Surfactin's structure consists of a peptide loop of seven amino acids, and a hydrophobic fatty acid chain thirteen to fifteen carbons long which allows it to penetrate cellular membranes. Glutamic acid and aspartic acid residues at positions 1 and 5 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 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.
Physical properties
Surface tension
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.
Molecular mechanisms
There are three prevailing hypotheses for how surfactin works.
Cation-carrier effect
The cation-carrier effect is characterized by surfactin's ability to drive monovalent and divalent cations through an organic barrier. The two acidic residues aspartate and glutamate 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.
Pore-forming effect
The pore-forming 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.
Detergent effect
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.
Biological properties
Surfactin has a nonspecific mode of action, which originates both benefits and disadvantages. It is 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. For surfactin to carry out its antibacterial property successfully, the bacterium needs to be treated with a high concentration. In fact, surfactin needs to be in concentrations between 12–50 μg/ml in order for it to carry out minimal antibacterial effects. This is also known as the minimum inhibitory concentration. The antiviral effects of surfactin distinguish this antibiotic from others. This property is such because surfactin has been found to disintegrate enveloped viruses. Surfactin not only disintegrates the viral lipid enveloped, but also the capsid of the virus through ion channel formations. This process has been proven through test on several envelop viruses such as HIV and HSV. Also, the isoforms of the fatty acid chain containing 14 or 15 carbon atoms exhibited an improvement in inactivation of the viral envelops. Unfortunately, surfactin only affected cell-free viruses and those that had penetrated the cell were unaffected. Concurrently, if surfactin were exposed to a high medium of protein or lipid concentrations, its antiviral activity would be limited. This is also known as the buffer effect and is a significant drawback in surfactin's antiviral activity.
Toxicity
Surfactin has one major drawback: its non-specific cytotoxicity. This is seen as surfactin has the ability to lyseanimal cells as well as pathogen cells. 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 for this type of cells. At concentrations below 25 μM, toxicity effects of surfactin are not significant.
