Editing Peripheral membrane protein (section)

==Binding to the lipid bilayer==
[[Image:1mai.png|thumb|right|250px| PH domain of phospholipase C delta 1. Middle plane of the lipid bilayer – black dots. Boundary of the hydrocarbon core region – blue dots (intracellular side). Layer of lipid phosphates – yellow dots.]]
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.<ref name="Johnson_2002">{{cite journal | vauthors = Johnson JE, Cornell RB | title = Amphitropic proteins: regulation by reversible membrane interactions (review) | journal = Molecular Membrane Biology | volume = 16 | issue = 3 | pages = 217–235 | year = 2002 | pmid = 10503244 | doi = 10.1080/096876899294544 | doi-access = free }}</ref>
Some proteins, such as [[G-protein]]s and certain [[protein kinase]]s, interact with [[transmembrane protein]]s 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.{{cn|date=May 2025}} <!-- I think? -->

The [[phospholipid bilayer]] that forms the cell surface membrane consists of a [[hydrophobic]] inner core region sandwiched between two regions of [[hydrophilic]]ity, one at the inner surface and one at the outer surface of the cell membrane (see [[lipid bilayer#Structure and organization|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 [[Ångström|Å]], although this may be wider in [[biological membrane]]s that include large amounts of [[ganglioside]]s or [[lipopolysaccharide]]s.<ref name="McInosh">{{cite book| vauthors = McIntosh TJ, Vidal A, Simon SA |chapter=The energetics of peptide-lipid interactions: modification by interfacial dipoles and cholesterol |title=Current Topics in Membranes |volume=52 |pages=205–253 |publisher=Academic Press |year=2003 |isbn=978-0-12-643871-0}}</ref>
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)]].<ref name="Mitra">{{cite journal | vauthors = Mitra K, Ubarretxena-Belandia I, Taguchi T, Warren G, Engelman DM | title = Modulation of the bilayer thickness of exocytic pathway membranes by membrane proteins rather than cholesterol | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 101 | issue = 12 | pages = 4083–4088 | date = March 2004 | pmid = 15016920 | pmc = 384699 | doi = 10.1073/pnas.0307332101 | doi-access = free | bibcode = 2004PNAS..101.4083M }}</ref>
The boundary region between the hydrophobic inner core and the hydrophilic interfacial regions is very narrow, at around 3 Å, (see [[lipid bilayer#Structure and organization|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 2 [[Concentration#Molality|M]].<ref name=Marsh_2001>{{cite journal | vauthors = Marsh D | title = Polarity and permeation profiles in lipid membranes | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 98 | issue = 14 | pages = 7777–7782 | date = July 2001 | pmid = 11438731 | pmc = 35418 | doi = 10.1073/pnas.131023798 | doi-access = free | bibcode = 2001PNAS...98.7777M }}</ref><ref name=Marsh_2002>{{cite journal | vauthors = Marsh D | title = Membrane water-penetration profiles from spin labels | journal = European Biophysics Journal | volume = 31 | issue = 7 | pages = 559–562 | date = December 2002 | pmid = 12602343 | doi = 10.1007/s00249-002-0245-z | s2cid = 36212541 }}</ref>
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.<ref name=Nagle_2000>{{cite journal | vauthors = Nagle JF, Tristram-Nagle S | title = Structure of lipid bilayers | journal = Biochimica et Biophysica Acta (BBA) - Reviews on Biomembranes | volume = 1469 | issue = 3 | pages = 159–195 | date = November 2000 | pmid = 11063882 | pmc = 2747654 | doi = 10.1016/S0304-4157(00)00016-2 }}</ref>

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-associated X protein|BcL-2 like protein]] <!-- reference? -->, in some amphiphilic [[antimicrobial peptides]] <!-- reference? -->, and in certain [[annexin]]s <!-- reference? -->. These proteins are usually described as peripheral as one of their conformational states is water-soluble or only loosely associated with a membrane.<ref name="Goñi_2002">{{cite journal | vauthors = Goñi FM | title = Non-permanent proteins in membranes: when proteins come as visitors (Review) | journal = Molecular Membrane Biology | volume = 19 | issue = 4 | pages = 237–245 | year = 2002 | pmid = 12512770 | doi = 10.1080/0968768021000035078 | s2cid = 20892603 }}</ref>