Antimicrobial peptides (AMPs) are diminutive, naturally occurring peptides that are integral to the immune defense system, demonstrating significant efficacy against many pathogens, including bacteria, viruses, fungus, and some cancer cells. Given the escalating issue of antibiotic resistance, AMPs have garnered interest as possible replacements or adjuncts to conventional antibiotics. This document provides an overview of the properties, methods of action, and uses of antimicrobial peptides.
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The majority of AMPs with known secondary structures fall into one of four categories: β-sheet, α-helix, extended, or loop. The α-helix and β-sheet structures are the most prevalent among these structural groupings, whereas α-helical peptides have been the focus of the majority of AMP research up to this point. The distance between two neighboring amino acids in α-helix structures is around 0.15 nm, and their angle with respect to the center is approximately 100 degrees when seen from above. Cyclic indolicin, protegrin, magainin, and coiled indolicin are the most well-known AMPs in this class. β-sheet peptides have disulfide linkages that connect at least two β-strands.
Fig.1 AMP structures illustrating α-helix, β-sheet, and extended coil1,2.
Antimicrobial peptides exert their antimicrobial effects through interactions with microbial cell membranes. The mechanism of membrane targeting typically involves the following key steps:
Electrostatic Interactions: AMPs are generally positively charged, while bacterial cell membranes are rich in negatively charged phospholipids. The electrostatic attraction between positive and negative charges allows AMPs to bind to the bacterial membrane surface.
Membrane Penetration: After interaction with the membrane surface, AMPs enter the membrane structure through various mechanisms. Common mechanisms include: The mechanisms by which antimicrobial peptides target membranes include the Barrel-Stave Model, Carpet Model, and Toroidal-Pore Model. In the Barrel-Stave Model, AMP molecules arrange themselves into a barrel-like structure and insert into the membrane, forming ion channels that increase membrane permeability, leading to leakage of cellular contents and ultimately cell death. In the Carpet Model, AMPs align parallel to the membrane surface, covering a large area, which alters the membrane's fluidity and causes it to rupture. In the Toroidal-Pore Model, AMPs insert into the membrane and form a ring-like structure, pulling lipid molecules into the pore and creating a circular channel that disrupts membrane integrity. These models illustrate the different ways in which AMPs interact with and disrupt microbial membranes, resulting in cell damage and death.
Membrane Disruption: Through these mechanisms, AMPs cause localized damage to the membrane, leading to leakage of cytoplasmic contents, rupture of the cell membrane, and ultimately cell death.
Selective Toxicity: Antimicrobial peptides typically exhibit a high affinity for bacterial membranes and a lower affinity for human cell membranes. This selectivity is closely related to the differences in the lipid compositions of bacterial and host cell membranes, allowing AMPs to effectively kill bacteria without significantly damaging host cells.
In summary, the membrane-targeting mechanism of AMPs primarily relies on their interaction with bacterial membranes, disrupting membrane integrity and inhibiting or killing bacterial cells.
Fig.2 Diverse actions of antimicrobial peptides in cellular defense2,3.
A key component of bacterial cell walls is peptididoglycan. Lipid II, an important part of peptidoglycan, is required for the translocation of bacterial cell wall subunits across the plasma membrane. In addition to preventing cell wall development, AMPs can damage the structure of already-formed cell walls when they react directly with lipid II. When it comes to killing gram-positive bacteria, glycopeptides have a wide range of effects. Vancomycin, a glycopeptide, inhibits cell wall formation by binding to the C-terminal D-Ala-D-Ala region of lipid II and blocking the binding of penicillin-binding proteins (PBPs), according to studies. Nisin is a lantibiotic that employs a series of five lanthionine rings. A pore complex can be produced on the cell wall once lanthionine rings A and B have combined with lipid II's pyrophosphate.
Permeating the bacterial cell membrane directly, some AMPs disrupt fundamental cellular processes such DNA replication, translation, transcription, folding, and cell division. AMPs primarily cross the plasma membrane via two pathways: energy-dependent endocytosis and energy-independent direct permeation (immediate hole creation or direct translocation via membrane instability). Once within the cell membrane and building up, it begins to target intracellular macromolecules and biological processes in order to gain further activity. This includes targeting proteases, nucleic acids, and proteins, among others.
Antimicrobial properties of AMPs were initially identified. Thus, AMPs are seen to provide great prospects for research into creating a new category of antibiotics to deal with the growing problem of bacteria that are resistant to many drugs. The antimicrobial activity of AMPs is attributed to their ability to disrupt bacterial membranes, leading to cell lysis and death. Their broad-spectrum efficacy extends to both Gram-positive and Gram-negative bacteria, as well as fungi and viruses. Due to their unique mechanism of action, AMPs hold significant potential in overcoming the challenges posed by multi-drug-resistant pathogens, offering a promising alternative to traditional antibiotics.
Structured bacterial clumps known as biofilms exhibit extraordinary resistance to standard antibiotic treatments. Up to two-thirds of all human illnesses are caused by biofilms, which are a significant environmental bacterial growth condition. Recent research has demonstrated that certain AMPs have antibiofilm properties apart from their antimicrobial action against planktonic cells that are free to swim. These antimicrobial peptides are capable of targeting biofilm formation at various stages, including preventing initial adhesion and disrupting mature biofilms, thereby enhancing the effectiveness of existing antibiotics and offering a novel approach to combating persistent infections.
The host immune response can be modulated and enhanced positively by AMPs. Amps can promote cellular recruitment by increasing leukocyte chemokine expression, which is a key immunomodulatory function of AMPs. As amphipathic cations, AMPs have many properties with certain chemokines; moreover, AMPs can display direct chemokine action at concentrations high enough. The capacity of AMPs to control proinflammatory responses through interference with TLR-ligand-induced proinflammatory pathways is another key characteristic of AMPs. In bacterial infections, when septic shock and organ failure are symptoms of excessive inflammation and cytokine production, it may be helpful to control inflammatory pathways. By stimulating the migration of keratinocyte and epithelial cells and the creation of remodeling metalloproteinases, certain AMPs can also aid in wound healing.
Some antimicrobial peptides, known as anticancer peptides (ACPs), exhibit selective cytotoxicity against tumor cells. Unlike traditional chemotherapy, which targets rapidly dividing cells and causes harmful side effects, ACPs target cancer cells more precisely, minimizing damage to normal tissue. ACPs exploit electrostatic interactions, binding to the anionic surface of cancer cells, which is enriched with substances like heparan sulfate and gangliosides. Their flexibility allows ACPs to penetrate cancer cell membranes, whereas normal cells, with higher cholesterol content, are more rigid. However, some ACPs, like melittin and defensins, can still affect healthy cells at high concentrations.
Defensin, found in humans and other organisms, disrupts bacterial membranes. Another example is Magainin from frogs, effective against both Gram-positive and Gram-negative bacteria.
AMPs are explored for treating chronic infections, wound healing, and as alternatives to antibiotics. They're used topically for skin infections and in oral care products to fight drug-resistant bacteria.
Disadvantages include instability in physiological environments, potential toxicity to human cells, high production costs, and the risk of microbial resistance over time.
AMPs disrupt microbial membranes, causing leakage and cell death. They also inhibit bacterial enzymes, DNA/protein synthesis, and modulate immune responses to help fight infections.
Some antimicrobial peptides also exhibit activity against fungi and certain viruses by disrupting fungal cell membranes or interfering with viral replication. Their effectiveness depends on the specific peptide structure and mechanism of action.
Antimicrobial peptides are sensitive to temperature, pH, and enzymatic degradation. They should be stored at low temperatures, protected from light, and used according to instructions. Avoid prolonged exposure to high temperatures or extreme pH to maintain activity and efficacy.
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