4. ion channels

• Ion channels are passive transporters allowing ions to move down their electrochemical gradients without ATP
• They exhibit high selectivity for specific ions and have fast transport rates
• Ion channels feature hydrophilic pores and may be gated, controlling ion passage
• Ion channels transport ions at rates of 10^7 to 10^8 ions/s, much faster than transporters
• The selectivity of ion channels ensures specific ions pass through efficiently, akin to diffusion rates in solution
• Ion channels have hydrophilic pores facilitating ion transport in aqueous environments
• Ion channels may be gated, allowing them to be activated or deactivated by changes in voltage or specific molecules
• Ion channels vary based on the ions they transport, with common types including potassium (K+), sodium (Na+), calcium (Ca2+), chloride (Cl-), and proton (H+) channels, along with non-selective ion channels
• Potassium channels regulate membrane potential and play a role in action potential recovery
• Sodium channels underlie the initiation of action potentials by facilitating depolarization
• Calcium channels increase cytoplasmic calcium levels, crucial for inducing secretion processes
• Chloride channels regulate ionic balance within cells
• Proton channels rapidly modulate intracellular pH levels
• Non-selective ion channels include receptors like acetylcholine and glutamate receptors, contributing to various cellular functions
• The pore of ion channels is lined with polar regions of the protein, rendering it hydrophilic
• Conformational changes in ion channels result from changes in the protein structure
• The selectivity filter of ion channels ensures only specific ions can pass through, crucial for precise regulation of ion movement
• The gate of the ion channel can be in an open or closed state, controlling ion flow
• Voltage-gated channels open and close in response to changes in membrane potential
• Ligand-gated channels open in response to the binding of extracellular or intracellular ligands
• Mechanically-gated channels open in response to mechanical stress or force applied to the channel or surrounding membrane
• Potassium channels are highly conserved across different cell types, making them an excellent model for studying ion channels
• The KcsA potassium channel is the first known structure of an ion channel, providing valuable insights into ion channel function
• The selectivity filter of the KcsA channel prefers potassium ions (K⁺) over other ions like sodium (Na⁺)
• The KcsA channel has a pore with five rings of oxygen atoms, each about 3 Å in diameter
• When a K⁺ ion enters the KcsA channel, it loses its water coating and sticks to the oxygen atoms inside
• The oxygen atoms in the KcsA channel create spots where K⁺ ions can briefly stop and stabilize
• Due to their positive charge, only two K⁺ ions can hang out in the KcsA channel pore at once
• The KcsA channel allows preferential passage for K⁺ ions over Na⁺ ions
• The size of the KcsA channel fits K⁺ ions perfectly, while Na⁺ ions are too small to interact with all the oxygen atoms
• Both K⁺ and Na⁺ ions exchange their water coating for oxygen atoms inside the KcsA channel
• Na⁺ ions can only interact with a few oxygen atoms in the KcsA channel, making it less likely for them to pass through
• Eukaryotic potassium (K⁺) channels exhibit voltage sensitivity, allowing them to regulate ion flow across the cell membrane
• Potassium (K⁺) channels exhibit voltage sensitivity, unlike some channels that are always open
• The sensitivity to changes in voltage allows potassium channels to regulate ion flow across the cell membrane
• The voltage sensor in potassium channels is formed by transmembrane segments S1 to S4 within each subunit
• S4 segment in potassium channels contains positively charged amino acids, making it sensitive to changes in membrane potential
• In a resting state when the cell interior is negatively charged, S4 remains near the cytosol, keeping the channel closed
• During depolarization, S4 in potassium channels is pushed across the membrane, inducing a conformational change in the channel
• The movement of S4 in response to changes in membrane potential triggers a series of conformational changes in the channel protein, leading to the opening of a gate for ion passage