Ch 13 Text 4

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Ch 13 Text 4
2014-11-08 15:07:57
Test Three
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  1. ·         Channels engage in __; and ion channels are capable of __. These channels are highly sophisticated molecular machines that respond to __ and __ in the environments and undergo precisely timed conformational changes
    • passive transport
    • ion-transport
    • chemical and physical changes
  2. ·         __ are mediated by transient changes in Na+ and K+ permeability
    o   A __ (an electrical signal produced by the flow of ions across the plasma membrane) provides the communication
    o   The interior of a neuron has a high concentration of __ and low concentration of __; and, the ion gradients are generated by the __
    o   The cell membrane has an __ determined by the ratio of the internal to the external concentration of ions
    §  At rest, membrane potential is __
    • Action potentials
    • nerve impulse
    • K+
    • Na+
    • Na+-K+ ATPase
    • electrical potential
    • -60 mV
  3. ·         A nerve impulse (__) is generated when the membrane potential is __ (60 to 40 for ex)
    o   The membrane potential becomes __and has a value of about 30 mV before __
    • action potential
    • depolarized beyond a critical threshold value
    • positive 
    • repolarization
  4. ·         Depolarization of the membrane beyond the threshold level leads to __, which flow in due to the large __. This flow further depolarizes the membrane, leading to __, leading to a large change in membrane potential from -60 to 30 mV
    • an increase in permeability to sodium ions
    • electrochemical gradient
    • further Na+ permeability
  5. ·         Afterwards, the membrane spontaneously becomes __ and __, which flows outwardà membrane potential is now negative and resting level of -60 mV is respond
    ·         For this process to occur, channels must exist and __in response to changes in membrane potential and then __
    • less permeable to Na+
    • more permeable to K+
    • open 
    • close after opening
  6. ·         Experiments to find this include the __, which enables the measurement of the ion conductance through a small patch of cell membrane
    o   Technique:__
    • patch-clamp technique
    • clean glass pipette with tip diameter is pressed against intact cell to form a seal. Suction leads to a tight seal forming so that resistance between the inside of the pipette and the bathing solution forms, called a gigaohn seal (gigaseal)
  7. §  This seal ensures that an __
    o   The flow of ions through a __and transition between __ can be monitored; the activity of a channel in its native membrane environment can be observed as well
    • electric current flowing through the pipette is identical with the current flowing through the membrane covered by the pipette
    • single channel 
    • open and closed states of a channel
  8. ·         To examine Na+ channels specifically they purified the protein from an eel and purified based on its ability to bind __
    o   Na+ channel structure: __ chain; channels have __, each with a similar __; each repeat has __. Each repeat also has a __. __ are __; and, the __ act as __
    • tetrodotoxin (affinity chromatography)
    • 260 kd
    • four internal repeats
    • amino acid sequence
    • five hydrophobic segments (S1, S2, S3, S5, and S6)
    • positively charged S4 segment
    • S1 through S6 are membrane spanning alpha helices
    • positively charged residues in S4
    • voltage sensors of the channels
  9. ·         To examine K+ channels, they studied shaker fruit flies; they contained a shaker gene that was responsible for the shaking due to an amino acid sequence encoded by a K+ channel gene
    o   The shaker gene encodes a 70 kd protein that contains sequences corresponding to __in one of the repeated units of the Na+ channel
    §  __ came together to form a __ 
    • segments S1 through S6 
    • Four shaker polypeptides
    • functional channel
  10. o   For K+ channels, , __ and __form the actual pore in the K+ channel; __ have the apparatus that contains the __
    o   Structure: __of identical subunits, each which includes __; the __subunits come together to form a __in the shape of a cone that runs through the center of the structure
    • S5 and S6 
    • segments S1 through S4 
    • open pore
    • tetramer 
    • two membrane-spanning alpha helices
    • four 
    • pore
  11. ·         The structure of K+ channel suggests its selectivity
    Explain it.
    o   Beginning from inside the cell, the pore starts with a diameter of approximately 10 angstroms and then constricts to a smaller cavity with a diameter of 8 angstroms. Both the opening to the outside and the central cavity of the pore are filled with water, and a K+ ion fits in without losing its shell of bound water.
  12. o   At the __, the potassium must __ and __
    §  To relinquish their water molecules, __ must replace those with water
    ·         The restricted part of the pore is built from __contributed by the __. In particular, a five amino-acid stretch within this region functions as the __ that determines the preference for K+ over the ions. The stretch has a sequence (__), and is nearly conserved in all K+ channels.
    o   This sequence lies in an __ and is oriented such that the __ are directed into the channel to interact with the potassium
    • constriction point
    • give up water
    • interact with the side chains
    • other polar interactions
    • residues 
    • two transmembrane alpha helices
    • selectivity filter
    • TVGYB
    • extended conformation
    • peptide carbonyl groups
  13. o   The selectivity of the channel: ions having radii larger than 1.5 angstroms cannot pass into the narrow diameter of the selectivity filter of the K+ channel. Though Na+ has a size that allows it to fit through, the __is way less favorable than that of K+
    §  The channel pays the cost of __ by providing __ with the __; these oxygens don’t react favorably with Na+
    • free energy cost of dehydrating it 
    • dehydrating K+
    • compensating interactions with the carbonyl oxygen atoms lining the selectivity filter
  14. ·         In Ca2+ channels, one __residue of the region between __ and __ in each of the four repeated units plays a major role in determining __
    o   Residues in the positions corresponding to glutamate residues in Ca2+ channels are major components of the __ of the __
    • glutamate 
    • segments S5 and S6 
    • ion selectivity
    • selectivity filter of the Na+ channel.
  15. §  These residues—__(4)__—are located in each of the internal repeats of the Na+ channel, forming a region termed the __.
    ·         Thus, the __ of the channel is clearly broken in this region, which explains why Na+ channels consist of a __ rather than a __. 
    • aspartate, glutamate, lysine, and alanine
    • DEKA locus
    • potential fourfold symmetry
    • single large polypeptide chain
    • noncovalent assembly of four identical subunits
  16. o   The preference of the Na+ channel over K+ depends on __; the diameter of the pore determined by these residues and others is __ that small ions can pass through, but larger ions like K+ are hindered.
    • ionic radius
    • restricted
  17. ·         __ crucial for rapid ion flow are present in the __
    o   A hydrated potassium ion enters the channel through the __.
    o   The ion then gives up its __ and binds to a site within the __. The ion can move between the __ because of similar ion affinities. 
    • Four K+ binding sites
    • constricted region of the K+ channel
    • unrestricted part of the channel
    • coordinated water molecules
    • selectivity-filter region
    • four sites within the selectivity filter
  18. o   The positive charges will do what, causing it to do what?
    o   Thus, each ion that binds anew favors the __
    • repel the potassium ion at the nearest site
    • shift to a site farther up the channel and push any potassium up. 
    • release of an ion from the other side of the channel
  19. ·         Some Na+ and K+ channels are gated by __; they change conformation to a highly conducting form in response to __. These __ include __in addition to the __.

    • membrane potential
    • changes in the voltage across the membrane
    • voltage-gated channels
    • segments S1 through S4 
    • pore itself formed by S5 and S6
  20. o The segments S1 through S4 form __, called __that extend from the core of the channel; they include the __, the __ itself. Segment S4 forms an __.

    § Segments S1 through S4 are not __, but, instead, are positioned to __
    • domains
    • paddles 
    • segment S4
    • voltage sensor
    • alpha helix lined with positively charged residues
    • enclosed within the protein
    • lie in the membrane itself
  21. ·         Model for voltage gating:
    • o   In the closed state, the paddles lie in a “down” position.
    • On membrane depolarization, the cytoplasmic side of the membrane becomes more positively charged, and the paddles are pulled through the membrane into an “up” position.
    • In this position, they pull the four sides of the base on the pore apart, increasing access to the selectivity filter and opening the channel
  22. ·         A channel can be inactivated by occlusion of the pore: the __
    o   The K+ channel and the Na+ channel undergo inactivation when?
    §  Experiments done with the shaker fly (a synthetic peptide corresponding to the first 20 residues of the native channel allowed it to inactivate the channel) and cleavage by trypsin producing trimmed channels that stayed open after depolarization support the __. 
    • ball-and- chain model
    • shortly after opening
    • ball and chain model
  23. ·         The first 20 residues of the K+ channel form a __that is attached to a __.
    ·         When the channel is closed, the ball does what?.
    ·         When the channel opens, the ball does what?
    ·         Hence, the channel opens for only a brief interval before undergoing __
    • cytoplasmic unit (ball) 
    • flexible segment of the polypeptide (the chain)
    • rotates freely in the aqueous solution
    • quickly finds a complementary site in the open pore and occludes it
    • inactivation by occlusion
  24. §  Shorter chains __; lengthening chains __
    §  In some senses, the “ball” domains can be thought of as __
    • speed inactivation
    • slows inactivation
    • large, tethered cations that are pulled in to the open channel but get stuck and block further ion conductance
  25. ·         The __ is an archetype for ligand-gated ion channels
    o   Nerve impulses are communicated across synapses by small, diffusible molecules called __, such as __
    §  The presynaptic membrane of a synapse is separated from the postsynaptic membrane by a gap of about 50 nm called the __
    ·         The arrival of a nerve impulse at the end of an axon leads to the __
    • acetylcholine receptor
    • neurotransmitters
    • acetylcholine
    • synaptic cleft
    • export of acetylcholine vesicles into the cleft
  26. o   The binding of acetylcholine to the postsynaptic membrane markedly changes its __, triggering an __.
    §  Acetylcholine opens a single kind of cation channel, called the __, which is almost equally permeable to Na+ and K+
    • ionic permeability
    • action potential
    • acetylcholine receptor
  27. ·         The acetylcholine receptor is the __, gated by the presence of __. The binding of acetylcholine to the channel is followed by its __
    o   To study, it was isolated from a membrane by nonionic detergent and purified by __
    • ligand-gated channel
    • specific ligands
    • transient opening
    • affinity chromatography using cobratoxin
  28. o   The four kinds of subunits have similar sequences; the genes for __ arose by duplication and divergence of a common ancestral gene
    §  Each subunit has a large __, followed by the __ by four predominantly hydrophobic segments that span the lipid bilayer
    ·         Acetylcholine binds at the __ and __ interfaces.
    • alpha, beta, gamma, and sigma subunits
    • extracellular domain
    • carboxyl end
    • alpha-gamma and alpha-sigma
  29. o   The basis of channel opening is the __ to the __, which causes a __ that initiates __
    §  The amino acid sequences of these helices point to the presence of alternating ridges of __ and __
    • binding of acetylcholine
    • extracellular domain
    • structural alteration
    • rotations of the alpha-helical rods lining the membrane-spanning pore
    • small polar or neutral residues and large nonpolar ones
  30. ·         In the closed state, the large residues may __ by __
    o   Indeed, each subunit has a __at a critical position
    §  The binding of acetylcholine could __ so that the pore would be lined by __rather than by __. The wider, more polar pore would then be open to the passage of Na+ and K+ ions
    • occlude the channel by forming a tight hydrophobic ring
    • bulky leucine residue 
    • allosterically rotate the membrane-spanning helices
    • small polar residues 
    • large hydrophobic ones
  31. ·         Equilibrium potential: __
    o   In the absence of stimulation, the resting potential for a typical neuron is __, which is close to the equilibrium potential for K+ owing to the fact that a small number of K+ channels are open
    • driving force due to the concentration gradient is balanced by the electrostatic force resisting the motion of an additional charge; the membrane potential at equilibrium
    • –60 mV
  32. ·         So, what happens in the generation of an action potential?
    o   A __ is released from an adjacent cell, which binds to the __, causing it to open
    o   Since it is nonspecific, what happens?
    • neurotransmitter like acetylcholine
    • acetylcholine receptor
    • sodium goes in and potassium goes out
  33. o   As the membrane potential approaches -40 mV, the __ are pulled into the membrane, opening the __, causing __ and __
    o   The __ of K+ channels are pulled into the membrane by the changed membrane potential, but more slowly than Na+ channel paddles
    • voltage-sensing paddles of Na+ channels
    • Na+ channels
    • sodium ions to rapidly flood the cell and membrane potential to rise rapidly toward the Na+ equilibrium potential
    • voltage sensing paddles
  34. o   K+ channels start to open at the same time the __, decreasing the Na+ current. __ are also inactivated at this point
    o   With only the K+ channels open, the membrane potential __. The open K+ channels can be inactivated by their “__ so they are __
    o   __ are released when the membrane potential returns to initial value. The domains are released and channels return to their original closed states
    • inactivation ball plugs Na+ channels
    • Acetylcholine receptors

    drops rapidly toward the K+ equilibrium potential

    • ball” domains
    • blocked

    Inactivation domains
  35. ·         A robust action potential is generated because __
    ·         Mutations of ion channels can be life threatening, as is the case with __, a genetic disorder in which recovery of the action potential is delayed
    • the membrane potential is very sensitive to even a slight change in the distribution of charge, making the action potential very efficient for signaling
    • Long QT syndrome

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