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A membrane protein is a protein molecule that is attached to, or associated with the membrane of a cell or an organelle. More than half of all proteins interact with membranes. Membrane proteins can be classified into two groups, based on the strength of their association with the membrane.
Integral membrane proteins are permanently attached to the membrane. They can be defined as those proteins which require a detergent (such as SDS or Triton X-100) or some other apolar solvent to be displaced. They can be classified according to their relationship with the bilayer:
Peripheral membrane proteins are temporarily attached either to the lipid bilayer or to integral proteins by a combination of hydrophobic, electrostatic, and other non-covalent interactions. Peripheral proteins dissociate following treatment with a polar reagent, such as a solution with an elevated pH or high salt concentrations.
There are also numerous membrane-associated peptides, some of which are nonribosomal peptides. They can form transmembrane channels (for example, gramicidins and peptaibols), travel across the membrane as ionophores (valinomycin and others), or associate with lipid bilayer surface, as daptomycin and other lipopeptides. These peptides are usually secreted. So, they probably should be classified as amphitropic, although some of them are poorly soluble in water and associate with membrane irreversibly.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Membrane_protein". A list of authors is available in Wikipedia.|