Everything about Integral Membrane Proteins totally explained
An
Integral Membrane Protein (
IMP) is a
protein molecule (or assembly of proteins) that's 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
Three-dimensional 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 α-helices have been removed to facilitate the extraction and crystallization.
IMPs can be divided into two groups:
- Transmembrane proteins
- Integral monotopic proteins
Integral transmembrane protein
The most common type of IMP is the
transmembrane protein (TM), which spans the entire
biological membrane. Such a protein may cross the membrane only once or it may weave in and out, crossing several times. TM proteins can be categorized as Type I, which are positioned such that their amino-terminus is outside of the membrane, or Type II, which have their carboxy-terminus outside of the membrane.
Integral monotopic proteins
Integral monotopic proteins are permanently attached to the membrane from one side.
Three-dimensional structures of the following integral monotopic proteins have been determined:
prostaglandin H2 syntheses 1 and 2 (cyclooxygenases) (External Link
),
lanosterol synthase and squalene-hopene cyclase (External Link),
microsomal prostaglandin E synthase (External Link),
carnitine O-palmitoyltransferase 2 (External Link).
There are also structures of integral monotopic domains of transmembrane proteins:
monoamine oxidases A and B (External Link),
fatty acid amide hydrolase (External Link),
mammalian cytochrome P450 oxidases (External Link),
corticosteroid 11-beta-dehydrogenases (External Link).
Such domains require detergents for extraction or crystallization, even after removal of their transmembrane helices. Therefore, they are often classified as integral monotopic proteins (External Link)
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. Classification of transporters can be found in TCDB database.
Further Information
Get more info on 'Integral Membrane Proteins'.
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