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Membrane Proteins

Integral and peripheral membrane proteins are both usually found in biological membranes. Membrane proteins that are integral include lipid-anchored proteins and transmembrane proteins. These types of integral proteins have two types of membrane-spanning domains. In transmembrane proteins 2 types of membrane spanning domains can be found : 1) One or more alpha helices 2) multiple beta strands (less commonly found). Proteins containing 7 membrane spanning alpha helices form a major class that include many cell surface recptors and bacteriorhodopsin.When a transmembrane proteins’ polypeptide chain spans the membrane many times, the core of the protein is hydrophilic (allowing the passage of water soluble molecules) and the surface of the protein is hydrophobic (permitting interaction with the interior of the lipid bilayer). Amino acid residues modified with long chain hydrocarbons anchor some integral proteins to one membrane leaflet. Peripheral membrane proteins interact with integral membrane proteins or with the polar head groups of membrane phospholipids. They do not enter the hydrophobic core of the membrane.


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