MEMBRANE CHANNELS AND PUMPS

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MEMBRANE CHANNELS AND PUMPS
1. Transport 1) simple diffusion - ?G
RTln(c2/c1) 2) passive transport (facilitated
diffusion) 3) active transport - coupled with
energy 4) ion channels - highly selective - open
and closed states - regulation for opening and
closing - inactivation 5) gap junction

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• 2. Na-K pump
• 1) Na-K ATPase
• - requirement of Mg2
• - P-type ATPase
• - ß-aspartyl phosphate enzyme intermediate
• - Ca2 ATPase
• - H-K ATPase

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• 3. Ca2-ATPase
• 1) sarcoplasmic reticulum (SR)
• - Ca2 concentrations inside (cytosol), 100 nM
  outside (SR), 1.5 mM
• 2) structure
• - N ATP binding
• - P phosphorylation site 
• - A actuator for the N-domain
• 3) mechanism
• - phosphorylation at Asp 351 at relative high
  cytosolic Ca2 concentrations
• - at least two conformations (E1, E2) total four
  conformational states (E1, E1 -P, E2-P, E2)
• - binding of ATP and two Ca2 molecules at E1
• - phosphorylation in the presence of bound
  Ca2(E1-P state)
• - conformational equilibrium to E2 (E2-P state)
• - eversion of the ion-binding site
• - low affinity for Ca2 in E2-P state
• - release of Ca2
• - dephosphorylation of E2-P
• - conformational equilibrium to E1

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• 4) P-type ATPase
• - flippase P-type ATPase transporting lipids
• - Na-K pump
• 5) cardiotonic steroids
• - Na-K pump inhibitors
• - digitoxigenin from dried leaf of foxglove plant
• - dephosphorylation on extracellular face

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Kinetic scheme for the active transport of Na
and K by (NaK)ATPase
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Putative dimeric structure of the (NaK)ATPase
indicating its orientation in the plasma membrane
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• 4. ABC (ATP-binding cassette domain) proteins
• 1) multidrug resistance protein (MDR)
• - P-loop NTPase 
• - transporter of toxic drugs
• 2) mechanism
• - couple ATP hydrolysis to conformational change

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• 5. Secondary transporters or cotransporters
• 1) symport vs. antiport
• - coupled to the downhill flow of a different
  species
• 2) lactose permease

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• 6. Action potential
• 1) Membrane potential
• - resting potential -60 mV
• - depolarization ? action potential
• - permeability to Na ? K
• 2) questions
• - ion selectivity
• - voltage gating
• - mechanism of inactivation
• 3) patch-clamp techniques
• - high-resolution current measurement
• - whole-cell mode
• - excised-patch mode

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• 7. Sodium channel
• 1) sodium channel blockers positively charged
  groups
• - tetrodotoxin
• 2) reconstituted sodium channel
• - Na-selectivity
• - voltage-dependent
• 3) four repeating units in the amino acid
  sequence
• - each repeating unit five hydrophobic segments
  (S1, S2, S3, S5, S6),
•   a highly positively charged S4
• 4) selectivity
• - steric factors of ions with a water molecule

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• 8. Potassium channel
• 1) selectivity
• - diameter of the pore 3Å
• - Li, Na higher free-energy cost of
  dehydration
• - optimal for interaction with K
• 2) inactivation
• - the ball and chain model

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K channel
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Predicted secondary structure and membrane
orientation of voltage-gated K channels
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• 9. Acetylcholine receptor
• 1) transmission of nerve impulses
• - neurotransmitters acetylcholine
• - synaptic cleft the presynaptic membrane-the
  postsynaptic membrane
• - secretion of acetylcholine from synaptic
  vesicles
• - binding of acetylcholine to acetylcholine
  receptor
• - a large inward current of Na
• - a smaller outward current of K
• - depolarization of the postsynaptic membrane
  -60 mV ? -20 mV
• - generation of a nerve impulse an action
  potential (above -40 mV)

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• 2) structure of acetylcholine receptor
• - fivefold symmetry (five subunits)
• - three domains an extracellular domain, a
  transmembrane domain,
•   a cytosolic domain
• 3) transient opening of acetylcholine receptor
• - opening ? desensitization
• - acetylcholinesterase kcat/KM 2 108 M-1S-1
• - organic phosphates (ex, DIPF) insecticides,
  nerve gases
• 4) a plausible mechanism
• - rotation and sliding of helices

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• 10. Gap junction
• 1) cell-to-cell channels
• - 20 Å central hole
• - small hydrophilic molecules sugars, amino
  acids, nucleotides
• - ions
• 2) structure
• - 12 molecules of connexin
• - connexon hemichannel
• - transverse two membranes
• 3) closing of the channel
• - Ca2, H

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• 11. Specific channels for water molecules
• 1) rapid water transport
• - kidney
• - saliva, tears
• 2) aquaporin

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