Presentation on theme: "Ion Channels. Active Transporters: The proteins that created and maintain ion gradients Ion channels : give rise to selective ion permeability changes."— Presentation transcript:
Active Transporters: The proteins that created and maintain ion gradients Ion channels : give rise to selective ion permeability changes
Ion channels are transmembrane proteins that contain a specialized structure, THE PORE that that allow particulars ions to cross the membrane. Some ion channels contain voltage sensor ( voltage gated channels) that open or close the channel in response to changes in voltage. Other gated channels are regulated by extracellular chemical signals such as neurotransmitter or by intracellular signals as a second messengers. ION CHANNELS
ACTIVE TRANSPORTERS Membrane proteins that produce and maintain ion concentration gradients. For example the Na+ pump which utilizes ATP to regulate internal concentration of Na+ and K+. Transporters create the ionic gradient that drive ions through open channels, thus generating electric signals
What is the mechanism for ion movement across the membrane? K + and Na + currents were distinct, suggesting distinct mechanisms Mechanism is voltage dependent (must sense voltage) Voltage clamp recordings showed that ions move across membrane at high rates (~ 600,000 /s) – inconsistent with an ion pump mechanism Ion selectivity of Na + and K + currents – size dependent permeability suggests pore of certain diameter. Armstrong (1965-6) – TEA block could be overcome by adding excess K + to the extracellular fluid and stepping to hyperpolarized potentials (K + comes into cell) suggesting that K + ions dislodge TEA from pore
Ion channels share several characteristics The flux of ions through the channel is passive. The kinetic properties of ion permeation are best described by the channel conductance (g) that is determinate by measuring the current flux (I) that flows through the channel in repose to a given electrochemical driving force. (Electrochemical driving force is determinate by difference in electric potential across membrane and gradient of concentration of ions). At the single channel level, the gating transitions are stochastic. They can be predicted only in terms of probability.
Ion channels share several characteristics In some channels the current flow varies linearly with the driving force ( channels behave as resistors) In other channels, current flow is a non-linear function of driving force ( Rectifiers) I (pA) V (mV) Ohmic channel ( I=Vm/R) Rectifying Channel Low conductance (γ) High conductance (γ)
Ion channels share several characteristics The rate of ion flux (current) depends on the concentration of the ions in solution ( At low concentrations the current increases linearly with the concentration, at higher concentrations the current reach a saturation point ). The ionic concentration at which current flow reaches half its maximum defines the dissociation constant for ion binding. Some ion channels are susceptible to occlusion by free ions or molecules
The Opening and closing of channels involve conformational changes In all channel so far studied, the channel protein has two or more conformational states that are relatively stable. Each stable conformation represents a different functional state.. Each channel has an open state and one or two closed states. The transition between states is calling gating.
The Opening and closing of channels involve conformational changes Three major regulatory mechanisms have evolved to control the amount of time that a channel remains open and active. Under the influence of these regulators,channels enter one of three functional states: closed and activable (resting), open (active) or closed and nonactivable ( refractory). The signal that gate the channel also controls the rate of transition between states.
The Opening and closing of channels involve conformational changes Ligand -gated and voltage gated channels enter refractory states through different process. Ligand-gated channels can enter refractory state when the exposure to ligand sis prolonged (desensitization) Voltage-gate channels enter a refractory state after activation. The process is called inactivation. Activation is the rapid process that opens Na+ channels during a depolarization. Inactivation is a process that closes Na+ channels during depolarization. The membrane needs to be hyperpolarized for many milliseconds to remove inactivation.
The Opening and closing of channels involve conformational changes Exogenous factors such as drugs and toxins can affect the gating control sites.
Structure of Ion Channels Ion channels are composed of several subunits. They can be constructed as heterooligomers from distinct subunits, as homooligomers from a single type of subunit o from a single polypeptide chain organized into repeated motifs. In addition to one or more pore forming unties, which comprise a central core, some channels contain auxiliary subunits which modulate the characteristics of the central core
Structure of voltage gated ion channels Repeated series of 6 TM helices S4 helix is voltage sensor Loop between S5 & S6 composes selectivity filter
Gating currents Movement of + charges in S4 segment produces small outward current that precedes ion flux through channel
Role of auxiliary subunits Auxiliary (non pore) subunits affect: Surface expression Gating properties
Voltage gated sodium channels Blocked by: TTX, STX, *cain local anesthetics A large alpha subunit that forms the core of the channel and its functional on its own. It can associate with beta subunits
Persistent (non- inactivating) Na + currents are produced by an alternative channel gating mode
Protein nameGeneExpression profileAssociated channelopathies Na V 1.1SCN1ACentral and peripheral neurons and cardiac myocites Febrile epilepsy, severe myclonic epilepsy of infancy, infantile spasms, intractable childhood epilepsy, familial autisms Na v 1.2SCN2ACentral and peripheral neurons Febrile seizures and epilepsy Na v 1.4SCN4ASkeletal musclePeriodic paralysis, potassium agravated myotonia Na v 1.5SCN5ACardiac myocites, skeletal muscle, central neurons Idiopathic ventricular fibrillation Na v 1.7SCN9ADorsal root ganglia, peripheral neurons. Heart, glia Insensitivity to pain. Functions of voltage-gated Na+ channel alpha subunits
Voltage gated Ca 2+ channels Gene ProductCa v Ca v 2.1Ca v 2.2Ca v 2.3Ca v Tsien Type“L”“P/Q”“N”“R”“T” CharacteristicsHigh voltageMod voltage High voltageMod voltageLow voltage activated,activated, activated,activatedactivated slow inactivation moderate moderatefastfast (Ca2+ dependent)inactivation inactivationinactivationinactivation Blocked by dihydropiridinesAgatoxin Conotoxin SNX 482Mibefridil (nimodipine)IVAGVIAHigh Ni 2+ Form by different subunits:α1, α2δ,β and γ. The α1 subunit forms the pore, the other subunits modulate gating.
Ca 2+ dependent Ca 2+ channel inactivation Ca 2+ - channel CaM Ca 2+
Inwardly-rectifying and “leak” K + channels Inwardly-rectifying channels subunits: Kir 1.X - 7.X Rectifying character due to internal block by Mg 2+ and polyamines Roles: Constitutively active resting K+ conductance (eg. Kir1, Kir2) G-protein activated (Kir3) ATP sensitive (Kir6) 2 pore “leak” channels many different subunits, nomenclature still argued Outwardly rectifying due to unequal [K + ] across the membrane Roles: Constitutively active resting K + conductance pH sensing Mechanosensitive Thermosensitive Second messenger sensitive (cAMP, PKC, arachadonic acid) Inwardly-rectifying2 pore “leak”
Voltage gated K + channels K v 4 (“A type”) K v 1 (“D type”) K v 2 (“DR type”) K v 3 (“DR type”) Gene ProductK v 1.X (1-8)K v 2.X (1-2)K v 3.X (1-4)K v 4.X (1-4)Kv7.X (1-5) “D type”“Delayed“Delayed “A type”“M current” rectifier” Characteristics Low voltageHigh voltageHigh voltageLow voltageLow voltage activated (~50 mV),activated (0 mV),activated (-10 mV),activated (-60 mV)activated (-60 mV) fast activation mod activation fast activationfast activationslow activation ( 20 ms) (10-20 ms)(10-20 ms)(>100 ms) slow inactivation very slowvery slowfastno inactivationinactivationinactivationinactivation fast deactivation Blocked by 4-AP (100 µM) TEA (5-10 mM) TEA ( mM) 4-AP (5 mM) XE991 dendrotoxin4AP (1-5 mM)4AP (0.5-1 mM) BDS (50 nM)
Ca 2+ activated K + channels - role in repolarization following APs Spike frequency accommodation Voltage responsecurrents mediating AHP Role of IK Ca in burst duration
Ca 2+ activated K + channels Channel TypeBKSKsAHP “maxi K, I C fAHP”“mAHP”“sAHP” Gene productslo 1-3SK1-3???? Voltage dep?YesNoNo [Ca 2+ ] to activate1-10 µM0.1-1 µM0.1-1 µM Ca 2+ bindingdirect to subunitcalmodulinhippocalcin? Single channel pS5-20 pS5-10 pS Conductance Blocked by charybdotoxinapamin TEA (> 20 mM) TEA ( 20 mM)
Many drugs and toxins act on voltage gated ion channels
Effect of drugs and toxins Many toxins block ion channels directly either from the outer (TTX) or inner (lidocaine) surface of the channel Other toxins change the properties of the channel without blocking it –Delaying inactivation –Shifting voltage dependence TTX LA FUGU
Modulation of Ion Channels Example, enhancement of Ca 2+ channels in cardiac myocytes by NE
Dendritic ion channels participate in synaptic amplification and integration