Essential Biochemistry · 2016-03-09 · KEY CONCEPTS: Section 9-1 • During a nerve impulse, ion...
Transcript of Essential Biochemistry · 2016-03-09 · KEY CONCEPTS: Section 9-1 • During a nerve impulse, ion...
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Lecture Notes for
Chapter 9 Membrane Transport
Essential Biochemistry Third Edition
Charlotte W. Pratt | Kathleen Cornely
Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.
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KEY CONCEPTS: Section 9-1
• During a nerve impulse, ion movements alter membrane potential, producing an action potential that travels along the axon.
• Transporters obey the laws of
thermodynamics, providing a way for solutes to move down their concentration gradients or using ATP to move substances against their gradients.
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Na+ and K+ concentrations are inversely proportional in cells.
• [Na+] is greater outside of the cell than inside.
• [K+] is greater inside of the cell than outside.
• Proteins are required to transport ions across the membrane!
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The voltage across a membrane caused by ion transport is the
membrane potential.
Δψ has units of volts. R = Gas Constant = 8.3145 J �K-1 � mol-1 F = Faraday Constant = 96,485 J �V-1 � mol-1
Z = Net charge per ion T = Temperature in Kelvin
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Ion movements alter membrane potential.
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Propagation of a Nerve Impulse
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Propagation of a Nerve Impulse
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Action potentials propagate rapidly because axons in mammals are
insulated by a myelin sheath.
Electron micrograph of a myelinated axon © 2014 John Wiley & Sons, Inc. All rights reserved.
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KEY CONCEPTS: Section 9-2
• Porins are β barrel channels with some solute selectivity.
• Ion channels include a selectivity filter and may be gated.
• Aquaporins allow only water molecules to pass through.
• Transport proteins alternate between conformations to expose binding sites on each side of the membrane.
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Porins are trimers composed of β sheets.
Each subunit forms a 16-18 stranded membrane-spanning β barrel
Ribbon Diagram Stick Figure
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One of the loops in the β barrel constricts the
core and makes the porin specific for small cationic
solutes.
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The high selectivity for K+ reflects the geometry of the selectivity filter.
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A closer view of the K+ selectivity filter shows a backbone lined with carbonyl
groups with a geometry suitable
for coordinating a K+ ion (purple).
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Gated channels undergo conformational changes.
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In bacteria, α helices alter their packing arrangements in mechanosensitive ion
channels.
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Aquaporins are water specific pores.
Hydrophobic residues and two key Asn residues line aquaporin pores to prevent proton transport.
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Some transport proteins can bind more than one type of ligand.
• Uniport: moves one substance at a time • Symport: transports two different substances • Antiport: moves two different substances in different
directions across the membrane
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Lactose permease works by a rocking mechanism.
Lactose analog in dark gray spheres
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KEY CONCEPTS: Section 9-3
• Conformational changes resulting from ATP hydrolysis drive Na+ and K+ transport in the Na,K-ATPase.
• Secondary active transport of a
substance is driven indirectly by the ATP-dependent formation of a gradient of a second substance.
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Na,K-ATPase changes conformation as it pumps ions across the membrane.
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Na,K-ATPase changes conformation as it pumps ions across the membrane.
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The Structure of the
Na,K-ATPase
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Glucose transport is coupled with Na+ and K+ transport.
Energetically favorable
Energetically unfavorable
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KEY CONCEPTS: Section 9-4
• Neurotransmitters are released by the process of exocytosis.
• Membrane fusion, driven by the action of
SNARE proteins, requires changes in bilayer curvature.
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Acetylcholine is a common neurotransmitter.
Many neurotransmitters are derivatives of amino acids
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Acetylcholine is degraded at a neural synapse by acetylcholinesterase.
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Events at the Nerve-Muscle Synapse
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Events at the Nerve-Muscle Synapse
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Events at the Nerve-Muscle Synapse
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SNAREs link vesicle and plasma proteins.
• Soluble N-ethylmaleimide-sensitive-factor attachment protein receptor
• SNAREs are integral membrane proteins.
• Complex formation – Two SNAREs from plasma
membrane – One SNARE from synaptic vesicle
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Membrane fusion of SNAREs is spontaneous.
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Membrane fusion requires changes in bilayer curvature.
How might bilayer
curvature be facilitated?
Removal of an acyl chain could convert shape.
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Schematic of Membrane Fusion
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