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An Integral Membrane Protein (IMP) is a protein molecule (or assembly of proteins) that is permanently attached to the biological membrane. Such proteins can be separated from the biological membranes only using detergents, nonpolar solvents, or sometimes denaturing agents. IMPs comprise a very significant fraction of the proteins encoded in the genome.
Structure 3D structures of only ~160 different integral membrane proteins are currently determined at atomic resolution by X-ray crystallography or Nuclear magnetic resonance Spectroscopy , due to the difficulties with extraction and crystallization. In addition, structures of many water-soluble domains of IMPs are available in the Protein Data Bank. Their membrane-anchoring alpha helices have been removed to facilitate the extraction and crystallization. IMPs can be divided into two groups: Transmembrane Transmembrane proteins span the entire biological membrane. This is probably the most common type of IMP. Monotopic Integral monotopic proteins are attached to the membrane from one side. 3D structures of the following integral monotopic proteins have been determined: prostaglandin H2 synthases 1 and 2, lanosterol synthase, squalene-hopene cyclase, microsomal prostaglandin E synthase, and carnitine O-palmitoyltransferase 2. There are also structures of several integral monotopic domains of transmembrane proteins that required detergents for extraction or crystallization, even after removal of their transmembrane helices: Monoamine oxidases A and B, fatty acid amine hydrolase (both classified as monotopic proteins by Stephen White *), seven mammalian cytochromes P450, and two corticosteroid 11-beta-dehydrogenases. Function IMPs include transporters, channels, receptors, enzymes, structural membrane-anchoring domains, proteins involved in accumulation and transduction of energy, and proteins responsible for cell adhesion. Types Following is a list of types of integral membrane proteins: | ||||||||
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