Structure[ edit ] Potassium channel Kv1. Calculated hydrocarbon boundaries of the lipid bilayer are indicated by red and blue lines. Potassium channels have a tetrameric structure in which four identical protein subunits associate to form a fourfold symmetric C4 complex arranged around a central ion conducting pore i.
Structure[ edit ] Potassium channel Kv1. Calculated hydrocarbon boundaries of the lipid bilayer are indicated by red and blue lines.
Potassium channels have a tetrameric structure in which four identical protein subunits associate to form a fourfold symmetric C4 complex arranged around a central ion conducting pore i. Alternatively four related but not identical protein subunits may associate to form heterotetrameric complexes with pseudo C4 symmetry.
All potassium channel subunits have a distinctive pore-loop structure that lines the top of the pore and is responsible for potassium selective permeability. There are over 80 mammalian genes that encode potassium channel subunits.
However potassium channels found in bacteria are amongst the most studied of ion channels, in terms of their molecular structure. Using X-ray crystallography  profound insights have been gained into how potassium ions pass through these channels and why smaller sodium ions do not.
The protein is displayed as a green cartoon diagram. Finally potassium ions occupying the S2 and S4 sites and the oxygen atoms of water molecules S1 and S3 are depicted as purple and red spheres respectively.
Potassium ion channels remove the hydration shell from the ion when it enters the selectivity filter. The selectivity filter is formed by a five residue sequence, TVGYG, termed the signature sequence, within each of the four subunits.
This signature sequence is highly conserved, with the exception that a valine residue in prokaryotic potassium channels is often substituted with an isoleucine residue in eukaryotic channels. This sequence adopts a unique main chain structure, structurally analogous to a nest protein structural motif.
The four sets of electronegative carbonyl oxygen atoms are aligned toward the center of the filter pore and form a square anti-prism similar to a water-solvating shell around each potassium binding site.
The distance between the carbonyl oxygens and potassium ions in the binding sites of the selectivity filter is the same as between water oxygens in the first hydration shell and a potassium ion in water solution, providing an energetically-favorable route for de- solvation of the ions. This width appears to be maintained by hydrogen bonding and van der Waals forces within a sheet of aromatic amino acid residues surrounding the selectivity filter.
The next residue toward the extracellular side of the protein is the negatively charged Asp80 KcsA. This residue together with the five filter residues form the pore that connects the water-filled cavity in the center of the protein with the extracellular solution.
The carbonyl oxygens are strongly electro-negative and cation-attractive. The filter can accommodate potassium ions at 4 sites usually labelled S1 to S4 starting at the extracellular side.
In addition, one ion can bind in the cavity at a site called SC or one or more ions at the extracellular side at more or less well-defined sites called S0 or Sext.
Several different occupancies of these sites are possible. Since the X-ray structures are averages over many molecules, it is, however, not possible to deduce the actual occupancies directly from such a structure.
In general, there is some disadvantage due to electrostatic repulsion to have two neighboring sites occupied by ions.
Proposals for the mechanism of selectivity have been made based on molecular dynamics simulations,  toy models of ion binding,  thermodynamic calculations,  topological considerations,   and structural differences  between selective and non-selective channels.
The mechanism for ion translocation in KcsA has been studied extensively by theoretical calculations and simulation. MD simulations suggest the two extracellular states, Sext and S0, reflecting ions entering and leaving the filter, also are important actors in ion conduction.
Hydrophobic region[ edit ] This region is used to neutralize the environment around the potassium ion so that it is not attracted to any charges. In turn, it speeds up the reaction. The water-filled cavity and the polar C-terminus of the pore helices ease the energetic barrier for the ion. Repulsion by preceding multiple potassium ions is thought to aid the throughput of the ions.
Regulation[ edit ] Graphical representation of open and shut potassium channels PDB:This is a reference manual for the Go programming language. For more information and other documents, see vetconnexx.com Go is a general-purpose language designed with systems programming in mind. It is strongly typed and garbage-collected and has .
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