Ion channels are pore- forming transmembrane proteins that allow electrically charged particles, ions , to traverse biomembrane . Because of this function, they are also referred to as channel proteins or tunnel proteins . The transport takes place along the existing electrochemical gradient (the concentration and potential gradient). This distinguishes them from active transport proteins such as ion pumps , which in turn enable primarily active transport via ion channels while consuming energy . Ion channels are found both in the external cell membrane and in the membranes of cell organelles such as the tonoplast .
Ion channels, in combination with other transport proteins , are of universal importance for transport processes across the cell's membrane systems. This includes the regulation of osmotic activity , the acid-base balance, the absorption and excretion of substances and the conduction of excitation in nerves and muscle cells .
The patch-clamp technique made it possible to measure the ionic currents of just a few picoamps across individual channel proteins and thus to record their electrical, kinetic and other properties.
Selectivity and conductivity
Ion channels can be described on the basis of their pronounced or lack of selectivity for certain ions. The higher the selectivity for one type of ion, the lower the conductivity of the opened pore for other ions. We know highly specific channels for
- Cations (positively charged) such as:
- Anions (negatively charged) such as:
Ion channels are often named for their selectivity: potassium , sodium , calcium or chloride channel .
There are also the so-called unspecific cation channels such as the TRP channels . transient receptor potential channels , which have a similar conductivity for potassium, sodium and calcium ions.
There are a number of ion channels for protons such as thermogenin or voltage-gated proton channels . Protons are also actively transported by proton pumps , consuming ATP ; the latter do not belong to the ion channels. The connexons of the gap junctions , which can allow molecules up to approx. 1 kDa to pass, are also not counted among the ion channels in the narrower sense .
Control ( gating )
The conductivity of most ion channels is drastically influenced by the existing environment or directed signals; such channels are called gated .
Voltage gated ion channels
A large class of ion channels is controlled by the membrane potential (voltage-dependent ion channels). So are z. B. typical voltage-activated sodium channels are not conductive during the resting membrane potential, but only when they are activated by a depolarization .
Ion channel receptors
Another large class of ion channels is activated by ligands , i.e. by molecules that function as messenger substances (ligand-controlled ion channels). So z. B. the acetylcholine receptor , which plays a role in signal transduction from nerve to muscle, conductive in the presence of the neurotransmitter acetylcholine .
Other control mechanisms
- Mechanosensitive ion channels can be activated by mechanical stimuli (e.g. pressure, vibrations).
- light-gated ion channels (light gated channels), for example. B. the channelrhodopsins , are activated by light of a specific wavelength
- temperature-controlled ion channels are activated from specific temperatures
When open, ion channels have the highest rate of passage of all transport proteins; it is usually given as 10 6 to 10 8 ions per second. This makes them the fastest membrane transport medium compared to co-transporting proteins ( symporters and antiporters ) (10 2 to 10 4 molecules per second) or ATP-driven pumps (10 0 to 10 2 ions per second).
Roderick MacKinnon , who together with Peter Agre received the Nobel Prize for Chemistry in 2003 for his structural analysis of ion channels, commissioned the German-American artist Julian Voss-Andreae to create a sculpture based on his experimental data.
- Membrane transport
- Sodium channel , potassium channel , calcium channel , chloride channel , HERG channel
- nicotinic acetylcholine receptor , ionotropic glutamate receptor , ionotropic GABA receptor
- Blood sugar sensor system
- Ulrich Koert: Synthetic Ion Channels. In: Chemistry in Our Time . 31, 1997, pp. 20-26, doi : 10.1002 / ciuz.19970310105 .
- ↑ Textbook of botany for universities ; by E. Strasburger. 35th edition. Spectrum, Akad. Verl., 2002.
- ↑ Non-active transport ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. , Script Medical Physiology of the University of Vienna , 2003 (PDF, 445kb).
- ↑ Philip Ball: The crucible: Art inspired by science should be more than just a pretty picture . In: Chemistry World . 5, March 2008, pp. 42-43. Retrieved January 12, 2009.