Lecture 13: Membrane Structure Flashcards
1
Q
Membrane functions
A
- Act as barrier to regulate import/export of molecules
- Compartmentalization
- Cell-cell recognition
- Signaling is mediated by proteins and lipids
2
Q
Membrane properties
A
- Composed of lipid bilayers that are impermeable to polar or charged molecules
- Hydrophobic effect drives membrane formation of amphipathic molecules due to non covalent interactions
- Membranes may/may not have carbohydrates covalently bound
- Play key role in transport of molecules and transduction of information across membrane
3
Q
Membrane fluidity
A
- Proteins carry the movement of molecules across the membrane
- Lipid and cholesterol affect membrane fluidity
4
Q
Fluid-Mosaic Model
A
- “Fluid” - membrane components can move rapidly in plane of membrane
- “Mosaic” - diverse mixture of lipids on peripheral proteins and carbohydrates on the surface
5
Q
Fluorescence Recovery After Photobleaching(FRAP)
A
- Label cell surface molecule
- Use laser to bleach fluorophore in small part of membrane
- Measure mobilty of other molecules in region
6
Q
What effects do detergents play on FRAP
A
- detergents lyse the cell and unable to observe FRAP
- crosslinker decrease recovery rate
7
Q
Asymmetry of biological membranes
A
- Two leaflets have different lipid/protein compositions
- Addition of sugars to lipids is form of PTM and is important for membrane insertion and cell recognition
- Flip-flop movement across hydrophobic membrane unfavorable
8
Q
Enzymes mediate membrane assembly
A
- Flippase: move PE and PS from outer membrane to cytosolic leaflet
- Floppase: Moves phospholipids from cytosolic to outer leaflet
- Scramblase: Moves lipids in either direction
- Enzymes are ATP dependent
9
Q
Membrane assembly
A
- Enzymes assist with distribution of lipids and proteins on either side of bilayer
- ATP hydrolysis of flippases and floppases drive movement of lipids from one membrane to another
- Scramblase moves all lipids down concentration gradient producing a symmetrical membrane
- Enzymes catalyze addition of oligosaccharides to proteins and lipids
- Membrane synthesis in ER to ensure proper orientation
10
Q
Lipids
A
- Class of molecules providing structural support for cells and organelles, and can play a role in information transduction and signaling
- Defined by physical properties
- low solubility in water
- high solubility in non polar solvents
- Properties arise from presence of functional groups(amphipathic)
11
Q
Lipid Bilayer Formation
A
- Lipids aggregate in water to bury hydrophobic groups while polar groups interact with water
- Non covalent forces drive assembly
- Structure forms is based off lipid structure and chemical interactions
12
Q
Biological Lipids
A
- Storage: Triacylglycerides
- Structural: Phospholipids, Sphingolipids, Glycolipids, Sterols
- Signalling: Sterols, Omegas, Eicosanoids
13
Q
Fatty Acids
A
- Fatty acid chains are either saturated(no double bonds) or unsaturated(double bonds)
- Doubled bonds are usually numbers relative to carboxylic acid but can also be numbered relative to terminal methyl group for omega fatty acidss
- Can be free or bound to head group/backbone via ester bonds
14
Q
Fatty Acid Nomenclature
A
- # C : #double bonds
- Monounsaturated: 1 C double bond
- Poly: more than 1 double bond
15
Q
Omega Fatty Acids
A
- Omega 3 and omega 6 fatty acids are polyunsaturated
- Humans cannot synthesize these omega fatty acids
- Must be obtained by diet
16
Q
Chain Saturation and Membrane Fluidity
A
- Saturated: low fluidity, and high melting temperature
- Unsaturated: high fluidity, low melting temperature
17
Q
Phase Transitions
A
- Melting of membrane lipids is a phase transition from a solid phase to a liquid crystalline phase
- Melting temperature an index of membrane fluidity
- Measures how fluid the membrane is
- Pure lipid samples have sharp, well-defined temperatures, while native membranes have broad peaks
18
Q
Membrane lipid diversity
A
- Amphipathic molecules made by attaching fatty acyl chains to polar head groups
- Glycerol-phospholipids
- Sphingolipids
- Variability exists in structure of fatty acid tails and polar head groups
- Cholesterol most common steroid
19
Q
Cholesterol
A
- Can be metabolized to other hormones needed for dietary lipid absorption
- Can form complexes with sphingolipids, glycolipids, and some lipid-anchored proteins to form lipid rafts
- Lipid rafts help moderate membrane fluidity and signal transduction
- Modulate membrane fluidity
20
Q
Peripheral Membrane Proteins
A
- Adhere to surface of lipid membranes or integral membrane proteins through non-covalent interactions
- Can be removed by increasing salt/changing pH
21
Q
Integral Membrane Proteins
A
- Completely span the membrane(transmembrane segments)
- Require harsh conditions for purification(detergents or organic solvents)
22
Q
Lipid Anchored Proteins
A
- Also require harsh detergents for membrane removal
- Lipid chains are covalently attached to amino functional groups and side chains
- GPI anchored proteins covalently linked through sugar chain and lipid anchor
23
Q
Membrane Protein Purification
A
- Some membrane proteins can be removed using mild conditions that disrupt non-covalent interactions, while others need detergents to disrupt hydrophobic interactions
- Detergents create micelles around hydrophobic regions to solubilize membrane proteins
- Proteins can then be purified and analyzed
24
Q
Studying Membrane Proteins
A
- Critical micelle concentration is concentration at which detergent spontaneously forms stable micelle structures
- Some detergents are more mild and preserve structure(TX-100, Tween 20, and digitoxin), while others are more harsh and denature proteins(SDS)
25
Q
Predicting Membrane Spanning Segments
A
- DNA sequencing usd to deduce primary sequence
- Transmembrane segments composed on primarily hydrophobic amino acids
- In alpha helix, 20 amino acids needed to span the membrane
26
Q
Determining Protein Topology
A
- Hydropathy index for a stretch of amino acids can be determined by averaging the Kyte-Doolittle hydrophobicity values of all amino acids found in the segment
- Move window one residue to the right and recalculate hydropathy index
- Plot hydropathy index vs central amino acid in the sequence
27
Q
Identifying TM Segments
A
- Glycophorin A: Contains a single transmembrane segment
- Bacteriorhodopsin: Contains 7 TMs
