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Surfactin is a very powerful surfactant commonly used as an antibiotic. It is a bacterial cyclic lipopeptide, largely prominent for its exceptional surfactant power.  Its amphiphilic properties help this substance to survive in both hydrophilic and hydrophobic environment. It is one of the 24 types of antibiotics produced by the Gram-positive endospore-forming bacteria Bacillus subtilis. In the course of various studies of its properties, surfactin was found to exhibit effective characteristics like anti-bacterial, anti-viral, anti-fungal, anti-mycoplasma and hemolytic activities.
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Structure and Synthesis
Surfactin's structure consists of a peptide loop of seven amino acids (L-asparagine, L-leucine, glycine, L-leucine, L-valine and two D-leucines), and a hydrophobic fatty acid chain thirteen to fifteen carbons long which allows its ability to penetrate cellular membranes. Glycine and asparagine residues at positions 1 and 6 respectively, constituting a minor polar domain. On the opposite side, valine residue at position 4 extends down facing the fatty acid chain, making up a major hydrophobic domain. Below critical micellar concentrations (CMCs) the fatty acid tail can extend freely into solution, and then participate in hydrophobic interactions within micelles. This antibiotic is synthesized by a linear nonribosomal peptide synthetase, surfactin synthetase, and has, in solution, a characteristic "horse saddle" conformation that explains its large spectrum of biological activity.
Surfactin, like other surfactants, affects the surface tension of liquids in which it is dissolved. It can lower the water's surface tension from 72 mN/m to 27 mN/m at a concentration as low as 20 µM. Surfactin accomplishes this effect as it occupies the intermolecular space between water molecules, decreasing the attractive forces between adjacent water molecules, mainly hydrogen bonds, creating a more fluid solution that can go into tighter regions of space increasing water’s wetting ability. Overall, this property is significant not only for surfactin but for surfactants as a whole, as they are primarily used as detergents and soaps.
There are three prevailing hypotheses for how surfactin works. These are described below.
The cation-carrier effect is characterized by surfactin’s ability to drive monovalent and divalent cations through an organic barrier. The two acidic residues asparagine and glycine form a "claw" of sorts which easily stabilizes divalent cations. Calcium ions make for the best-fitting cations stabilizing the surfactin conformation and functioning as an assembly template for the formation of micelles. When surfactin penetrates the outer sheet, its fatty acid chain interacts with the acyl chains of the phospholipids, with its headgroup in proximity to the phospholipids polar heads. Attachment of a cation to causes the complex to cross the bilipidic layer undergoing a flip-flop. The headgroup aligns itself with the phospholipids of the inner sheet and the fatty acid chain interacts with the phospholipids acyl chains. The cation is then delivered into the intracellular medium.
The pore-forming (ion channel) effect is characterized by the formation of cationic channels. It would require surfactin to self-associate inside the membrane, since it cannot span across the cellular membrane. Supramolecular-like structures by successive self-association could then form a channel. This hypothesis for the most part applies only to uncharged membranes where there is a minimal energy barrier between outer and inner membrane leaflets.
The detergent effect draws on surfactin's ability to insert its fatty acid chain into the bilipidic layer causing disorganization leading to membrane permeability. Insertion of several surfactin molecules into the membrane can lead to the formation of mixed micelles by self-association and bilayer influenced by fatty chain hydrophobicity ultimately leading to bilayer solubilization.
Surfactin has a nonspecific mode of action, which originates both benefits and disadvantages. It’s advantageous in the sense that surfactin can act on many kinds of cell membranes, both Gram-positive and Gram-negative. Its non-specificity also has bearing on its tendency to not produce resistant strains of bacteria. Consequently, this efficient mode of cell destruction is indiscriminate, and attacks red blood cells with deadly efficiency.
Antibacterial and antiviral properties
Surfactin, true to its antibiotic nature, has a very significant antibacterial property, as it is capable of penetrating the cell membranes of all types of bacteria. There are two main types of bacteria and they are Gram-negative and Gram-positive. The two bacteria types differ in the composition of their membrane. The Gram-negative bacteria have an outer lipopolysaccharide membrane and a thin peptidoglycan layer followed by a phospholipids bilayer, whereas the Gram-positive bacteria lack the outer membrane and carry a thicker peptidoglycan layer as well as a phospholipids bilayer. This is an essential factor that contributes to surfactin’s detergent-like activity as it is able to create a permeable environment for the lipid bilayer and causes disruption that solubilizes the membrane.
Surfactin has only one drawback: its non specific cytotoxicity. This is seen as surfactin has the ability to lyse a cell. The hemolytic effect has been the result of surfactin having the ability to lyse red blood cells that is enough to warrant caution if used intravascularly. Fortunately, these results were seen at high concentrations of about 40 µM to 60 µM. These concentrations also exhibited the effect of proliferating cells in vitro though it also was the LD50 for this type of cells. At concentrations below 25 µM, toxicity effects of surfactin are not significant.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Surfactin". A list of authors is available in Wikipedia.|