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Unformatted text preview: Electrochemical Potential Ions can cross membranes through Ions channels channels Form of Passive Transport Ion Movement Controlled by Concentration Ion Gradient and Electrical Force Gradient Membrane Potential Voltage Across Membrane Resting potential is - 60 mV (more negative Resting inside) inside) ∆G = ZF∆V (Z = charge; V = voltage; F = Faradays’ ZF∆V
constant) constant) Chemical Potential Concentration Gradient Across Membrane ∆G = RT ln C1/C2 (R= gas constant; T = temperature RT
°K) °K) Electrochemical Potential Channels Channels Ions cannot move across a lipid bilayer Channels are integral membrane proteins with a Channels hydrophilic pore that provides a path for ion movement movement Selective for passage of specific ions Most channels can exist in open or closed states Most so that ion movement can be regulated so Function in every type of eucaryotic cell brain muscle contraction some types also present in eubacteria Patch clamp is used to study channels
Patch Clamp Patch Clamp Method
(Nobel Prize in 1992) Measure Channel Properties
5Duration of Opening 5Current/Voltage Function 5Conductance 5Ion Selectivity 5Regulation 5Structure/Function
V = IR = I/C; I = CV
(Voltage, V; Resistance, R; Current, I; Conductance, C) Plot Current vs Voltage Slope gives conductance. If not a simple straight line, then voltage sensitive. Units: Volts, pAmps (pA), pSiemens (pS) Nernst Nernst Equation and the K+ Leak Channel
[K+] out out 5 mM mM Electrical Force l G = ZFl V h h Electrical Chemical Force l G = RT ln l C h Chemical RT h Nernst Equation ZF∆V = RT ln(K+ out/K+ in) /K+ in Relates electric force & chemical force on Relates ion ion K+ Leak Channel Always Open Can pass 6 million K+/sec Inhibited by triethylammonium (TEA) Sets membrane potential K+ flows until chemical and electrical forces are in balance are - 70 mV [K+] in [K in 145 mM 145 ∆V = RT ln (K+ RT out/K+ in) out in ZF ZF Gated Channels Gated Channels
Ion Specific Regulated Opening Conserved Structure Voltage Gated Na+, K+, Ca++ Ligand Gated Acetylcholine
Neuromuscular junction cGMP
Vision Mechanically Gated least well studied
Hearing Pressure Touch Pain Action Potential Action Potential: Voltage Gated Na+ Channels Unidirectional Propagation Three Structural States Open Closed Inactivated Channel Closes Itself Structural Features: Pore Voltage Sensor Ion Selectivity Filter Inactivation Gate to Close 3 States 3 States of the Voltage Gated Na+ Channel During an Action Potential Closed
state in resting cell at - 60 mV Depolariz a-tion opens channel for a short time Open Channel inactivates itself Inactivate d channel reverts to the closed state
Structure Structure/Function of Voltage Gated Channels Pore: 24 Transmembrane Pore: Spans Spans 4 Sets of 6 Na+ channel- 24 in 1 peptide K+ channel- 4 peptides Quaternary structure Voltage Sensor: + charges M4 Channel Closing: Channel NH2-terminus on K+ channel IFM motif in Na+ channel Ion selectivity: Ion Loop between M5 &M6 is Loop inserted in the bilayer and lines the pore the Computer did NOT predict Computer correctly correctly Pore Helix Selectivity Filter Loop Channel Closing Closing of Voltage Gated Channels
Ball and Chain Model Na+ Channel IFM Motif IFM Isoleucine Phenylalanine methionine K+ Channel NH2 terminal leucine 3 states Three Conformations of Voltage Gated Channel: Open, Closed, Refractory. Lodish 5th. 7-33 Lodish Channel: Bacterial Channel Structure of a Bacterial K+ Channel Structure Bacterial channel has only 2 TM regions with loop between (TM 5 and 6 of mammalian) Selectivity loop determines which ions can pass channel. Characteristic amino acid side chains do not contact the ion (K+ channel: (T V G Y G) (T Side chains interact with hydrophobic core and set diameter of pore. Carbonyl groups interact with ion, and help to strip water. Polar pore helix promotes flow of ion through pore. Ion Selectivity Size difference of Na+ and K+ is basis for selectivity.
Atomic Radius: K+ 1.33 Å Na+ 0.97 Å Selectivity filter strips water from ion. In pore, carbonyl groups replace water. Sodium is too small to pass K+ channel. Cannot strip water from sodium due to its smaller diameter. Water of hydration helps to explain ion selectivity of channels. Neuromuscular Junction Neuromuscular Junction ...
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This note was uploaded on 04/03/2011 for the course CBIO 3400 taught by Professor Shen,kipreos during the Spring '08 term at University of Georgia Athens.
- Spring '08