Peripheral Membrane Protein - Binding of Peripheral Proteins To The Lipid Bilayer

Binding of Peripheral Proteins To The Lipid Bilayer

Peripheral membrane proteins may interact with other proteins or directly with the lipid bilayer. In the latter case, they are then known as amphitropic proteins. Some proteins, such as G-proteins and certain protein kinases, interact with transmembrane proteins and the lipid bilayer simultaneously. Some polypeptide hormones, antimicrobial peptides, and neurotoxins accumulate at the membrane surface prior to locating and interacting with their cell surface receptor targets, which may themselves be peripheral membrane proteins.

The phospholipid bilayer that forms the cell surface membrane consists of a hydrophobic inner core region sandwiched between two regions of hydrophilicity, one at the inner surface and one at the outer surface of the cell membrane (see lipid bilayer article for a more detailed structural description of the cell membrane). The inner and outer surfaces, or interfacial regions, of model phospholipid bilayers have been shown to have a thickness of around 8 to 10 Å, although this may be wider in biological membranes that include large amounts of gangliosides or lipopolysaccharides. The hydrophobic inner core region of typical biological membranes may have a thickness of around 27 to 32 Å, as estimated by Small angle X-ray scattering (SAXS). The boundary region between the hydrophobic inner core and the hydrophilic interfacial regions is very narrow, at around 3Å, (see lipid bilayer article for a description of its component chemical groups). Moving outwards away from the hydrophobic core region and into the interfacial hydrophilic region, the effective concentration of water rapidly changes across this boundary layer, from nearly zero to a concentration of around 2M. The phosphate groups within phospholipid bilayers are fully hydrated or saturated with water and are situated around 5 Å outside the boundary of the hydrophobic core region (see Figures ).

Some water-soluble proteins associate with lipid bilayers irreversibly and can form transmembrane alpha-helical or beta-barrel channels. Such transformations occur in pore forming toxins such as colicin A, alpha-hemolysin, and others. They may also occur in BcL-2 like protein, in some amphiphilic antimicrobial peptides, and in certain annexins . These proteins are usually described as peripheral as one of their conformational states is water-soluble or only loosely associated with a membrane.