The Lipid Library

LIPOPEPTIDES

STRUCTURE, OCCURRENCE AND BIOLOGY

A number of bacterial species produce lipopeptides or peptidolipids, most of which have important biological functions. Many have antibacterial or antifungal properties. They can consist of short linear chains or cyclic structures of amino acids, linked to a fatty acid via ester or amide bonds or both. Often the amino acids are of the D- rather than the usual L-configuration, presumably to resist the action of proteases.

1.   Glycopeptidolipids of Mycobacteria

The glycopeptidolipids or ‘C-mycosides’ from Mycobacteria are amongst the best known and most studied of the lipopeptides, as they are both species and type specific. The illustration below is of a typical member of the glycopeptidolipids of the Mycobacterium avium complex, an important human pathogen that is frequently associated opportunistically with acquired immunodeficiency syndrome (AIDS).

The fatty acid component is often 3-hydroxy-octacosanoate (C₂₈), but it can consist of a range of constituents with an average chain-length of C₃₀ and with variable numbers of double bonds. This is linked to the N-terminus of a tripeptide of hydrophobic amino acids of the D-configuration and thence to L-alaninol and dimethyl-rhamnose. A complex oligosaccharide is linked to the peptide via a disaccharide (deoxy-talose−rhamnose).

Mycobacterial glycopeptidolipids can be classified within two groups – polar and non-polar. Within the M. avium complex, all have in common an N-acylated lipopeptide core attached to a rhamnosylated alaninyl C-terminus. The two groups differ in the structure of the oligosaccharide attached to the allo-threonine residue, which can carry additional O-acyl moieties at undefined locations. In other species of Mycobacteria, the basic structure of the lipopeptide unit does not vary appreciably, but the nature of the carbohydrate moieties does differ, importantly in the degree of substitution of the deoxy-talose and rhamnose units. However, it is the complex and more variable oligosaccharide unit that carries most of the antigenicity and type specificity.

These are variable, distinctive and highly antigenic molecules, and they are located on the external membrane of the organisms. This contains an assortment of extracellular polysaccharides and lipids. The lipid components include phthiocerol dimycocerosates, triacylglycerols and acylated trehaloses, which are common to most species of Mycobacteria, and the glycopeptidolipids, which are variable in structure and are specific to each species. A number of models have been put forward to describe the associations of these various components within the membrane, but a popular model places the lipopeptides in the outermost region of the layer, where they interact with the mycolic acids via hydrophobic attractions.

2.   Lipopeptides from Bacillus species

Bacteria of the genus Bacillus produce a number of cyclic lipopeptides, which are biologically active. For example, various strains of B. subtilis produce more than twenty different molecules with antibiotic activity including many lipopeptides. For example, surfactin (illustrated) in addition to its antibiotic properties is one of the most powerful biosurfactants known. It is composed of seven different amino acids of both the D- and L-configurations, which form a cyclic structure incorporating 3-hydroxy-13-methyl-tetradecanoic acid.

Very similar molecules are produced by other Bacillus species, and the various isoforms have been described under different synonyms, such as bacircine, halo- and isohalobactin, lichenysin, daitocin and pumilacidin.

Rather than the normal ribisomal mechanism of protein synthesis, they are produced by a linear non-ribosomal peptide synthetase, surfactin synthetase, which is a large multi-enzyme complex consisting of four modular building blocks, i.e. the multicarrier thiotemplate mechanism. Such enzyme systems typically contain an enzyme component that activates the initial substrate, one that tethers the covalent intermediates as an enzyme-bound thioester (peptidyl-carrier-protein), an enzyme that carries out peptide bond formation (condensation or C-domain), and a thioesterase domain (tedomain) to ensure the cleavage of the thio ester bond to the nascent peptide and usually to effect cyclization.

The amino acids glycine and asparagine are the main polar components that counterbalance the fatty acyl moiety and give the molecule its amphiphilic character, also explaining its antibiotic activity. Thus, various mechanisms have been proposed, all of which depend on the fact that the hydrocarbon tail of the molecule can insert itself readily into the membranes of both Gram-positive and Gram-negative bacteria where it forms associations with the hydrophilic fatty acid chains of the phospholipids. One suggestion is that the two acid residues are arrange spatially so that they can stabilizes divalent cations, such as Ca²⁺. The proximity of this to the polar head group of the phospholipids in the membrane causes the complex to cross the lipid bilayer via a flip-flop mechanism, delivering the cation into the intracellular medium. Alternatively, self association of surfactin molecules on both sides of an uncharged membrane may create a pore through which cations can pass. A third hypothesis is that such self association of surfactin molecules leads to the formation of mixed micelles and ultimately causes disruption of the bilayer. These effects are non-specific so do not produce resistant strains of bacteria. Indeed, at high concentrations surfactin can disrupt most membranes including those of erythrocytes.

Because of its detergent properties, surfactin has been investigated as a potential bio-remediation agent to assist in the degradation of oil spills and to mop up heavy metals from contaminated soils.

B. subtilis produces two further families of lipopeptide antibiotics, the iturins (bacillomycins, iturins and mycosubtilins) and fengycins (plipastatins). The iturins especially are unusual in that they contain long-chain fatty acids with an amine group in position 3. I am not aware of such fatty acids being present in any other living organism. All of these peptidolipids are under investigation as agents for the control of plant diseases. Not only do they have the potential to act against phytopathogens, including bacteria, fungi and oomycetes, but they also stimulate defence mechanisms in the plant hosts.

B. brevis produces a family of linear pentadecapeptides (gramicidins) with alternating L- and D-amino acids. They enter membranes readily and form ion channels specific for monovalent cations. In addition, this organism produces lipopeptides, consisting of decapeptides (cyclic peptides linked to a linear peptide) and thence to a fatty acid such as 6-methyl-octanoic acid, and termed ‘polymyxins’. These also have antibiotic actions. Six of the amino acids in polymyxin B are the uncommon L-2,4-diaminobutyric acid. Polymixins act by binding to the lipid A moiety of lipopolysaccharides, and they are used to treat a variety of infections caused by Gram-negative bacteria, especially in topical applications, such as wound creams and eye or ear drops. While polymixins have been considered to be too toxic to be used as systemic antibiotics, they are now finding application against multi-drug-resistant Gram-negative bacilli.

3.   Other Lipopeptides

The genus Streptomyces is the source of a large number of antifungal and antibiotic compounds. Streptomyces roseosporus (Actinobacteria), for example, produces daptomycin, which is a cyclic lipopeptide consisting of 13 amino acids, which includes three D-amino acid residues (D-asparagine, D-alanine, and D-serine), linked to decanoic acid. It is licensed for use as an antibiotic against skin infections by a number of Gram-positive organisms. The molecule is too large for absorption by the gastrointestinal tract, and it must be administered intravenously to act against internal infections. The mechanism of action involves calcium-dependent binding of the lipophilic tail of daptomycin to the bacterial plasma membrane, probably in conjunction with an interaction with phosphatidylglycerol, and this results in potassium efflux, membrane disruption, cessation of the synthesis of macromolecules and eventually cell death.

Recommended Reading

• Brennan, P.J. Mycobacterium and other actinomycetes. In: Microbial Lipids. Volume 1. pp. 203-298 (Ed. C. Ratledge & S.G. Wilkinson, Academic Press, London) (1988).
• Chatterjee, D. and Khoo, K.-H. The surface glycopeptidolipids of mycobacteria: structures and biological properties. Cell. Mol. Life Sci., 58, 2018-2042 (2001).
• Kirkpatrick, P., Raja, A., LaBonte, J., Lebbos, J. Daptomycin. Nat. Rev. Drug. Discov., 2, 943–944 (2003).
• Landman, D., Georgescu, C., Martin, D.A. and Quale, J. Polymyxins revisited. Clin. Microbiol.Rev., 21, 449-465 (2008).
• Ongena, M. and Jacques, P. Bacillus lipopeptides: versatile weapons for plant disease biocontrol. Trends Microbiol., 16, 115-125 (2008).
• Stein, T. Bacillus subtilis antibiotics: structures, syntheses and specific functions. Mol. Microbiol., 56, 845-857 (2005).

Updated: 1/9/2008 Credits/disclaimer	Scottish Crop Research Institute (and MRS Lipid Analysis Unit), Invergowrie, Dundee (DD2 5DA), Scotland	Right click on the icon to open in a new window