Transmembrane proteins are a group of membrane proteins and cross both sheets of the phospholipid bilayer of a biomembrane , in contrast to the single membrane proteins . The transmembrane proteins include functionally very different proteins such as transmembrane receptors , porins , ion channels , transporters , ATP-dependent pumps and cell adhesion molecules . They can be anchored in the plasma membrane or in the membrane of an organelle and can be structurally classified according to their protein components, the transmembrane domain , which cross the membrane .
In order to penetrate the lipid bilayer , a polypeptide chain must have hydrophobic side chains and shield its polar backbone group . This is achieved, among other things, by the formation of a secondary structure that contributes to the formation of hydrogen bonds. All known transmembrane segments therefore consist of either α-helices (very common) or β-barrels (rare). The complete structure is only known for a few membrane proteins.
A distinction is made between the single-pass -Transmembranproteinen that pass through the membrane only once, and the multi-pass -Transmembranproteinen that pass through the membrane several times. The single- pass transmembrane proteins are further divided into different types according to the type of anchoring and the position of their ends. The N -terminus is extracellular in type 1 transmembrane proteins that are anchored in the cell membrane. In the case of type 2 transmembrane proteins, on the other hand, the C terminus is extracellular.
In the transmembrane area (transmembrane domain of about eight to twelve amino acids), transmembrane proteins are built up from the less polar or more hydrophobic amino acids that enter into a protein-lipid interaction with the lipids of the membrane .