Polypeptide Hormones, Toxins, and Antimicrobial Peptides
Many hormones, toxins, inhibitors, or antimicrobial peptides interact specifically with transmembrane protein complexes. They can also accumulate at the lipid bilayer surface, prior to binding their protein targets. Such polypeptide ligands are often positively charged and interact electrostatically with anionic membranes.
Some water-soluble proteins and peptides can also form transmembrane channels. They usually undergo oligomerization, significant conformational changes, and associate with membranes irreversibly. 3D structure of one such transmembrane channel, α-hemolysin, has been determined. In other cases, the experimental structure represents a water-soluble conformation that interacts with the lipid bilayer peripherally, although some of the channel-forming peptides are rather hydrophobic and therefore were studied by NMR spectroscopy in organic solvents or in the presence of micelles.
||Well known types of biotoxins include neurotoxins, cytotoxins, hemotoxins and necrotoxins. Biotoxins have two primary functions: predation (snake, scorpion and cone snail toxins) and defense (honeybee and ant toxins).|
|Sea anemone toxins||
||Inhibition of sodium and potassium channels and membrane pore formation are the primary actions of over 40 known Sea anemone peptide toxins. Sea anemone are carnivorous animals and use toxins in predation and defense; anemone toxin is of similar toxicity as the most toxic organophosphate chemical warfare agents.|
||Microbial toxins are the primary virulence factors for a variety of pathogenic bacteria. Some toxins, are Pore forming toxins that lyse cellular membranes. Other toxins inhibit protein synthesis or activate second messenger pathways causing dramatic alterations to signal transduction pathways critical in maintaining a variety of cellular functions. Several bacterial toxins can act directly on the immune system, by acting as superantigens and causing massive T cell proliferation, which overextends the immune system. Botulinum toxin is a neurotoxin that prevents neuro-secretory vesicles from docking/fusing with the nerve synapse plasma membrane, inhibiting neurotransmitter release.|
||These peptides are characterized by the presence of an unusual amino acid, α-aminoisobutyric acid, and exhibit antibiotic and antifungal properties due to their membrane channel-forming activities.|
||The modes of action by which antimicrobial peptides kill bacteria is varied and includes disrupting membranes, interfering with metabolism, and targeting cytoplasmic components. In contrast to many conventional antibiotics these peptides appear to be bacteriocidal instead of bacteriostatic.|
||Defensins are a type of antimicrobial peptide; and are an important component of virtually all innate host defenses against microbial invasion. Defensins penetrate microbial cell membranes by way of electrical attraction, and form a pore in the membrane allowing efflux, which ultimately leads to the lysis of microorganisms.|
||These proteins excite neurons, evoke behavioral responses, are potent vasodilatators, and are responsible for contraction in many types of smooth muscle.|
||Members of the Bcl-2 family govern mitochondrial outer membrane permeability. Bcl-2 itself suppresses apoptosis in a variety of cell types including lymphocytes and neuronal cells.|
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