1.D.11 The Cyclic Lipopeptide Surfactin (Surfactin) Family
Iturins belong to a family of lipopeptides extracted from the culture media of Bacillus subtilis. These amphiphilic compounds are characterized by a peptide ring of seven amino acid residues including an invariable D-Tyr2, with the constant chiral sequence LDDLLDL closed by a C14-C17 aliphatic beta-amino acid (Maget-Dana and Peypoux 1994). They exhibit strong antifungal activities against a wide variety of pathogenic yeasts and fungi but their antibacterial activities are restricted to some bacteria such as Micrococcus luteus. The biological activity of the iturin lipopeptides is modulated by the primary structure of the peptide cycle as illustrated by the methylation of the D-Tyr2 residue which dramatically decreases the activity or by the inversion of the two adjacent Ser6-Asn7 residues which makes mycosubtilin more active than iturin A. The antifungal activity is related to the interaction of the iturin lipopeptides with the cytoplasmic membrane of target cells, the K+ permeability of which is greatly increased. The ability of iturin compounds to increase the membrane cell permeability is due to the formation of ion-conducting pores, the characteristics of which depend both on the lipid composition of the membrane and on the structure of the peptide cycle. From monolayer experiments it has been suggested that these ionic pores are the consequence of the presence of aggregates in the phospholipid membrane. When active, iturins interact strongly with sterols, forming lipopeptide/cholesterol complexes. Therefore, the biologically efficient structure might be a ternary structure: iturin/phospholipid/sterol (Maget-Dana and Peypoux 1994).
Surfactin is an acidic lipopeptide produced by Bacillus subtilis strains. At high concentrations (above its critical micellar concentration (CMC) of 9 μM) it is a biosurfactant. It is also an antibacterial, antiviral, antitumor agent that lyses erythrocytes and inhibits formation of fibrin clots. At low concentrations (below its CMC) it forms pores in membranes (Carrillo et al., 2003), allowing leakage of carboxyfluorescein from unilamellar vesicles. Its activity is dependent on the lipid composition of the vesicles. It interacts with the phospholipid acyl chains.
Surfactin is a cyclic lactone peptide of 7 aas. The structure is provided in Carrillo et al. (2003). It is made by surfactin synthetase (BAA02523), a 3072 aa, multidomain protein in which each domain catalyzes the addition of a different D- or L-amino acid to the growing chain.
Bacillus subtilis and other Bacilllus species make a large number of non-ribosomally synthesized (lipo)peptides related to surfactin. These include plipistatin A1 and B1, inhibitors of phospholipase A2, bacillomycins D and L, which inhibit growth of the aflatoxin-producing fungus Aspergillus flavus as well as other fungi and bacteria, Iturin A, which has been shown to inhibit fungal and bacterial growth by forming membrane pores that cause K+ permeability (Thaniyavarn et al. 2003), and Mycosubtilin, which resembles iturin A (Moyne et al. 2004). Intergenic module repoacements have occurred between the varous lipopeptide synthetases (Thaniyavarn et al. 2003).
Bacillomycin L, a natural iturinic lipopeptide produced by Bacillus amyloliquefaciens, is characterized by strong antifungal activities against a variety of agronomically important filamentous fungi including Rhizoctonia solani Kühn. The permeabilization of R. solani hyphae by bacillomycin L was investigated and compared with that by amphotericin B, a polyene antibiotic which is thought to act primarily through membrane disruption (Zhang et al. 2013). The results derived from electron microscopy, various fluorescent techniques and gel retardation experiments revealed that the antifungal activity of bacillomycin L may not be solely a consequence of fungal membrane permeabilization, but related to the interaction of it with intracellular targets.
A mixed population of native fengycins (FEs) forms micelles which solubilize individual FEs such as agrastatin 1 (AS1) that are otherwise rather insoluble. Fluorescence lifetime-based calcein efflux measurements and cryo transmission electron microscopy show that these FEs exhibit membrane permeabilization (Patel et al. 2011). B. velezensis produces several secondary metabolites including surfactins, iturins, and fengycins. Oxydifficidin was the most active against E. amylovora S435, the causitive agent of fire blight. Pseudomonas poae FL10F produces an active extracellular compound against E. amylovora S435 that can be attributed to a cyclic lipopeptide belonging to the viscosin subfamily (massetolide E, F, L, or viscosin) (Dagher et al. 2021). homologues of these compounds found in B. velezensis include: difficidin, macrolactin, bacillaene, bacillibactin and bacilysis. Those found in P. poae include: pyochelin, bananamide 1, safracins A and B and pyoverdin (Dagher et al. 2021).
The transport reaction catalyzed by surfactin and other similar lipopeptides is:
small molecules (in) small molecules (out)