Transmembrane protein
A transmembrane protein (TP) is a type of membrane protein spanning the entirety of the biological membrane to which it is permanently attached. That is, transmembrane proteins span from one side of a membrane through to the other side of the membrane. Firm attachment of TP to biological membrane is aided by a special class of membrane lipids, called annular lipid shell. Many TPs function as gateways or "loading docks" to deny or permit the transport of specific substances across the biological membrane, to get into the cell, or out of the cell as in the case of waste byproducts. As a response to the shape of certain molecules these "freight handling" TPs may have special ways of folding up or bending that will move a substance through the biological membrane.
Transmembrane proteins are polytopic proteins that aggregate and precipitate in water. They require detergents or nonpolar solvents for extraction, although some of them (beta-barrels) can be also extracted using denaturing agents. All transmembrane proteins are integral membrane proteins (IMPs), but not all IMPs are transmembrane proteins.
G protein–coupled receptor
G protein–coupled receptors (GPCRs), also known as seven-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptor, and G protein–linked receptors (GPLR), constitute a large protein family of receptors, the G-protein-coupled receptor (GPCR) Family (TC# 9.A.14) (a superfamily/family within the TOG Superfamily ), that sense molecules outside the cell and activate inside signal transduction pathways and, ultimately, cellular responses. Coupling with G proteins, they are called seven-transmembrane receptors because they pass through the cell membrane seven times.
G protein–coupled receptors are found only in eukaryotes, including yeast, choanoflagellates, and animals. The ligands that bind and activate these receptors include light-sensitive compounds, odors, pheromones, hormones, and neurotransmitters, and vary in size from small molecules to peptides to large proteins. G protein–coupled receptors are involved in many diseases, and are also the target of approximately 40% of all modern medicinal drugs.
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