- Surfactin
Surfactin is a very powerful
surfactant commonly used as anantibiotic . It is abacteria lcyclic lipopeptide , largely prominent for its exceptional surfactant power. [ Mor, A. Peptide-based antibiotics: A potential answer to raging antimicrobial resistance. "Drug Develop. Res. (2000) 50": 440–447.] Itsamphiphilic properties help this substance to survive in bothhydrophilic andhydrophobic environment. It is one of the 24 types of antibiotics produced by theGram-positive endospore -forming bacteria "Bacillus subtilis ". [Peypoux F, Bonmatin JM, Wallach J. Recent trends in thebiochemistry of Surfactin; "Applied Microbiol Biotechnol. (1999)" 51:553–63] In the course of various studies of its properties, surfactin was found to exhibit effective characteristics likeanti-bacterial ,anti-viral ,anti-fungal , anti-mycoplasma andhemolytic activities. [Pooja Singh and Swaranjit Singh Cameotra; Potential applications of microbial surfactants inbiomedical sciences ; "Institute of Microbial Technology", Sector 39 A,Chandigarh 160036,India .]tructure and Synthesis
Surfactin's structure consists of a
peptide loop of sevenamino acids (L-asparagine , L-leucine ,glycine , L-leucine, L-valine and two D-leucines), and a hydrophobicfatty acid chain thirteen to fifteencarbon s 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 criticalmicella r concentrations (CMCs) the fatty acid tail can extend freely intosolution , and then participate in hydrophobic interactions within micelles. [Grau, A, J C. Gomez Fernandez, and R Peypoux. A Study on the Interactions of Surfactin With Phospholipid Vesicles. "BBA (1999)" 1418: 307–319. ] This antibiotic is synthesized by a linearnonribosomal peptide synthetase ,surfactin synthetase , and has, in solution, a characteristic "horse saddle"conformation that explains its large spectrum of biological activity. [Nathalie Hue, Laurent Serani, Olivier Laprévote; Structural investigation of cyclic peptidolipids from "Bacillus subtilis" by high-energy tandemmass spectrometry ; "Institut de Chimie des Substances Naturelles", CNRS, avenue de la Terrasse, 91198 Gif-sur-Yvette, France. ]Physical properties
urface tension
Surfactin, like other surfactants, affects the surface tension of liquids in which it is dissolved. It can lower the
water 'ssurface tension from 72 mN/m to 27 mN/m at a concentration as low as 20 µM. [Yeh MS, Wei YH, Chang JS. Enhanced Production of surfactin from "Bacillus subtilis" by addition of solid carriers (2005) "Biotechnol Prog. 4":1329–34] Surfactin accomplishes this effect as it occupies the intermolecular space between watermolecule s, decreasing the attractive forces between adjacent water molecules, mainlyhydrogen bond s, creating a more fluid solution that can go into tighter regions of space increasing water’s wetting ability. [Dufour S, Deleu M, Nott K, Wathelet B, Thonart P, Paquot M. Hemolytic activity of new linear surfactin analogs in relation to their physico-chemical properties. (2005) "BBAGEN 25949":1–9] Overall, this property is significant not only for surfactin but for surfactants as a whole, as they are primarily used as detergents andsoap s.Molecular mechanisms
There are three prevailing
hypotheses for how surfactin works. [Deleu, M, O Bouffioux, H Razafindralambo, and M Paquot. Interaction of Surfactin with Membranes: A Computational Approach. "(2003) Langmuir 19": 3377–3385. ] These are described below.Cation-carrier effect
The cation-carrier effect is characterized by surfactin’s ability to drive
monovalent anddivalent cations through an organic barrier. The twoacid ic residues asparagine and glycine form a "claw" of sorts which easily stabilizes divalentcation s.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 theacyl chains of thephospholipid s, with its headgroup in proximity to the phospholipids polar heads. Attachment of a cation to causes the complex to cross the bilipidic layer undergoing aflip-flop . The headgroup aligns itself with the phospholipids of the inner sheet and the fatty acid chain interacts with the phospholipids acyl chains. [Heerklotz, H, T Wieprecht, and J Seelig. Membrane Perturbation oby the Lipopeptide Surfactin and Detergents as Studied by Deuterium NMR. "J. Phys. Chem. (2004)108": 4909–4915] The cation is then delivered into the intracellular medium.Pore-forming effect
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. [Deleu, M, O Bouffioux, H Razafindralambo, and M Paquot. Interaction of Surfactin with Membranes: A Computational Approach. (2003) "Langmuir 19": 3377–3385. ]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. [Kragh-Hansen, U, M Maire, and J Moller. The Mechanism of Detergent Solubilization of Liposomes and Protein-Containing Membranes. "Biophys. J. (1998) 75": 2932–2946. ] 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. [Maire, M, P Champeil, and J V. Moller. Interaction of Membrane Proteins and lipids with Solubilizing Detergents. "BBA (2000) 1508": 86–111. ]
Biological properties
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 outerlipopolysaccharide membrane and a thin peptidoglycan layer followed by a phospholipids bilayer, whereas the Gram-positive bacteria lack the outer membrane and carry a thickerpeptidoglycan layer as well as a phospholipids bilayer. [Bergey, John G. Holt, Noel R. Krieg, Peter H.A. Sneath (1994). "Bergey's Manual of Determinative Bacteriology", 9th ed., Lippincott Williams & Wilkins. ISBN 0-683-00603-7. ] 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. [Heerklotz H, Seelig J. Detergent-like action of the antibiotic peptide surfactin on lipid membranes. (2001) "Biophysical J 81(3):"1547–54] This is also known as the minimum inhibitory concentration (MIC).
The antiviral effects of surfactin distinguish this antibiotic from others. This property is such because surfactin has been found to disintegrate envelopedvirus es. Surfactin not only disintegrates the viral lipid enveloped, but also thecapsid of the virus through ion channel formations. This process has been proven through test on several envelop viruses such asHIV and HSV. [Jung M, Lee S, Kim H, Kim H. Recent Studies on Natural Products as Anti-HIV Agents. (2000). "Current Medicinal Chemistry 7":000-000] Also, theisoform s 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 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
intravascular ly. 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. [ [http://www.cibasc.com/index/ind-index/ind-per_car/ind-pc-ah/ind-pc-triclosan.htm Triclosan Information @ Ciba Specialty Chemicals.] ] At concentrations below 25 µM, toxicity effects of surfactin are not significant.References
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