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Lecture Notes for Chapter 9 Membrane Transport

Essential Biochemistry Third Edition Charlotte W. Pratt | Kathleen Cornely. Lecture Notes for Chapter 9 Membrane Transport. KEY CONCEPTS: Section 9-1. During a nerve impulse, ion movements alter membrane potential, producing an action potential that travels along the axon.

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Lecture Notes for Chapter 9 Membrane Transport

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  1. Essential Biochemistry Third Edition Charlotte W. Pratt | Kathleen Cornely Lecture Notes for Chapter 9 Membrane Transport

  2. 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.

  3. 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!

  4. The voltage across a membrane caused by ion transport is the membrane potential. Δy 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

  5. Ion movements alter membrane potential.

  6. Propagation of a Nerve Impulse

  7. Propagation of a Nerve Impulse

  8. Action potentials propagate rapidly because axons in mammals are insulated by a myelin sheath. Electron micrograph of a myelinated axon

  9. 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.

  10. Porins are trimers composed of β sheets. Ribbon Diagram Stick Figure Each subunit forms a 16-18 stranded membrane-spanning β barrel

  11. One of the loops in the β barrel constricts the core and makes the porin specific for small cationic solutes.

  12. The high selectivity for K+ reflects the geometry of the selectivity filter.

  13. 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).

  14. Gated channels undergo conformational changes.

  15. In bacteria, α helices alter their packing arrangements in mechanosensitive ion channels.

  16. Aquaporins are water specific pores. Hydrophobic residues and two key Asn residues line aquaporinpores to prevent proton transport.

  17. 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

  18. Lactose permease works by a rocking mechanism. Lactose analog in dark gray spheres

  19. 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.

  20. Na,K-ATPase changes conformation as it pumps ions across the membrane.

  21. Na,K-ATPase changes conformation as it pumps ions across the membrane.

  22. The Structure of the Na,K-ATPase

  23. Glucose transport is coupled with Na+ and K+ transport. Energetically favorable Energetically unfavorable

  24. 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.

  25. Acetylcholine is a common neurotransmitter. Many neurotransmitters are derivatives of amino acids

  26. Acetylcholine is degraded at a neural synapse by acetylcholinesterase.

  27. Events at the Nerve-Muscle Synapse

  28. Events at the Nerve-Muscle Synapse

  29. Events at the Nerve-Muscle Synapse

  30. 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

  31. Membrane fusion of SNAREs is spontaneous.

  32. Membrane fusion requires changes in bilayer curvature. How might bilayer curvature be facilitated? Removal of an acyl chain could convert shape.

  33. Schematic of Membrane Fusion

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