eurobiology Biomed 509 Ion hannels References: Luo 2.4, 2.4 2., (3.20 3.2), M 2.8 I. ores vs. carriers In order for hydrophilic ions to transit the hydrophobic membrane, they need to have a specific pathway that can be provided by either a water-filled channel (pore) or by a binding site that can translocate across the membrane (carrier). Both of these mechanisms play important roles in excitable cells, but they each have distinct properties: pores can move ions rapidly while carriers can move ions up their concentration gradient. The following calculation demonstrates the first of these properties namely the differences in turnover rate. Ion channel turnover rate: typical ion channel current = pa = 0-2 coul/s F = 0 5 coul/mol pa = 0-7 mol/s Avogadro s number, = 0 23 pa = 0 ions/s arrier turn-over rate e.g. the a-k-atase is 5 0 2 ions/s II. hannel properties Ion channels play a central role in the ability of excitable cells to change the potential across their membranes. In addition to providing a hydrophilic channel across the membrane, they generally have the select for specific ions, sense local signals, and gate the movement of ions. selectivity filter sensor lipid membrane gate These 3 central properties of ion channels are outlined below: A. Sensor voltage sensor ligand sensor ionotropic: nahr, iglur, gly, GABAA second messenger: cam, cgm, a 2+
(metabotropic: mglur, mahr, GABAB, etc.) mechanical sensor stretch, pressure, touch, vibration B. Selectivity filter Selectivity filters allow selected ions to overcome the energy barrier posed by the hydrophobic lipid membrane. Importantly they increase the rate of ion movement, but the ions still need a driving force, which determines the direction of their movement. ommon examples of selectivity filters are those for a, K, l, a, and nonselective cations. This selectivity is often non-selected ion determined by either steric factors or by selected ion binding energies to sites within the channel. Since ion channels conduct ions in single file, they have a rather high electrical resistance (low conductance) when driving force compared with typical electronic components: γ=/r e.g. for a a channel γ = 0-2 A/00 0-3 V = 0 ps. Gate To a first approximation gates are either open (selected ions can move freely) or closed (no ions can move). For voltage-sensing channels, there are two general categories of gates: activation/deactivation - open on depolarization/close on repolarization inactivation - close on depolarization free energy activation deactivation activation deactivation inactivation
III hannel structure Recent X-ray crystallographic studies have produced fine resolution detail of a few ion channels, notably certain K channels and the nicotinic acetylcholine channel. These molecular pictures have provided a detailed understanding of how the channel protein can produce properties of selectivity, sensing, and gating. Standard nomenclature for channel proteins is: subunit a peptide domain a repeated portion of a peptide (indicated with Roman numerals) segment a membrane spanning sequence (indicated with S or M followed by Arabic numerals) A. a, a channels I II III IV 2 3 5 4 S4: voltage sensor S5 linker ( loop): pore liner
B. K channel (Tetrameric assembly with the same functional relationships as the a channel.). nahr superfamily (nahr, 5HT3, GABAA, gly) 4 2 3 2 2 2 2 M2 M2 (entemeric assembly pore-lining M2 also acting as gate.) D. Ionotropic glutamate channels (iglur)
IV. Structure function relationships A. Voltage sensor Two types of experiments have demonstrated the role of the charged S4 segment of a, a, and K channels as voltage sensors:. hanging the number of + charged amino acids in this segment change the voltage dependence of gating in a very predictable manner. conductance decreased Arg or Lys in S4 V m 2. Specific amino acids in S4 can be fluorescently labeled and then they can be tracked as the gating charge moves in response to changes in transmembrane potential. B. Inactivation gate One common type of inactivation gating is provided by a + charged ball on a chain at the terminal of the channel protein. Depolarization forces this + charged region into the pore which blocks further ion conduction. +. Selectivity filter MacKinnon s lab has shown specific binding sites in the loop that select for K ions in a K channel. In closely related a and a channels, point mutation of Lys Glu in loop of a channel changes its selectivity from a to a. V. hannel evolution Voltage gated channels appear to have evolved from early prokaryotic nonselective cation channels, first as K channels, then with a channels branching off, and most recently with the evolution of a channels.