If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Locations of Mapped Genes:
|Superclasses:||Biosynthesis → Fatty Acids and Lipids Biosynthesis|
Bacterial lipopolysaccharides (LPS) are unique and complex glycolipids that provide characteristic components of the outer monolayer of the outer membranes of gram-negative bacteria. Wild type LPS is composed of a hydrophobic domain lipid A (or endotoxin), a phosphorylated "core" oligosaccharide (core OS) and a distal O-specific polysaccharide (or O-antigen). Strains of E. coli K-12 normally do not make O-antigen, unless a mutation in the O-antigen operon is corrected [Stevenson94]. O antigens are not needed for growth in the laboratory, but they protect bacteria from antibiotics and complement-mediated lysis. The lipid A and Kdo domains of LPS are required for growth [Raetz07, Raetz02].
The lipid A (endotoxin) domain of LPS is a unique, glucosamine-based saccharolipid that serves as the hydrophobic anchor of LPS. It is the bioactive component of the molecule that is associated with gram-negative septic shock [Trent04]. There are approximately 106 lipid A residues in an E. coli cell [Raetz07]. Lipid A of E. coli consists of a β (1'->6)-linked D-glucosamine disaccharide that carries four (R)-3-hydroxytetradecanoyl groups in positions 2, 3, 2', and 3' and two phosphoryl residues in positions 1 and 4'. The hydroxy fatty acids at positions 2' and 3' are acylated at their 3-hydroxyl groups by dodecanoic acid and tetradecanoic acid, respectively [Rietschel87]. Given their conserved architecture, most types of lipid A molecules are detected at picomolar levels by TLR4 (toll-like receptor 4) of the innate immune system present on macrophages and endothelial cells of animals [Raetz02].
The core oligosaccharides are conceptually divided into two regions: the inner core (lipid A proximal) and the outer core. The inner core is highly conserved. It comprises three 3-deoxy-D-manno-oct-2-ulosonic acid (Kdo) and L-glycero-D-manno-heptose (Hep) residues and it is often phosphorylated. Structural diversity in the core oligosaccharide is limited by the constraints imposed by its essential role in outer membrane stability [Whitfield03, Heinrichs98]. E. coli mutants that lack the heptose region of the inner core display the "deep rough" phenotype characterized by changes in both the structure and the composition of the outer membrane leading to its instability [Irvin79]. Deep rough mutants of E. coli activate expression of colanic acid exopolysaccharide, lose expression of pili and flagella and secrete a form of hemolysin with reduced hemolytic activity [Parker92]. The outer core comprises a tri-hexose backbone modified with varying side-branch substitutions of hexose and acetamidohexose residues. The outer core region provides an attachment site for O antigen [Heinrichs98]. Mutations that eliminate synthesis of the outer core result in increased susceptibility to some hydrophobic compounds due to an indirect effect on core phosphorylation [Austin90].
About This Pathway
E. coli lipid A is synthesized on the cytoplasmic surface of the inner membrane by a conserved pathway of nine constitutive enzymes. The first reaction of lipid A biosynthesis is the acylation of the sugar nucleotide UDP-GlcNAc by LpxA. Deacetylation of the product, UDP-3-O-(acyl)-GlcNAc by the zinc metalloenzyme LpxC is the committed reaction of the pathway. LpxC is an excellent target for the design of novel, gram-negative specific antibiotics. Following deacetylation, a second beta-hydroxymyristate moiety is incorporated by LpxD to generate UDP-2,3-diacylglucosamine. UDP-2,3-diacylglucosamine is cleaved at its pyrophosphate bond by the highly selective pyrophosphatase LpxH to form 2,3-diacylglucosamine-1-phosphate (lipid X). A beta,1'-6 linked disaccharide is then generated by the condensation of another molecule of UDP-2,3-diacylglucosamine with lipid X by LpxB. A specific kinase, LpxK next phosphorylates the 4' position of the disaccharide to form lipid IVA. The kinase product, lipid IVA, is of great interest because it possesses some of the properties of endotoxins [Raetz02].
E. coli LPS contains two Kdo residues that are transferred to lipid IVA by a bifunctional enzyme, encoded by waaA. The chromosomal waa region (formerly rfa) contains the major core-oligosaccharide assembly operons, and E. coli K-12 provided the first waa region sequenced in its entirety [Whitfield03]. The last steps of E. coli lipid A biosynthesis involve the addition of lauroyl and myristoyl residues to the distal glucosamine unit (generating acyloxyacyl moieties by LpxL and LpxM respectively. The completed lipid A-Kdo2 serves as the acceptor on which the core oligosaccharide and lipo-oligosaccharide chains are assembled.
The lipid A-core portion (known as rough LPS (R-LPS)) is synthesized by a conserved pathway. The mechanism involves sequential glycosyl transfer from nucleotide sugar precursors with a co-coordinated complex of membrane-associated glycosyltransferases (Waa proteins) acting at the cytoplasmic surface of the inner membrane (IM). The modification of the core region of E. coli LPS requires the action of three enzymes, viz. WaaP (an LPS kinase), WaaY (an enzyme required for a second phosphorylation) and WaaQ (a transferase that adds the side-branch heptose). The WaaP enzyme is the most important of these activities since the modifications must proceed in the strict order WaaPQY. Mutants lacking WaaP activity exhibit the deep rough phenotype and are avirulent. Mutant LPSs lacking the outer core (waaG) are phosphorylated inefficiently by WaaP [Yethon00]. Conversely, the loss of phosphorylation results in a decreased efficiency of core extension, leading to more truncated cores than seen in the wild type.
The lipid A-core is then flipped to the outer surface of the inner membrane by the ATP-binding cassette (ABC) transporter, MsbA. An additional integral membrane protein, YhjD, has recently been implicated in LPS export across the IM. The smallest LPS derivative that supports viability in E. coli is lipid IVA. However, it requires mutations in either MsbA or YhjD, to suppress the normally lethal consequence of an incomplete lipid A [Tran08]. Recent studies with deletion mutants implicate the periplasmic protein LptA, the cytosolic protein LptB, and the IM proteins LptC, LptF, and LptG in the subsequent transport of nascent LPS to the outer membrane (OM), where the LptD/LptE complex flips LPS to the outer surface [Ma08].
Austin90: Austin EA, Graves JF, Hite LA, Parker CT, Schnaitman CA (1990). "Genetic analysis of lipopolysaccharide core biosynthesis by Escherichia coli K-12: insertion mutagenesis of the rfa locus." J Bacteriol 172(9);5312-25. PMID: 2168379
Heinrichs98: Heinrichs DE, Yethon JA, Whitfield C (1998). "Molecular basis for structural diversity in the core regions of the lipopolysaccharides of Escherichia coli and Salmonella enterica." Mol Microbiol 30(2);221-32. PMID: 9791168
Irvin79: Irvin RT, Lam J, Costerton JW (1979). "Structural and biochemical examination of ghosts derived from a deep rough (heptose-deficient lipopolysaccharide) strain and a smooth strain of Escherichia coli." Can J Microbiol 25(4);436-46. PMID: 385128
Ma08: Ma B, Reynolds CM, Raetz CR (2008). "Periplasmic orientation of nascent lipid A in the inner membrane of an Escherichia coli LptA mutant." Proc Natl Acad Sci U S A 105(37);13823-8. PMID: 18768814
Parker92: Parker CT, Kloser AW, Schnaitman CA, Stein MA, Gottesman S, Gibson BW (1992). "Role of the rfaG and rfaP genes in determining the lipopolysaccharide core structure and cell surface properties of Escherichia coli K-12." J Bacteriol 174(8);2525-38. PMID: 1348243
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Rietschel93: Rietschel ET, Kirikae T, Schade FU, Ulmer AJ, Holst O, Brade H, Schmidt G, Mamat U, Grimmecke HD, Kusumoto S (1993). "The chemical structure of bacterial endotoxin in relation to bioactivity." Immunobiology 187(3-5);169-90. PMID: 8330896
Stevenson94: Stevenson G, Neal B, Liu D, Hobbs M, Packer NH, Batley M, Redmond JW, Lindquist L, Reeves P (1994). "Structure of the O antigen of Escherichia coli K-12 and the sequence of its rfb gene cluster." J Bacteriol 1994;176(13);4144-56. PMID: 7517391
Tran08: Tran AX, Trent MS, Whitfield C (2008). "The LptA protein of Escherichia coli is a periplasmic lipid A-binding protein involved in the lipopolysaccharide export pathway." J Biol Chem 283(29);20342-9. PMID: 18480051
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Yethon00: Yethon JA, Vinogradov E, Perry MB, Whitfield C (2000). "Mutation of the lipopolysaccharide core glycosyltransferase encoded by waaG destabilizes the outer membrane of Escherichia coli by interfering with core phosphorylation." J Bacteriol 182(19);5620-3. PMID: 10986272
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Anderson85a: Anderson MS, Bulawa CE, Raetz CR (1985). "The biosynthesis of gram-negative endotoxin. Formation of lipid A precursors from UDP-GlcNAc in extracts of Escherichia coli." J Biol Chem 1985;260(29);15536-41. PMID: 3905795
Anderson87a: Anderson MS, Raetz CR (1987). "Biosynthesis of lipid A precursors in Escherichia coli. A cytoplasmic acyltransferase that converts UDP-N-acetylglucosamine to UDP-3-O-(R-3-hydroxymyristoyl)-N-acetylglucosamine." J Biol Chem 262(11);5159-69. PMID: 3549716
Anderson88b: Anderson MS, Robertson AD, Macher I, Raetz CR (1988). "Biosynthesis of lipid A in Escherichia coli: identification of UDP-3-O-[(R)-3-hydroxymyristoyl]-alpha-D-glucosamine as a precursor of UDP-N2,O3-bis[(R)-3-hydroxymyristoyl]-alpha-D-glucosamine." Biochemistry 1988;27(6);1908-17. PMID: 3288280
Anderson93: Anderson MS, Bull HG, Galloway SM, Kelly TM, Mohan S, Radika K, Raetz CR (1993). "UDP-N-acetylglucosamine acyltransferase of Escherichia coli. The first step of endotoxin biosynthesis is thermodynamically unfavorable." J Biol Chem 1993;268(26);19858-65. PMID: 8366124
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Babinski02: Babinski KJ, Ribeiro AA, Raetz CR (2002). "The Escherichia coli gene encoding the UDP-2,3-diacylglucosamine pyrophosphatase of lipid A biosynthesis." J Biol Chem 277(29);25937-46. PMID: 12000770
Babinski02a: Babinski KJ, Kanjilal SJ, Raetz CR (2002). "Accumulation of the lipid A precursor UDP-2,3-diacylglucosamine in an Escherichia coli mutant lacking the lpxH gene." J Biol Chem 277(29);25947-56. PMID: 12000771
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Bartling09: Bartling CM, Raetz CR (2009). "Crystal structure and acyl chain selectivity of Escherichia coli LpxD, the N-acyltransferase of lipid A biosynthesis." Biochemistry 48(36);8672-83. PMID: 19655786
Belunis92: Belunis CJ, Raetz CR (1992). "Biosynthesis of endotoxins. Purification and catalytic properties of 3-deoxy-D-manno-octulosonic acid transferase from Escherichia coli." J Biol Chem 1992;267(14);9988-97. PMID: 1577828
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