BIOLCHEM 415: LECTURE 32, LIPID MEMBRANES, PROTEIN CHANNELS AND PUMPS
37 Cards in this Set
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2 characteristicsof lipid membranes
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1. proteins carry out most of the functions in the membranes
2. phospholipids, glycolipids, and cholesterol make up the basic structure of membranes
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Common structural theme of membrane lipids
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amphipathic molecules: contain a hydrophobic and hydrophilic moiety
1. hydrophilic (polar) head and hydrophobic tails (hydrocarbon)
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What do hydrophobic interactions drive?
DRIVING FORCE!!!
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the self-assembly of micelles and bilayers. (phospholipids and glycolipids form bilayers)
the hydrophobic tails want to interact with each other (associate)
polar heads face the water
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Definition of the membrane and function of the bilayer
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are 3D solutions of oriented lipids and globular proteins
Functions of the bilayer:
1. as a solvent for integral membrane proteins
2. as a permeability barrier
3. self-healing
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Fact: membrane lipids form a permeability barrier:
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Highly impermeable to ions and most polar molecules (except water).
Exchange of chemicals or information across membrane requires proteins
Na+ and K+ take much longer to cross membrane than water
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Why is membrane asymmetry preserved?
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Proteins and lipids diffuse only laterally. Lateral diffusion is quick process, but transverse (flip-flop) diffusion occurs quickly
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Biological membranes are structurally and functionally asymmetric
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1. outer and inner surfaces of membranes have different components and enzymatic activity
2. proteins are asymmetrically oriented because of function (receptors for growth factors, Na+-K+ pump)
3. lipids are asymmetrically distributed: outer leaflet: sphingomyelin, phosphatidylcholine
…
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Membrane fluidity depends on what?
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by fatty acid chains --
1. long hydrocarbon chains interact more strongly than short (Tm increases with increasing chain length)
2. a cis double bond decreases Tm, bend that interferes with packing of the hydrocarbon chain
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What is a key membrane fluidity regulator in animals?
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Cholesterol, it reduces membrane fluidity by interacting with hydrocarbon tails of the PLs at high temps.
2. at low temps cholesterol maintains membrane fluidity (rings prevent gelling)
SMALL CHANGES IN TEMP COULD CAUSE LARGE CHANGES IN MEMBRANE FLUIDITY WITHOUT CHOLESTEROL
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How would you tell if a helical segment would be more stable in a hydrocarbon/non-polar or aqueous environment?
What does a positive change in free energy mean?
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1. Calculate the free energy for the transfer of an alpha-helix from the membrane interior to water [alpha-helices are in hydrophobic part of membrane]
2. means the amino acid is more hydrophobic/wants to stay within the membrane
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What else determines the free energy change? hydropathy plot
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1. it is a visual example of indicating transmembrane protein (membrane spanning alpha-helix)
2. measures a window of 20 residues
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What is an integral membrane protein?
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they interact with the hydrocarbon chains of membrane lipids and they can be released by agents that compete for the nonpolar interactions
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Bacteriorhodopsin
What does it do?
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use light energy to transport protons from inside the cell to outside generating a proton gradient
1. made up of alpha helices
2. the amino acids are nonpolar and few are charged
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Beta-barrel membrane proteins
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1. comprised of anti-parallel beta strands
2. each strand h-bonded to neighbor to form beta sheet
3. side chains facing lipid are nonpolar while those facing into the barrel are often polar
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Curious feature of monotopic integral membrane proteins?
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They don't span the membrane. They lie on the surface firmly bound by alpha helices with hydrophobic surfaces that extend from the bottom of protein into the membrane
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Example of how membrane protein localization is crucial for function
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in prostaglandin synthase, arachidonic acid gains access to the active site of the enzyme through a hydrophobic channel, without entering an aqueous environment
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Types of hydrophobic interactions that anchor proteins to membrane
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1. palmitoyl group attached to cysteine by thioester bond
2. farnesyl attached to cysteine residue at carboxyl terminus
3. glycolipid structure termed a glycosylphosphatidylinositol anchor attached to carboxyl terminus
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3 types of membrane transport
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1. simple diffusion
2. passive transport and facilitated diffusion
3. active transport
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Pump action is an example of active transport
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Uphill, needs an extra source of energy such as light absorption or ATP hydrolysis
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Definition of Passive Transport
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downhill reaction, through channels, enable ions to flow rapidly down membranes
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5 different types of pumps and transporters
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1. ATP-driven pumps
2. P-type ATPase
3. ABC transporters
4. Secondary transporters or co-transporters
5. lactose permease
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4 Different types of channels
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1. Potassium ion channel
2. ligand-gated channels
3. voltage gated channels
4. gap junctions
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What causes transformational changes of membrane proteins?
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ATP input (when it binds or during hydrolysis), causes a bound ion to be transported across the membrane
the different conformation states have a binding site open to a different side of the membrane and of different affinity for the transported compund
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P-type ATPase is a pump driven by ATP
one example is SERCA (Sarcoplasmic Reticulum Ca2+ ATPase)
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1. its muscle relaxation depends on rapid removal of Ca2+ into the SR
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How does SERCA work?
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ATP binds to (N) domain, (P) domain accepts phosphoryl group on conserved aspartate residue, (A) activator links changes in N and P to transmembrane part of enzyme
There is a 6part catalytic cycle
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6 part catalytic cycle of P-type ATPase
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1. 2 Ca2+ ions bind to unphosphorylated state [E1]
2. Domains undergo rearrangement as they close around bound ATP, Ca2+ ions trapped
3. phosphoryl group transferred to asp 351 (P)
4. As ADP is released, overall conform. change occurs [E2] Ca2+ released on other side of membrane
5. ph…
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What is the multi-drug resistance protein? ABC transporter
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ATP dependent pump that extrudes a wide range of small molecules from cells that express it, MDR pumps drugs outside cell b4 the drug can exert its affects
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What are the domains of the protein MDR?
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in ABC transporters: 2 ATP binding cassettes
(2) membrane spanning domains
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Catalytic cycle of ATP binding cassettes
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1. transporter free of both ATP & substrate can interconvert between forms (open & closed)
2. substrate enters cavity from cell interior which induces ABC affinity for ATP
3. ATP binds changing conform. so 2 domains interact
4. substrate releases b/c of strong interaction
5. the hydro…
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Curious fact about co-transporters, they do not need ATP
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1. antiporters: couple downhill flow of 1 molecule to uphill flow of another
2. symporters: both molecules flow the same way across the membrane
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What is a specific type of symporter? lactose permease and what does it do?
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uses the H+ gradient across E. coli membrane (outside E. coli generated by oxidation of fuel molecules) to drive the uptake of lactose and other sugars against concentration gradient
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Structure of lactose permease and mechanism?
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2 halves joined by single stretch of polypeptide
Mechanism:
1. cycle begins with the 2 halves oriented so that the opening to the binding pocket faces outside the cell, proton binds to a residue in the permease
2. in protonated form lactose binds
3. eversion and release
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The structure of the potassium ion channel reveals the basis of ion specificity
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1. The opening to the outside and the central cavity of the pore are filled with water and the K+ ion can fit in the pore without losing its shell of bound water molecules
2. the pore becomes more constricted 2/3rd through the membrane (3Angstrom), and interacts directly with groups from…
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Why is K+ accepted but not Na+?
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Potassium channels are 100 fold more permeable to K+ because of the free energy of dehydrating the Na+ is much higher and this cost would not be rediscovered. and the ion is really small
K+ interacts with the carbonyl oxygen atoms of the TVGYG sequence
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Ligand gated ion channel and function of acetylcholine receptor
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The acetylcholine receptor, a cation channel equally permeable to Na+ and K+, is gated by binding of a ligand, acetylcholine to the receptor. Binding opens the channel
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Characteristicsof gap junctions: cell-to-cell channels
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They differ from other membranes because they traverse 2 membranes not just 1 and connect cytoplasm to cytoplasm instead of extracellular to intracellular
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Function of gap junctions?
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1. Transport ions and metabolites (sugars, amino acids, nucleotides) - proteins nucleic acids and polysaccharides are not transported via gap junctions
2. provide nutrients to cells that are distal from blood vessels
3. provide communication between cells
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BIOLCHEM 415: LECTURE 8