Flashcards in chapter 13 Deck (55):
1
pumps
use a source of ree energy such as atp or light to drive thermodynamically uphill transport of ions or molecules (active transport
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channels
enable ions to flow rapidly through membranes in a downhil direction (passive/faciliatited transport)
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3 type of pumps
1. p-type ATPase
2. atp binding casette (abc) transporters
3. secondary transporters
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2 energy sources for pumps
atp or concentration gradient
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p-type atpases
use atp to drive active transport of a specific ion
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3 p-type atpases
ca2+ atpase: transports calcium from cytoplasm to sarcoplasmic reticulum of muscle cells
2. H+ K+ atpase enzyme responseible for pumping sufficient protons into the stomach to lower the pH below 1.0
2. na+ k+ atpase - pumps na+ out of the cell and k+ into the cell
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structure of sarcoplasmic reticulum ca2+ ATPase (SERCA)
110-kd polypeptide with a transmembrane domain consisting of 10
ALPHA helices. transmembrane domaine includes sites for binding 2 calcium ions. each calcium ion is coordinated to 7 oxygen atoms
3 domains: N binds atp nucleotide, P accepts phosphoryl group on aspartate residue, and A serves as an actuator for N domain
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what does SERCA do?
maintains a calcium ion concentration of about .1 mM in the cytosol compared with 1.5 mM in the sarcoplasmic reticulum
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what is muscle contraction and relaxation triggered by?
contraction: Abrupt rise in cytosolic calcium level. relaxation depends on rapid removal of ca2+ from cytosol into sarcoplasmic reticulum
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sarcoplasmic reticulum:
contains large stores of calcium
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what happens to SERCA after atp hydrolysis
N and P domains move toward one another during catalytic clycle. closure is facilitated by atp binding and binding of ca2+ to the membrane spanning helices
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what happens when a phosphoryl analog binds, but no calcium
very different compared to structure without atp but with calcium
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simple pump action mechanism
pump interconverts between 2 conformational states, each with a binding site acccessible to a different side of the membrane
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serca mechanism
1. atp binds into n domain, atp cleaved to adp, clacium is released
finally calcium goes back in
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digitalis
plants that inhibit na+ K+ pump.
digitoxigenin and ouabain are membres of this class as inhibitors
they inhibit dephosphorylation of ep-2 form of the atpase when applied on the extracellular face of the membrane.
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digitoxigenin and ouabain are known as?
cardiotonic streoids becasue of their strong effects on the heart
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how is digitalis used for congestive heart failure
increases force of contraction of heart muscle. inihibition of the na k+ pump leads to higher Na+ inside the cell. this risults in slower extrusion of CA2+ by sodium calcium exchanger. increase in intracellular level of ca2+ enhances contractility of cardiac muscle
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multidrug resistance
tumor cells in culture often becoem resistant to drugs that were intialy quite toxic to the cells. development of resistance to one drug also makes the cells less sensitive to a range of other compounds
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what causes multidrug resistance
multidrug resistance protein acts as an atp dependent pump that extrudes a wide range of small molecules from cells that express it. when cells are xposed to the drug, the MDR pumps the drug out of the cell before the drug can exerti ts effects
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common architecture of MDR and homologous proteins
each protein has 4 domains: 2 membrane binding domains and 2 atp binding domains (abc transporters atp binding casettes). they are members of the p-loop NTPase super family
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bacterial abc transporters are usually what compared to eukaryotic ones?
Bacterial ABC transporters are usually multimeric whereas
eukaryotic ones are monomeric
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Mechanism of abc transporter mechanism
1. opening of channel toward inside of teh cell
2. substrate binding and conformational changes in the atp-binding casettes
3. atp binding and further conformational changes
4. speapartion of the membrane binding domains and relase of the substrate to the other side of the membrane
5. atp hydrolysis to reset the transporter to its intitial state
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how do abc transporters differ from p-type atpases?
different mechanism
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what is a secondary transporter
membrane proteins that pump ions or moelcules uphill by coupling with the dowhill flow of a different species
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2 types of secondary transporters/cotransporters
antiporters or symporters
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what are antiporters
couple the downhill flow of one species to the uphill flow
of another in the opposite direction across the membrane
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what are symporters
Symporters use the flow of one species to drive the flow of a
different species in the same direction across the membrane
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what are uniporters
Uniporters are governed by the concentration of a single species
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lactose permease
symporter uses the h+ gradient across e.coli membrane generated by oxidation of fuel molecules to drive uptake of lactose and other sugatrs against a concentration gradient. has a proton binding site and al actose binding site
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lactose permease structure
2 halves each with 6 membrane spanning alpha helices. 2 halves surround sugar and are linked to each other by a single polypeptide. the sugar lies in a pocket in the center of the protein and is accesible from a path that leads from the interior of the cell
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mechanism of lactose permease
1. h+ from outside binds to coo-,
2. lactose binds between two halves at the active site.
3. eversion
4. lactose pumped in
5. h+ pumped in
5. eversion
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properties of ion channels
1. ion channels can be highly selective for particular ions
2. ion channels exist in open and closed states
3. transitions between the open and closed states are regulated
4. open states of channels often spontaneously convert into inactive state
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ligand gated channels
channels open and close in response to the bding of specific chemicals
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voltage gated channels
open and close in resposne to the elctrical potential across the membrane in which they are found
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patch clamp technique
enables measurment of the activity of asingle ion channel to be measured.
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how do you do the patch clamp technique
a clea glass pipette with a tip diameter of about 1mm is preast against an intact cell to form a seal. slight suction ldeads to the formation of a very tight seal. (gigaseal) ensures that an electric current flowing through pipette is identical to current flowing through membrane covered by pipette
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cell attached
high resistance gigaseal formed
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whole cell mode
breaking of the membrane patch by increased suction produces low resistance pathway between pipette and interior of the cell
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excised patch mode
pieptte is pulled away from the cell
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acetylcholine receptor channel
ligand-gated channel which communicates the nerve impuse between certain neurons
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voltage gated na+ and k+ channels
conduct the nerve impuse down the axon of a neuron
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potassium ion channel
structure
both sodium and potassium channels contain 4 repeats, each repeat has 5 hydrophobic segments and one positively charged segment. hydrophobic segments wer thought to be membrane spanning while positively charged segment was a voltage sensor. it is tetrameric with 2 membrane spanning alpha helices. the subunits come together to form a pore in the same of a cone that runs through center of structure. opening to the outside and central cavity of the pore are filled with water so that k+ ion can fit in the pore without losing solvation shell of water
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stucture of potassium ion channel pore
inside of the cell diameter is about 10 angstroms, constricts to 8 angstroms and finally down to 3 angstroms where the k+ ions must give up water molecules and interact with carbonyl groups of the protein
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what must happen for potassium ions to relinquish their water molecules?
other polar interactions must replace those with water
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what functions as a selectivity filter that determines preference for k+ over other ions?
5 amino acid stretch within region including thr, val, gly, tyr, gly which is observed in all k+ channels
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how is selectivity of potassium ion channel achieved
narrow diameter 3 angstomrs rpevents having ions with a radius greater than 1.4 angstroms. but na+ is small enough to fit. it does not pass through, however because it is energetically unfavorable to become unsolvated because it is too small. protein only compensates for k+ losing its solvation shell
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how does the potassium ion channel manage to have tight binding but quick transport rate?
4 k+ binding sites present in the constristed region of the k+ channel. one hydrated k+ ion proceeds into the channel and through the relatively unrestriscted part of the channel
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potassium ion channel mechanism
1. 1 hydrated k+ ion proceeds into the channel and through the relatively unrestrsicted part of the channel
2. it gives up most or all of its coordinated water molecules and binds to the first site of the slectivity filter region
3. the binding energy of the second site presents a free energy barrier or trap preventing the ion from completing its journey
4. if a second ion moves through the channel into the first site, the electrostatic repulsion betwee 2i ons will stabilize initially bound ion and push it into solution
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what are s1 and s4 in voltage gated channels
paddles that extend from core of the channel. s4 is the voltage sensor. s1 5o s4 lie within the membrane. positively charged s4 region lies on the outside of the structure at the bottom of the pore
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mechanism of voltage gated channel
voltage sensing paddles lie in the down position below the closed channel. membrane depolarization pulls these paddles through the membrane. this motion pulls the base of the channel apart, opening the channel
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what happens when exposing cytoplasm side of potassium or sodium channel to trypsin?
cleavage of trypsin produced a trimmed channel that stayed persistently open after depoalrization
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what happens when mutant potassium channel lacked 42 amino acids near amino terminus?
opened in response to depolarization, but did not inactivate. inactivation was restored by adding a syntehtic peptide corresponding to the first 20 resideus of the native channel
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ball and chain model of inactivation
first 2 residues of potassium channel form a cytoplasmic unit (ball) that is atached to a flexible segment of the polypeptid (chain). when channel is closed, ball rotates freely in the aqueous solution, when the channel opens, the ball quickly finds a complemetnary site in the pore and occludes it
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where is the ball located in the inactivated state?
located in the cytosol
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