IVa, and lipid A, as well as Kdo2-lipid IVa (as shown). The physiological substrate is probably lipid A, given that LpxY is an outer membrane protein.

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