This signature sequence is within a loop between the pore helix and TM2/6, historically termed the P-loop. The selectivity filter is formed by a five residue sequence, TVGYG, termed the signature sequence, within each of the four subunits. ![]() Potassium ion channels remove the hydration shell from the ion when it enters the selectivity filter. 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. In addition backbone carbonyl groups and threonine sidechain protein atoms (oxygen = red, carbon = green) are displayed. The protein is displayed as a green cartoon diagram. In this figure, only two of the four subunits of the tetramer are displayed for the sake of clarity. Selectivity filter Ĭrystallographic structure of the bacterial KcsA potassium channel ( PDB: 1K4C). The 2003 Nobel Prize for Chemistry was awarded to Rod MacKinnon for his pioneering work in this area. Using X-ray crystallography, profound insights have been gained into how potassium ions pass through these channels and why (smaller) sodium ions do not. However potassium channels found in bacteria are amongst the most studied of ion channels, in terms of their molecular structure. There are over 80 mammalian genes that encode potassium channel subunits. All potassium channel subunits have a distinctive pore-loop structure that lines the top of the pore and is responsible for potassium selective permeability. Alternatively four related but not identical protein subunits may associate to form heterotetrameric complexes with pseudo C 4 symmetry. Potassium channels have a tetrameric structure in which four identical protein subunits associate to form a fourfold symmetric ( C 4) complex arranged around a central ion conducting pore (i.e., a homotetramer). Top view of a potassium channel with potassium ions (purple) moving through the pore (in the center). ![]()
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