Pathway · 5 proteins
Lipid A and outer envelope
Kdo and lipid-A-related enzymes build distinct envelope components.
How it works
- The lipid-A route begins with acyl-ACP--UDP-N-acetylglucosamine O-acyltransferase and UDP-3-O-acyl-N-acetylglucosamine deacetylase, which set up the acylated sugar that will become the membrane anchor.
- UDP-3-O-(3-hydroxymyristoyl)glucosamine N-acyltransferase, lipid-A-disaccharide synthase and a 3-deoxy-D-manno-octulosonic acid transferase extend the disaccharide and attach the Kdo sugars, while LpxE removes the 1-phosphate.
- An MlaE family permease and a lysophospholipid acyltransferase maintain the lipid asymmetry of the outer membrane. These are annotated capabilities assembled from the genome; whether the complete outer-membrane assembly operates as it does in model Gram-negative bacteria is a separate experimental question.
Grouped by the atlas’s curated pathway map. The table order is not a proven reaction order.
Its proteins
| Protein | EC | Reaction (UniProt) | Metal or cluster | Amino acids | Certainty |
|---|---|---|---|---|---|
| Lipid-A-disaccharide synthase | 2.4.1.182 | a lipid X + a UDP-2-N,3-O-bis[(3R)-3-hydroxyacyl]-alpha-D-glucosamine = a lipid A disaccharide + UDP + H(+) | — | 356 | Step named |
| Phosphatidic acid phosphatase type 2/haloperoxidase domain-containing protein | — | — | — | 174 | System only |
| DUF374 domain-containing protein | — | — | — | 133 | System only |
| 2-acylglycerophosphoethanolamine acyltransferase | — | — | — | 211 | Step named |
| 3-deoxy-D-manno-octulosonic acid transferase | 2.4.99.12 | lipid IVA (E. coli) + CMP-3-deoxy-beta-D-manno-octulosonate = alpha-Kdo-(2->6)-lipid IVA (E. coli) + CMP + H(+) | — | 353 | Step named |
Also part of this pathway
These proteins are described under another pathway, but also take part here.
| Protein | EC | Reaction (UniProt) | Metal or cluster | Amino acids | Certainty |
|---|---|---|---|---|---|
| UDP-3-O-acyl-N-acetylglucosamine deacetylaseHome: Lipid A and outer envelope | 3.5.1.108 | a UDP-3-O-[(3R)-3-hydroxyacyl]-N-acetyl-alpha-D-glucosamine + H2O = a UDP-3-O-[(3R)-3-hydroxyacyl]-alpha-D-glucosamine + acetate | Zn(2+) | 282 | Step named |
| UDP-3-O-acylglucosamine N-acyltransferaseHome: Lipid A and outer envelope | 2.3.1.191 | a UDP-3-O-[(3R)-3-hydroxyacyl]-alpha-D-glucosamine + a (3R)-hydroxyacyl-[ACP] = a UDP-2-N,3-O-bis[(3R)-3-hydroxyacyl]-alpha-D-glucosamine + holo-[ACP] + H(+) | — | 326 | Step named |
| Acyl-[acyl-carrier-protein]--UDP-N-acetylglucosamine O-acyltransferaseHome: Lipid A and outer envelope | 2.3.1.129 | a (3R)-hydroxyacyl-[ACP] + UDP-N-acetyl-alpha-D-glucosamine = a UDP-3-O-[(3R)-3-hydroxyacyl]-N-acetyl-alpha-D-glucosamine + holo-[ACP] | — | 261 | Step named |
Origins
How could an outer membrane anchored by lipid A first form?
None of the books searched describes a prebiotic route to lipid A or to an outer membrane.
Today the barrel proteins of the outer membrane are folded and inserted by the BAM complex, whose core protein is itself an outer-membrane barrel. The machinery that builds the envelope sits inside the envelope it builds.
From the books: Alberts et al., Molecular Biology of the Cell, p. 733.
The wider question for this part of the cell: origins for building and maintaining the membrane.
Sources
- Atlas pathway grouping from the VF5 reference-genome system map (Deckert et al. 1998; Braakman & Smith 2014)
Other pathways in this group: Glycerolipid assembly · Fatty-acid elongation · Substrate recognition · Terpenoid backbone biosynthesis · Fatty acid biosynthesis · Lipopolysaccharide biosynthesis · Biosynthesis of various nucleotide sugars · Not yet placed in a pathway