Potassium Channel

8/8/2019 Potassium Channel

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POTASSIUMPOTASSIUMCHANNELCHANNEL


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potassium channels are the most widelydistributed type ofion channel and are found

in virtually all living organisms. They form potassium-selective pores that

span cell membranes.

Furthermore potassium channels are found inmost cell types and control a wide variety ofcell functions.


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Function

In excitable cells such as neurons, they shapeaction potentials and set the resting membranepotential.

By contributing to the regulation of the actionpotential duration in cardiac muscle, malfunctionof potassium channels may cause life-threatening arrhythmias.

They also regulate cellular processes such as thesecretion ofhormones (e.g., insulin release frombeta-cells in the pancreas) so their malfunctioncan lead to diseases (such as diabetes).


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Types

There are four major classes of potassiumchannels:

Calcium-activated potassium channel - openin response to the presence ofcalcium ions orother signalling molecules.

Inwardly rectifying potassium channel -passes current (positive charge) more easilyin the inward direction (into the cell).


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Tandem pore domain potassium channel -areconstitutively open or possess high basalactivation, such as the "resting potassium

channels" or "leak channels" that set thenegative membrane potential of neurons. Whenopen, they allow potassium ions to cross themembrane at a rate which is nearly as fast astheir diffusion through bulk water.

Voltage-gated potassium channel -are voltage-gated ion channels that open or close inresponse to changes in the transmembranevoltage.


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Structure

Potassium channels have a tetrameric structure inwhich four identical protein subunits associate toform a fourfold symmetric (C4) complex arranged

around a central ion conducting pore (i.e., ahomotetramer). Alternatively four related but not identical protein

subunits may associate to form heterotetramericcomplexes with pseudo C

4symmetry.

All potassium channel subunits have a distinctivepore-loop structure that lines the top of the pore andis responsible for potassium selective permeability.


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Top view of purple potassium ions moving throughpotassium channel


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Bacterial potassium channels shut (left, PDB 1k4c) and open(right, 1lnq).They can sense voltage differences across

membrane, and then change conformation


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There are over 80 mammalian genes that encodepotassium channel subunits.

However potassium channels found in bacteria

are amongst the most studied of ion channels, interms of their molecular structure. Using X-ray crystallography, profound insights

have been gained into how potassium ions passthrough these channels and why (smaller)sodium ions do not.

The 2003 Nobel Prize for Chemistry wasawarded to Rod MacKinnon for his pioneeringwork in this area.


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Selectivity filterSelectivity filter

Potassium ion channels remove the hydrationshell from the ion when it enters theselectivity filter.

The selectivity filter is formed by five residues(TVGYG-in prokaryotic species) in the P loopfrom each subunit which have their electro-negative carbonyl oxygen atoms alignedtowards the centre of the filter pore and forman anti-prism similar to a water solvatingshell around each potassium binding site.


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Crystallographic structure of thebacterial KcsA potassium channel(PDB1K4C).[11] In this figure, only twoof the four subunits of the tetramer aredisplayed for the sake of clarity. Theprotein is displayed as a green cartoon

diagram. In addition backbone carbonylgroups and threonine sidechain proteinatoms (oxygen = red, carbon = green)are displayed. Finally potassium ions(occupying the S2 and S4 sites) and theoxygen atoms of water molecules (S1and S3) are depicted as purple and redspheres respectively


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The distance between the carbonyl oxygensand potassium ions in the binding sites of theselectivity filter is the same as between wateroxygens in the first hydration shell and apotassium ion in water solution.

Passage of sodium ions would beenergetically unfavorable since the stronginteractions between the filter and pore helixwould prevent the channel from collapsing tothe smaller sodium ion size.


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The selectivity filter opens towards theextracellular solution, exposing four carbonyloxygens in a glycine residue (Gly79 in KcsA).

The next residue towards the extracellularside of the protein is the negatively chargedAsp80 (KcsA).

This residue together with the five filterresidues form the pore that connects thewater filled cavity in the centre of the proteinwith the extracellular solution


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The carbonyl oxygens are strongly electro-negative and cation attractive.

The filter can accommodate potassium ionsat 4 sites usually labelled S1 to S4 starting atthe extracellular side.

In addition one ion can bind in the cavity at asite called SC or one or more ions at theextracellular side at more or less well definedsites called S0 or Sext.


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Several different occupancies of these sites are possible. Since the X-ray structures are averages over many

molecules, it is, however, not possible to deduce the actualoccupancies directly from such a structure.

In general, there is some disadvantage due to electrostaticrepulsion to have two neighbouring sites occupied by ions.

The mechanism for ion translocation in KcsA has beenstudied extensively by simulation techniques.

A complete map of the free energies of the 24=16 states

(characterised by the occupancy of the S1, S2, S3 and S4sites) has been calculated with molecular dynamicssimulations resulting in the prediction of an ion conductionmechanism in which the two doubly occupied states (S1,S3) and (S2, S4) play an essential role.


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The two extracellular states, Sext and S0, were foundin a better resolved structure of KcsA at highpotassium concentration.

In free energy calculations the entire ionic pathwayfrom the cavity, through the four filter sites out to S0and Sext was covered in MD simulations.

The amino acids sequence of the selectivity filter of

potassium ion channels is conserved with theexception that an isoleucine residue in eukaryoticpotassium ion channels often is substituted with avaline residue in prokaryotic channels.


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Central Cavity

A 10 wide central pore is located near thecenter of the transmembrane channel where

the energy barrier is highest for thetransversing ion due to the hydrophobity ofthe channel wall.

The water-filled cavity and the polar C-

terminus of the pore helices ease theenergetic barrier for the ion.


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Repulsion by preceding multiple potassiumions is thought to aid the throughput of the

ions. The presence of the cavity can be understood

intuitively as one of the channel'smechanisms for overcoming the dielectric

barrier, or repulsion by the low-dielectricmembrane, by keeping the K+ ion in a watery,high-dielectric environment


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BlockersBlockers

Potassium channel blockers, such as 4-

aminopyridine and 3,4-diaminopyridine, havebeen investigated for the treatment ofconditions such as multiple sclerosis


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Muscarinic potassium channel

Some types of potassium channels are activatedby muscarinic receptors and these are calledmuscarinic potassium channels (I

KACh).

These channels are a heterotetramer composedof two GIRK1 and two GIRK4 subunits.

Examples are potassium channels in the heart,

which, when activated by parasympatheticsignals through M2 muscarinic receptors, causesan outward current of potassium which slowsdown the heart rate


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ATP-sensitivepotassium

channel An ATP-sensitive potassium channel is a type of

potassium channel that is gated by ATP.

ATP-sensitive potassium channels are composedof Kir6.x-type subunits and sulfonylurea receptor(SUR) subunits, along with additionalcomponents.

They can be further identified by their positionwithin the cell as being either sarcolemmal("sarcKATP"), mitochondrial ("mitoKATP"), ornuclear ("nucKATP").


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Discovery and structure

SarcKATP are composed of eight protein subunits. Four of these are members of),2.6irKor1.6irK.x (either6irthe inwardly rectifying potassium channel family K]2[).B2SUR, andA2SUR,1SURwhile the other four are sulfonylurea receptors (

The Kir subunits have two transmembrane spans and form the channels pore.The SUR subunits have three additional transmembrane domains, and contain

These allow for]3[side.cytoplasmicbinding domains on the-two nucleotide

nucleotide-mediated regulation of the potassium channel, and are critical in itsroles as a sensor of metabolic status. These SUR subunits are also sensitive tosulfonylureas, MgATP, and some other pharmacological channel openers. While

all sarcKATP are constructed of eight subunits in this 4:4 ratio, their precise]4[composition varies with tissue type.

MitoKATP were first identified in 1991 by single-channel recordings of the inneris less clearlyATPmitoKThe molecular structure of]5[mitochondrial membrane.

understood than that of sarcKATP. Some reports indicate that cardiac mitoKATPMore]7][6[.2nor SUR1subunits, but neither SUR2.6irand K1.6irconsist of Krecently, it was discovered that certain multiprotein complexes containing

]8[channels.ATPcan provide activity similar to that of Kdehydrogenasesuccinate

The presence of nucKATP was confirmed by the discovery that isolated patches ofnuclear membrane possess properties, both kinetic and pharmacological, similar

channelsATPKplasma membraneto


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Sensor of cell metabolism

Regulation of gene expression

1surgene family. TheATPhave been identified as members of the KgenesFourand kir6.2 genes are located in chr11p15.1 while kir6.1 and sur2 genes reside in

chr12p12.1. The kir6.1 and kir6.2 genes encode the pore-forming subunits of theKATP channel, with the SUR subunits being encoded by the sur1 (SUR1) gene or

]10[

B).2A and SUR2gene (SUR2surselective splicing of theATPof these genes, and thus the production of KtranscriptionChanges in the

channels, are directly linked to changes in the metabolic environment. HighmRNA2.6kirlevels, for example, induce a significant decrease in theglucose

Similarly,]11[an effect that can be reverse by lower glucose concentration.levelhours of reperfusion leads to an72to24followed byischemiaminutes of60

]12[.myocytestranscription in left ventricle rat2.6kirincrease in

andhypoxiareaction toAT

PA mechanism has been proposed for the cells KLow intracellular oxygen levels decrease the rate of metabolism by]13[ischemia.

in the mitochondria. Unable to transfer electronsTCA cycleslowing theratio decreases, activatingNADH/NAD+efficiently, the intracellular

. This, inkinasesregulated-and extracellular signalkinase-3-phosphotidylinositol2surtranscription, creating a protein which binds to thejun-cupregulatesturn,

promoter


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One significant implication of the linkbetween cellular oxidative stress and

increased KATP production is that overallpotassium transport function is directly

proportional to the membrane concentrationATP, Kdiabetesof these channels. In cases of

channels cannot function properly, and a

marked sensitivity to mild cardiac ischemiaand hypoxia results from the cells' inability to

adapt to adverse oxidative conditions


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Metabolite regulation

The degree to which particular compounds are able to regulate KATP channelopening varies with tissue type, and more specifically, with a tissues primary

metabolic substrate.

/ATP ratioADP, which are sustained primarily by ATP, thebeta cellspancreaticIn

determines KATP activity. Under normal conditions, when ATP is relativelyIf the beta cells are subjected to]15[), the channels are closed.nM20-10plentiful (oxygen or glucose deprivation, however, ADP levels increase and the channelsopen. The change from one state to the other happens remarkably quickly and

potential among-multimerizationterminus-Cwith great synchronization due to]16[channels.ATPproximate K

, on the other hand, derive the majority of their energy fromCardiomyocytesequivalents. Cardiac ischemia, as itCoA-acyland theirfatty acidschain-long

slows the oxidation of fatty acids, causes an accumulation of acyl-CoA andinduces KATP channel opening while free fatty acids stabilize its closed

mice,transgenicconformation. This variation was demonstrated by examiningbred to have ATP-insensitive potassium channels. In the pancreas, these

channels were always open, but remained closed in the cardiac cells


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Metabolite regulation

The degree to which particular compounds are able to regulate KATP channelopening varies with tissue type, and more specifically, with a tissues primary

metabolic substrate.

/ATP ratioADP, which are sustained primarily by ATP, thebeta cellspancreaticIn

determines KATP activity. Under normal conditions, when ATP is relativelyIf the beta cells are subjected to]15[), the channels are closed.nM20-10plentiful (oxygen or glucose deprivation, however, ADP levels increase and the channelsopen. The change from one state to the other happens remarkably quickly and

potential among-multimerizationterminus-Cwith great synchronization due to]16[channels.ATPproximate K

, on the other hand, derive the majority of their energy fromCardiomyocytesequivalents. Cardiac ischemia, as itCoA-acyland theirfatty acidschain-long

slows the oxidation of fatty acids, causes an accumulation of acyl-CoA andinduces KATP channel opening while free fatty acids stabilize its closed

mice,transgenicconformation. This variation was demonstrated by examiningbred to have ATP-insensitive potassium channels. In the pancreas, these

channels were always open, but remained closed in the cardiac cells


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Cardiovascular KATP channels and protection from ischemic injury

Cardiac ischemia, while not always immediately lethal, often leads to, causing permanent injury toapoptosisdeath bycardiomyocytedelayed

the heart muscle.One method, first described by Keith Reimer in 1986,involves subjecting the affected tissue to brief, non-lethal periods of

ischemia (35 minutes) before the major ischemic insult. This procedure("IPC"), and derives itsischemic preconditioningis known aseffectiveness, at least in part, from KATP channel stimulation.

Both sarcKATP and mitoKATP are required for IPC to have its maximaleffects. Selective mito KATP blockade with 5-hydroxydecanoic acid (5-

affordedcardioprotectioncompletely inhibits the]20[134-HD) or MCCin mice has been]21[genes.ATPsarcKofgenetic knockoutby IPC, and

shown to increase the basal level of injury compared to wild type mice.This baseline protection is believed to be a result of sarcKATPs ability toprevent cellular Ca2+ overloading and depression of force development

during muscle contraction, thereby conserving scarce energy]22[resources.


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Absence of sarcKATP, in addition to attenuating the benefits ofIPC,significantly impairs the myocytes ability to properly distribute Ca2+,

signals, and predisposingsympathetic nervedecreasing sensitivity toATPsarcKSimilarly,]23[and sudden death.arrhythmiathe subject to2suror2.6kirtone, and deletion of thesmooth muscleregulates vascular]24[

and death.vasospasmcoronary arterygenes leads toregulation,cardiac rhythmrole insATPsarcKUpon further exploration of

it was discovered that mutant forms of the channel, particularlydilatedsubunit, were responsible for2mutations in the SURIt is still]25[, especially after ischemia/reperfusion.cardiomyopathy

unclear as to whether opening of KATP channels has completely pro- orantiarrhythmic effects. Increased potassium conductance should

stabilize membrane potential during ischemic insults, reducing theactivity.On the other hand,ectopic pacemakerextent infarct andof the actionrepolarizationpotassium channel opening accelerates

potential, possibly inducing arrhythmic reentry

Potassium Channel

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