Download Understanding Membrane Channels and Pumps: Active and Passive Transport and more Slides Biochemistry in PDF only on Docsity! Chapter 13: Membrane Channels and Pumps Active or passive transport Pumps maintain ion gradients across membranes Voltage-gated and ligand-gated channels Learning Goals 1.Distinguish between passive and active transport. 2.Compare the mechanisms of the P-type ATPase pump and the ABC transporter pump. 3.Explain how channels are able to rapidly and selectively move ions across membranes. 4.Describe two mechanisms by which ion channels are gated. 5.Explain the steps that lead to the formation of an action potential. Ionic Concentration Gradients are Established by Pumps Unequal distribution of charged species across membrane creates electrical potential Pump Action
Energy input
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Conformation 1 Conformation 2
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© 2012 WH. Freeman and Company
Na+-K+ Pump (ATPase) So named because pumping of both ions is linked For each ATP hydrolyzed, three sodium ions and two potassium ions are transported Digitalis inhibits, which reduces the Na+ gradient and in turn Ca2+ extrusion Enhances heart muscle contraction P-Type ATPases Examples: Na+-K+ Pump, SERCA Na+-K+ Pump: Establishes ionic gradient between intracellular and extracellular environment (K+ high inside, Na+ high outside) SERCA: Pumps Ca2+ into stores in muscle cells (sarcoplasmic reticulum) Mechanism: Two states – open to either side of membrane Unphosphorylated pump binds ions ATP binds and pump is phosphorylated Phosphorylated pump releases ions on other side Pump is dephosphorylated Multidrug Resistance (MDR) Protein Pumps drugs out of cells ABC transporter ABC Transporter Mechanism 1-No drug 2-Drug enters 3-ATP binds 4-Drug extruded 5-ATP hydrolyzed Return to start Action Potential When membrane temporarily becomes more permeable to certain ions, those charge differences will change Membrane Gets Depolarized Once membrane potential is depolarized beyond threshold, it will reach +30 mV before turning negative Repolarization
, Repolarization (C, Action potential peoks
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Action Potentials Propagate
Plasma
Action | membrane
potential
Action |
Ke potential
Kt |
Action
Kt potential
Patch Clamp Technique
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Channel Proteins Much faster ion transport compared to pumps Near diffusion limit Channel opening may be triggered by: Change in voltage across membrane (voltage- gated) Binding of ligand (ligand-gated) Specific for particular ions Specificity for lons
K* reaches
selectivity
filter
Figure 13.18
Biochemistry, Seventh Edition
© 2012W.H. Freeman and Company
Selectivity Filter
Figure 13.19
Blochemistry, Seventh Edition
© 2012 W.H. Freeman and Company
How is Selectivity Achieved? Larger ions too big Smaller ions are rejected because the energy required to strip hydration shell too great Dehydrated K+ can interact with residues in selectivity filter; dehydrated Na+ too small K+ Channel Selectivity and Transport Selectivity filter discriminates between ions Ions need to be dehydrated to pass through Dehydrated K+ can interact with carbonyl oxygens Larger ions too big to fit through Smaller ions such as Na+ too small to interact with carbonyl oxygens S4 Senses Voltage Changes
(B)
Membrane Depolarization Causes S4 to “Stand Up” and Open Pore Sodium Channel Four domains within one protein chain Same general architecture Selective for Na+ because K+ too big Action Potential - Recap • Resting membrane potential: -60 mV (negative inside) • Action potentials triggered by an increase in membrane potential (depolarization): Voltage-gated sodium channels open • Positive potential: Voltage-gated potassium channels open (repolarization) • S4 – voltage sensor • Selective (selectivity filter) • Inactivate (ball-and-chain) Channelopathies Recovery to resting potential delayed Genetic disorders Cardiac arrhythmias Sudden death during exercise (K+) SIDS (Na+) Retinal Ca2+ - night blindness Neuronal Na+ - epilepsy CFTR – cystic fibrosis Neurotransmitters
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H2 CH3
Acetylcholine
Neuropeptide
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Acetylcholine Receptor Ligand-gated channel; binds acetylcholine Pore permeable to Na+ and K+ M2 Helices Rotate to Open Channel
Figure 13.28
Biochemistry, Seventh Edition
© 2012 W. H. Freeman and Company
