Editing Peripheral membrane protein (section)

===Polypeptide hormones, toxins, and antimicrobial peptides===
Many hormones, [[neurotoxin|toxins]], [[Enzyme inhibitor|inhibitor]]s, 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 [[Ionic bond|electrostatically]] with [[anionic]] membranes.

Some water-soluble proteins and peptides can also form [[Ion channel|transmembrane channel]]s. They usually undergo [[oligomer]]ization, significant [[conformational change]]s, and associate with membranes irreversibly. 3D structure of one such transmembrane channel, [[Hemolysis (microbiology)|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]].

{| class="wikitable" style="margin: 1em auto 1em auto"
!width="100"|Class
!width="275"|Proteins
!width="655"|Physiology
|-
| [[Venom]] [[toxin]]s ||
*[[Scorpion|Scorpion venom]]
*[[Snake venom]]
*[[Conotoxins]]
*[[Poneratoxin]] (insect)
|| Well known types of biotoxins include [[neurotoxins]], [[cytotoxin]]s, [[hemotoxin]]s and [[necrotoxin]]s. Biotoxins have two primary functions: predation ([[snake]], [[scorpion]] and [[cone snail]] toxins) and defense ([[honeybee]] and [[ant]] toxins).<ref>{{cite book | veditors = Rochat H, Martin-Eauclaire MF |title=Animal toxins: facts and protocols | url=https://books.google.com/books?id=kDDWg_oJYHIC&q=peptide+venom+toxin&pg=PA149| publisher=Birkhũser Verlag |location=Basel |year=2000 |isbn=3-7643-6020-8}}</ref>
|-
| [[Sea anemone]] toxins ||
*Sea anemone [[sodium channel]] inhibitory toxin
*[[Neurotoxin|Neurotoxin III]]
*[[Cytolysin]]s
||Inhibition of sodium and [[potassium channel]]s and [[Pore forming toxins|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.<ref>{{cite web | vauthors = Patocka J, Strunecka A | date = 1999 | url = http://www.asanltr.com/ASANews-99/991b.htm | title = Sea Anemone Toxins | archive-url = https://web.archive.org/web/20130615144536/http://www.asanltr.com/ASANews-99/991b.htm | archive-date= 15 June 2013 | work = The ASA Newsletter }}</ref>
|-
| [[Bacterial]] toxins ||
*[[Clostridium perfringens|Perfringolysin O]]
*[[Botulinum toxin]] B
*Heat-stable [[enterotoxin]] B
*δ-[[Endotoxin]]s
*[[Bacteriocin]]s, such as [[microcin]])
*[[Lantibiotic]] peptides, such as [[nisin]])
*[[Gramicidin S]]
||[[Microbial]] toxins are the primary [[virulence factor]]s for a variety of [[pathogenic]] bacteria. Some toxins, are [[Pore forming toxins]] that lyse cellular membranes. Other toxins inhibit [[Protein biosynthesis|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 [[superantigen]]s and causing massive [[T cell]] [[Cell growth|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.<ref>{{cite journal | vauthors = Schmitt CK, Meysick KC, O'Brien AD | title = Bacterial toxins: friends or foes? | journal = Emerging Infectious Diseases | volume = 5 | issue = 2 | pages = 224–234 | year = 1999 | pmid = 10221874 | pmc = 2640701 | doi = 10.3201/eid0502.990206 }}</ref>
|-
| [[Fungal]] toxins ||
*Cyclic [[lipopeptide]] antibiotics<br/> [[Surfactin]] and [[daptomycin]]
*[[Peptaibol]]s
||These peptides are characterized by the presence of an unusual amino acid, [[2-Aminoisobutyric acid|α-aminoisobutyric acid]], and exhibit [[antibiotic]] and [[Fungicide|antifungal]] properties due to their membrane channel-forming activities.<ref>{{cite journal | vauthors = Chugh JK, Wallace BA | title = Peptaibols: models for ion channels | journal = Biochemical Society Transactions | volume = 29 | issue = Pt 4 | pages = 565–570 | date = August 2001 | pmid = 11498029 | doi = 10.1042/BST0290565 }}</ref>
|-
|[[Antimicrobial peptide]]s ||
*[[Helicobacter pylori|HP]] peptide
*[[Saposin]] B and [[NK-lysin]]
*[[Lactoferricin]] B
*[[Magainin]], and [[Pleurocidin]]
||The modes of action by which antimicrobial peptides kill bacteria is varied and includes disrupting membranes, interfering with [[metabolism]], and targeting [[cytoplasm]]ic components. In contrast to many conventional antibiotics these peptides appear to be [[bacteriocidal]] instead of [[bacteriostatic]].
|-
| [[Defensin]]s ||
*[[Insect defensin]]s
*[[Plant defensin]]s, including [[Cyclotides]] and [[thionin]]s
||Defensins are a type of antimicrobial peptide; and are an important component of virtually all [[innate immune system|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.<ref>{{cite journal | vauthors = Oppenheim JJ, Biragyn A, Kwak LW, Yang D | title = Roles of antimicrobial peptides such as defensins in innate and adaptive immunity | journal = Annals of the Rheumatic Diseases | volume = 62 | issue = Suppl 2 | pages = ii17–ii21 | date = November 2003 | pmid = 14532141 | pmc = 1766745 | doi = 10.1136/ard.62.suppl_2.ii17 }}</ref>
|-
| [[Neuronal]] peptides ||
*[[Tachykinin]] peptides
||These proteins excite neurons, evoke [[behavioral]] responses, are potent [[Vasodilation|vasodilatator]]s, and are responsible for contraction in many types of [[smooth muscle]].<ref>{{Cite web |url=http://www.sanger.ac.uk//cgi-bin/Pfam/getacc?PF02202 |title=Pfam entry Tachykinin |access-date=2007-01-25 |archive-url=https://web.archive.org/web/20070926215842/http://www.sanger.ac.uk//cgi-bin/Pfam/getacc?PF02202 |archive-date=2007-09-26 |url-status=dead}}</ref>
|-
| [[Apoptosis]] regulators ||
*[[Bcl-2]]
|| Members of the Bcl-2 family govern [[mitochondria]]l outer membrane permeability. Bcl-2 itself suppresses apoptosis in a variety of cell types including [[lymphocyte]]s and [[neurons|neuronal cell]]s.
|}