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Biological Membrane & Transport
Table of contents
Membrane character molecular constituent of
membrane supramolecular architecture of
membrane membrane motion membrane protein peripheral protein integral protein types of integral protein topology of integral protein cell-cell interaction & adhesion
membrane fusion passive transport erythrocyte’s glucose transporter cotransport systems active transport types of active transporter Na+K+ATPase secondary active transport ion selective channels
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Membrane (boundary)
Flexible
Self-sealing
Selectively permeable
Two-dimensional
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Membrane components: proteins, polar lipids and carbohydrates
Ratio of protein/lipids: depends on type & role of membrane
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Supramolecular Architecture of Membranes
Common properties:
Impermeable
5 to 8 nm thick
Trilaminar
Asymmetric (structural & functional)
Fluid
Able to undergo fusion
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Lipid aggregates: depending on the size of head & tail
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Asymmetric Distribution of Phospholipids on Membrane
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Membrane Motion
Conformational motion
Lateral diffusion
Flip-flop diffusion
Paracrystalline, transition temperatureSterols moderate extremes of fluidity & solidity
High degree of regularity in one dimension &Great mobility in the other
Transbilayer diffusionflippases
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Membrane proteinsIntegral (intrinsic) proteins:
Firmly associated, removable only by agents that interfere with �hydrophobic interactions such as detergents, organic solvents or denaturants.
Peripheral (extrinsic) proteins
Associated through electrostatic interactions & hydrogen bonding with the hydrophilic domains of integral proteins and with the polar head groups of membrane lipids, readily removable by mild treatments
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Peripheral proteins anchored by covalently attached lipids
Lipids with long-chain fatty acids, isoprenoids, or glycosylated derivatives of phosphatidylinositol (GPI)
Proteins with GPI anchors are exclusively on the outer face (extracellular), whereas other types of lipid-linked proteins are found exclusively on the inner face (cytosolic)
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Integral proteinsIntegral proteins are held in the membrane by hydrophobic interactions with lipids, i.e., firmly fixed by interaction between membrane lipids and hydrophobic domain of proteins.
According to the spatial relationship of protein domains to the lipid bilayer, plasma membrane proteins fall into six categories.
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Types I & II have only one transmembrane helix, the amino terminal domain is outside the cell in type I proteins and inside in type II.
Type III proteins have multiple transmembrane helices in a single polypeptide.
Type IV proteins have several transmembrane domains from different polypeptide chains to form a channel through the membrane
Type V proteins are held to the bilayer primarily by covalently linked lipid
Type VI proteins have both transmembrane helices and lipid (GPI) anchors.
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Topology of an integral protein
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Hydropathy Plots
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Glycophorin in the erythrocyte
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For type III or IV proteins:
α-helical sequence of 20 to 25 amino acid residues
β-sheet sequence of 7 to 9 amino acid residues
α-helical: average 3.6 amino acids for 0.54 nm
β-sheet: average 2 amino acids for 0.65 to 0.7 nm
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Porin FhuA, with 22 antiparallel β strands forming channel for iron ion bound to the carrier ferrichrome
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Cell-cell interaction & adhesion
Integrins: heterodimeric, for binding collagen & fibronectin receptors & signal transducers, and regulate platelet aggregation at the site of a wound, tissue repair, activity of immune cells, and the invasion of tissue by a tumorCadherins for homophilic interaction
Immunoglobulin-like protein for both homophilic & heterophilic interaction
Selectins for binding polysaccharides.
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Membrane fusionSpecific fusion of two membranes requires:a) They recognize each otherb) Their surfaces become closely apposed, which
requires the removal of water molecules normally associated with the polar head groups of lipids
c) Their bilayer structures become locally disruptedd) The two bilayers fuse to form a single continuous
bilayere) The fusion process is triggered at the appropriate
time or in response to a specific signal
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Solute Transport across Membranes
Solute mediated by transmembrane channels, carriers or pumps
Passive transport & active transport
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Passive TransportMembrane potential (Vm) & electric gradient
Electrochemical gradient or electrochemical potential
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Membrane: selectively permeable
The energy of activation for translocation of a polar solute across the bilayer is so large that pure lipid bilayers are virtually impermeable to polar and charged species over periods of time relevant to cells.
Transmembrane passage of polar compounds and ions is made possible by membrane proteins that lower the activation energy for transport by providing an alternative path for specific solutes through the lipid bilayer. These proteins are called transporters or permeases.
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Transporters span the lipid bilayers at least once, and usually several times, forming a transmembrane channel lined with hydrophilic amino acid side chains. The channel provides an alternative path for a specific substrate to move across the lipid bilayer without its having to dissolve in the bilayer, further lowering the activation energy for transmembrane diffusion.
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Erythrocyte’s Glucose TransporterType III integral protein with MW of 45,000 and 12 hydrophobic segments, each of which form a membrane-spanning helix
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Hallmarks of passive transportHigh rates of diffusion down a concentration gradient
Saturability
Specificity
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Anion exchange protein
Obligatorily transport one bicarbonate and one chloride
Cotransport systems: simultaneously carry two solutes across a membrane
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Antiport systems: transporters that carry two substrates moving in opposite directions
Symport systems: transporters that carry two substrates moving simultaneously in the same direction.
Uniport systems: transporters that carry only one substrate
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Active transportActive transport results in solute movement against a concentration or electrochemical gradient, thus it is thermodynamically unfavorable or endergonic process, and must be coupled with other energy releasing processes.
Primary active transport (coupled with energy)
Secondary active transport (coupled with concentration flow)
(Major energy-consuming process)
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Four types of transportersDifferent in structure, mechanism, localization in specific tissues and intracelluar compartments
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1. P-type ATPase: ATP driven cation transporters, reversibly phosphorylated by ATP during the transport cycle, with similar amino acid sequence, can be inhibited by phosphate analog vanadate. Generally have two types of integral protein subunits. The α-subunit is essential, has Asp residue phosphorylated during transport.
2. V-type ATPase: responsible for acidifying intracelluar compartments in many organisms via proton-transporting, also called proton pump. To acidify the vacuoles of fungi and higher plants, as well as lysosomes, endosomes, the Golgi complex, and secretory vesicles in animal cells. All have an integral (transmembrane) domain as proton channel and a peripheral domain containing the ATP-binding site and the ATPase activity.
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3. F-type ATPase: central role in energy-conserving
reactions in bacteria, mitochondria and chloroplasts.
Catalyzes the uphill trans-membrane passage of
protons driven by ATP hydrolysis, as well as the
reverse reaction, in which downhill proton flow
drives ATP synthesis. (ATP synthases).
4. Multidrug transporter: responsible for removing
different drugs from tumor cell cytosol, preventing
their growth-inhibitory effect.
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Na+K+ ATPaseP-type ATPase that cotransport Na+ and K+
Na+: lower in the cell than in the surrounding medium
K+: higher in the cell than in the surrounding medium
ATP + EnzI ADP + P EnzII
H2O + P EnzII EnzI + Pi
ATP + H2O ADP + Pi
transmembrane potential of -50 to -70 mV
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Na+K+ ATPase
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Ion gradient for secondary active transportCells contain transport systems that couple the spontaneous, downhill flow of Na+, H+ ions to the simultaneous uphill pumping of another ion, sugar or amino acids.
For example, glucose:Chemical potentialElectrical potential
Poison that collapses the ion gradient across cellular membrane
Ionophores, ion bearers
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Ion selective channelsMove inorganic ions across membrane quickly.
Determine the plasma membrane’s permeability to specific ions, and together with ion pumps such as Na/K ATPase, regulate the cytosolic concentration of ions and the membrane potential.
Characters: the rate of flux through channels can be orders of magnitude greater than the turnover number for a transporter, 107 to 108 ions per channel per second.
Not saturable
“Gated”, open or close in response to some cellular event
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Ion channels
Ligand-gated channels: allosteric proteins change conformation when bind to some extracellular or intracellular small molecules
Acetylcholine receptor
Voltage-gated ion channels: response to a change in transmembrane electrical potential
K+ channel
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• LEHNINGER • PRINCIPLES OF BIOCHEMISTRY
• Fifth Edition
David L. Nelson and Michael M. Cox
© 2008 W. H. Freeman and Company
CHAPTER 12Biosignaling
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Biosignaling
Signal response pathways
Types of signals
Biosignaling characters
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Signals from receptor to cell response
Autocrine: acting on the same cell that produces the signals
Paracrine: acting on a near neighbour
Endocrine: carried in the bloodstream from the producer cell to a distant target cell.
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The end result of a signaling pathway is the phosphorylation of
a few specific target-cell proteins, which changes their
activities and thus the activities of the cell
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Characters of signal transduction
Ø Specificity: precise molecular complementarity between the signal and receptor molecules, mediated by weak forces occuring in the enzyme-substrate, protein-ligand and antigen-antibody interactions.
Ø Sensitivity
1) High affinity of receptors for signal molecules
2) Cooperativity in the ligand-receptor interaction
3) Amplification of the signal by enzyme cascades.
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Ø Adaptation/Desensitization (saturation): When receptor is continuously stimulated by
signal, the threshold would be leveled up.
ØIntegration: The ability of the system to receive multiple signals and produce a unified
response appropriate to the needs of the cell or organism.
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