Membrane Structure Flashcards
Recommended Reading: CH. 9 Lipid Classification: pp. 245-254, particularly 249-253 Lipid Bilayers: pp. 259-261 (5th) Membrane Proteins: pp. 262-268 Membrane Structure and Assembly: pp. 269-271, 274-276 Practice Problems Ch 9 Q's 3, 4, 6-8, 17, 18, 21, 22, 29 (5th)
Define the properties and roles of biological membranes.
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Explain why each type of membrane has an individual and characteristic protein and lipid composition.
Myelin Sheath: Low in protein, high in lipids, neutral glycolipids (lower polarity than phospholipids)
Inner mito membrane and E coli: Rel high protein, important in energy production
RBC 50/50 lipid/protein
Describe the phenomenon of lipid asymmetry in the bilayer.
Glycoproteins/Glycolipids will have carb aspects facing exterior to cell (extracellular face)
Lipids asymmetrically distributed, proteins have absolute orientations
Choline-containing lipids (sphingomyelin phsphatidylcholine) primarily in outer leaflet of RBC
Define the terms gel phase, liquid ordered and liquid disordered with respect to lipid fluidity.
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Describe the roles of flippases, floppases and scramblases in the distribution of lipids in a membrane and describe them as transport proteins.
Flippases: Move things In (inner leaflet)
- phosphatidylserine, phosphatidylethanolamine
Floppases: Move Out (outer leaflet)
- Phosphatidyl choline, sphingolipids
Scrmblases
- Move lipids down concentration gradient
- Not active transport
Outline how integral membrane proteins can be classified by structure or topology.
Integral:
classified based on structural composition of transmembrane region and N-Terminal Location
Single pass (bitopic)
- Single Helix
- 4 types (based on N-term location)
Multipass (polytopic):
- beta-barrels
- Helical Bundles:
N-terminal inside (cytosol) or outside (lumen)
Explain why the transmembrane segments of proteins fold into α-helices or β-barrels.
Transmembrane domains will have hydrophobic exteriors
Transmaembrane domains may have hydrophilic residues clustered in the interior or in channels
Exposed protein domains will have structures resembling soluble globular proteins
Describe the features of a hydropathy plot
Use primary sequence to predict folding
Detects/predicts transmembrane containing alpha-helix by looking for ~20 largely hydrophobic amino acids in sequence
Interior/cytoplasmic loops often enriched for Arginine and Lysine (Pos inside Rule)
Outline the steps in predicting the potential membrane-spanning segments of a protein from its amino acid sequence
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Describe the location of Trp and Tyr side-chains in integral membrane proteins and explain this observation
Trp/Tyr near interface, can be both Non-polar and form H-bonds
- interact with both polar heads and hydrophobic regions
Charged residues are found on surface/solvent exposed portions
Hydrophobes exposed to bilayer core
Solvent exposed surfaces are hydrophilic
Interior exposed loops of helical bundles are often enriched for Arg/Lys/His (Pos In)
Describe the key aspects of a lipid-anchored protein including: amino acids, timing of modification, specific chemical linkages, types of anchor and membrane location
May be via Fatty acid, prenyl group, or GPI linkage
- Anchors proteins to membranes
- May target proteins to specific membrane locations
Prenyl Anchors
Isoprene-based
- 5C group with Branch
- Irreversible modification
Lipid-Linked Membrane Proteins
Palmitoylation
Myristoylation
Palmitoylation (reversible)
Myristoylation (Irreveersible)
GPI anchors
- Post translation (because mod at C-terminal)
- Amide linkage to C-terminus
- Cleaved by phospholipases as regulation
- On outre face of PM (where large CHO structures are)
Phospholipase D cleaves GPI-anchored prtn (cleaves between inositol and phosphate)
- Membrane associated product = phosphatidate
Phospholipase C cleavage -> Diacylglycerol
- cleaves after phosphate
