ML1-2: Lipids and membranes Flashcards
What are the general principles of biological membranes?
- They surround every cell and some organelles within cells, e.g. mitochondria
- They form a highly selective barrier
- They are important in regulating cell function
What are the general principles of lipids?
- Structurally diverse
- Generally insoluble in water (hydrophobic)
- Most only contain C, H, O (phospholipids contain P, N)
What is the most simple type of lipid?
Fatty acid
What is the general formula for a fatty acid?
CH3(CH2)nCOOH
How many carbons can be present in a fatty acid?
Between 16 and 20
Which part of the general formula for fatty acids represents the hydrocarbon tail?
CH3(CH2)nCOOH
What is an amphipathic molecule?
One with hydrophobic and hydrophilic properties
Name one saturated and one unsaturated fatty acid.
Saturated: palmitate
Unsaturated: oleate
What effect on the properties of a fatty acid does a double bond have?
Melting point will be lower due to steric hindrance
What are the three types of lipids in biological membranes?
- Phospholipids
- Glycolipids
- Cholesterol
What is the general structure of a phospholipid?
What bond connects the fatty acid chains to the glycerol of a phospholipid?
Ester linkages
Name four phospholipids commonly found in membranes. Are they (overall) hydrophobic, amphipathic, or hydrophilic?
- Phosphatidylserine
- Phosphatidylcholine
- Phosphatidylethanolamine
- Phosphatidylinositol
Overall amphipathic
Are glycolipids (overall) hydrophobic, amphipathic, or hydrophilic?
Amphipathic
What is the role of cholesterol in eukaryotes? Is it present in prokaryotes?
- Keeps membrane fluid
- Important in signalling
- Not usually present in prokaryotic membranes
Which parts of the cholesterol molecule are hydrophilic and hydrophobic?
Is it (overall) hydrophobic, amphipathic, or hydrophilic?
All hydrophobic except the –OH group
What type of structure is cholesterol?
Steroid-based rings
What is the general shorthand form for membrane lipids?
Name six properties of biological membranes.
- Membrane lipids are all amphipathic molecules that spontaneously form bilayers in water
- Membranes are composed of sheets of lipids only two molecules thick
- Membranes are composed of lipids and proteins. Carbohydrates can be attached to these molecules
- Membranes are held together by non-covalent interactions
- Membranes are fluid structures. Lipids and protiens can diffuse readily in the plane of the membrane
- The two faces of a biological membrane are different – they are asymmetric
Describe hydrophobic interactions.
Hydrophobic (non-polar) molecules do not interact with water and cluster together to exclude it
What structure do amphipathic fatty acids readily form? Describe this structure.
Micelles
Polar heads are outside interacting with water
Hydrophobic tails are inside interacting with each other, thereby excluding water
What shape are individual units of micelles?
Wedge-shaped (cross-section of head is greater than that of the side chain)
Are micelles stable or unstable structures?
Stable
Do phospolipids normally form micelles?
No – they are more suited to forming bilayers to achieve stable structure in aqueous environment
Describe the structure of a phospholipid bilayer.
- 3nm thick (only two molecules)
- Hydrophobic core
- Hydrophilic surface
- Very stable with no gaps – self-assemble and self-heal if one is removed
- Fatty acyl chains are packed togehter
- Individual units are cylindrical (cross-section of head equals that of side chain)
What structure do liposomes mimic?
Cell membranes of a biological cell
What is the structure of a liposome?
A bilayer membrane trapped within two aqueous compartments
What are the different types of membrane protiens?
- Peripheral – only associate with one side and don’t span the membrane
- Integral – associate with both sides and do span the membrane
N.B. Peripheral proteins can also bind to integral proteins
Differntiate between peripheral and integral membrane proteins.
-
Peripheral:
- Largely hydrophilic surface so can interact with phospholipid heads
- Can be removed from the membrane fairly easily, e.g. by increasing [NaCl] to approx. 500 mM to disrupt ionic interactions
-
Integral:
- Amphipathic (with hydrophobic section through the phospholipid bilayer)
- Need detergent (disrupts membrane structure) to remove them and to solubilise
How do detergents work on biological membranes and low and high concentrations?
At low concentrations, they solubilise the membranes by inserting themselves into the membranes
At high concentrations, they also solubilise and shield integral proteins by surrounding them to protect them from water
Hoe are integral membrane proteins held in the membrane?
By hydrophobic interactions with the lipids
α-helices span the membrane in many transmembrane proteins. Why are there approximately 20 amino acids per membrane-spanning α-helix?
What is the most common structural motif in lipid proteins? What are the features of this motif?
Transmembrane α-helices
- Contain mostly hydrophobic amino acids
- Few charged amino acids
- Approximately 20 amino acids in length
How can you predict transmembrane α-helices from amino acid sequences?
- Calculate the average hydrophobicity for a window of amino acids
- Move one amino acid along and calculate for the next window
- Repeat for the entire pattern
- Plot a hydropathy plot – transmembrane α-helices are highly hydrophobic
What is a bacterial porin?
β-strands arranged in a barrel structure to form a pore
How can proteins be modified in biological membranes?
By the addition of carbohydrate or lipid groups
Explain the fluid mosaic model of membrane structure.
- Proposed by Singer and Nicholson in 1972
- Proteins diffuse freely in a 2D ‘sea’ of phospholipids (although some are locked into position by being anchored to cytoskeletal proteins)
- Proteins cannot readily flip from one side to the other, therefore asymmetric distribution is maintained
What are the two types of diffusion in membranes? Explain them.
Lateral diffusion – membrane lipids can move in the plane of the membrane, i.e. do not cross leaflets
Transverse diffusion – a.k.a. flip-flop, where membrane lipids move across leaflets. This has never been observed
What is the evidence for the fluid mosaic model (bleaching)?
- Studied using fluorescence recovery after photobleaching (FRAP)
- Cell surgace is labelled with fluroescent molecule
- Laser used to ‘bleach’ these molecules in a small area
- Follow the return of fluorescence to this area
- Proteins must move through the sea of phospholipids to replace the bleached ones
With reference to the fluid mosaic bleaching experiment, what is the rate of recovery a function of?
The diffusion coefficient
What is the evidence for the fluid mosaic model (cell fusion)?
- Mouse and human cells labelled with coloured antibodies against cell surface proteins
- Cells fused to produce heterokaryon – no mixing of labelled proteins initially
- Proteins mixed after several hours of incubation
What factors affect membrane fluidity?
-
Length and degree of saturation of the fatty acid chains
Melting temperature increases as fatty acyl chain length increases (i.e. stronger interactions)
Double bonds interfere with fatty acid chain packing so decrease TM -
The amount of cholesterol in a membrane
Cholesterol inserts between fatty acyl chains to interact with hydrophilic heads
Can effect membrane fluidity in different ways
Why is cholesterol described as a goldilocks molecule?
- Stops phospholipids coming too close together (prevents the membrane being too solid)
- Helps to hold the membrane together (hydrogen bonds stop it falling apart)
Are biological membrane symmetric or asymmetric? Why?
Asymmetric because inner and outer leaflets of the membrane have different lipids
What are the three roles of membrane proteins? What are the types of protein that allow these functions?
-
Transport
* Channels (work with concentration gradient but need to be selective)
* Pumps (work against concentration gradient so require energy) -
Recognition
*Receptors (e.g. hormone binding)
* Glycoproteins/glycolipids – carbohydrate important in recognition -
Cytoskeleton
* Binding to extracellular matrix
* Interaction with other cells
* Maintaining/changing cell shape
What are the five main different types of transport processes?
- Simple diffusion (nonpolar compounds only, down concentration gradient)
- Facilitated diffusion (down electrochemical gradient)
- Primary active transport (agains electrochemical gradient)
- Secondary active transport (against electrochemical gradient, driven by ion moving down its gradient)
- Ion channel (down electrochemical gradient; may be gated by a ligand or ion)
How do channels allow for passive transport?
They lower the activation energy for the transport across the membrane. They do not rotate across the membrane.
How do the glucose transporters (GLUT 1-12) work?
- T1 = external glucose binding site
T2 = internal glucose binding site - Glucose in plasma binds T1
- Activation energy lowered so conformational transformation to T2
- Glucose released to cytoplasm
- Transporter returns to T1 conformation
Describe a K+ ion channel.
- Composed of four subunits with two transmembrane sequences
- Hydrated K+ ion in solution
- K+ fits precisely in pore, stabilised by interaction with protein
How do pumps work? Give an example of a pump and its function.
- Exist in two states with ion binding sites on different sides of the membrane in each state
- The free energy from ATP hydrolysis is used to convert between the two states
- e.g. sarcoplasmic and endoplasmic reticulum calcium ATPase (SERCA) is a multi-domain protein that uses ATP hydrolysis to drive Ca2+ transport in the SR/ER
How does an insulin receptor work and why is it necessary?
- Insulin is a small peptide so cannot get across the membrane
- Insulin receptor is a dimeric protein of two identical units
- Insulin binds on the external surface which promotes cross-phosphorylation and activation fo the receptor
- Phosphorylated sites act as binding sites for scaffolding molecules
- This activates downstream kinases
What are integrins?
Integral membrane proteins that allow the adhesion of cells
They link the extracellular matric (ECM) with the intracellular cytoskeleton
