lecture 7 Flashcards
How thick are lipid bilayers?
5 nanometers, or around 40 atoms thick.
What is the difference between saturated and unsaturated acyl chains?
- Saturated: no double bbonds
- Unsaturated: cis-double bonds present, causing a ‘kink’ in the acyl chain.
What are the differences between glycolipids and phospholipids?
- Glycolipids have a hydrophilic sugar attatched to their chains.
- They have no phosphates (phospholipids do)
- Therefore, unlike phospholipids, they have no charge and are neutral
Explain why butter is solid at room temperature, and olive oil is liquid at room temperature. Explain this on the molecular level.
- Butter is composed of lipids that have saturated acyl chains. This means that they’re tightly packed, through van der Waals interactions. This is reflected in their melting temperature: aroung 40 degrees celcius.
- Oils are composed of lipids that have unsaturated acyl chains. These cause a kink, making packing through vander Waals interactions less favourable. They therefore have a lower melting temperature (-4 degrees) which is why they’re fluid at room temp.
On what two things does melting temperature of lipids depend?
- degree of saturation
- carbon number: short chain: lower melting temp, longer chain: higher melting temp
When a cell is growing at high temperatures, will it have more saturated, or unsaturated fatty acids in its membrane? Why?
It will have more saturated fatty acids, to maintain the crystalline structure of the membrane at high temperatures.
To which side are glycolipids always exposed? What important function do they have to the cell?
Glycolipids are always exposed to the exterior of the cell. They protect the membrane against harsh conditions like low PH for example.
What are four major phospholipids in the mammalian plasma membrane and which side to they face?
*Phosphatidylethanolami ne- inner leaflet
* phosphatidylserine- inner leaflet
* phosphatidylcholine- outer leaflet
* sphingolipids- outer leaflet
Which of the four phospholipids commonly present in mammalian cells has a charge?
Phosphatidylserine has a net negative charge, making the net charge of the membrane negative. All the other phospholipids have a net charge of zero.
Which organisms do not have sphingophospholipids?
Bacteria don’t have them. They’re only present in (higher) eukaryotes.
Which lipids form the insulation of nerve cells and how do they do this?
Glycolipids are very abundant in nerve cells. The myelin coating is filled with glycolipids, which form a natural insulation due to their hydrophilic character.
Which membrane contains a lot of cardiolipin? What is the general structure of cardiolipin?
The inner mitochondrial membrane has a lot of cardiolipin, which plays a key role in supporting the ETC proteins, for example.
It has four acyl chains, two of which are negatively charged.
What are the types of phospholipid movement?
- Rotation
- Flexion
- Flip-flop (rarely occurs, requires the help of special enzymes called flippases)
- Lateral diffusion (occurs spontaneously)
What is the importance of flippases to membrane proteins?
Flippases help maintain the asymmetry of the membrane lipids. Most membrane proteins rely on this asymmetry to function, making flippases essential to membrane protein function.
What is the relationship between membrane thickness and saturation of the lipids?
- Unsaturated hydrocarbon chains (cis-double bonds) : fluidity increases, thinner membrane
- Saturated hydrocarbon chains: fluidity decreases, thicker membrane
What does cholesterol do at different temperatures?
- High temperatures: cholesterol stiffens the membrane
- Low temperatures: cholesterol makes the membrane more fluid, preventint it from crystallizing
Why is membrane asymmetry important? What happens to membrane asymmetry in blood clotting?
Membrane asymmetry is essential to the functioning of membrane proteins, and also for the transfer of signals across the membrane (so the inner and outer surface can attract different kinds of proteins)
In blood clotting, scramblases become active to reduce this asymmetry, to form a recognition site for procoagulant factors to ensure fast and eficcient blood clotting at the site of injury.
What fraction of genes encode for membrane proteins?
About 30% of genes encode for membrane proteins.
Which membranes contain alpha-helix proteins? Which membranes contain beta barrel proteins?
- Alpha helix: Plasma membrane, inner mitochondrial membrane, inner chloroplast membrane
- Beta-sheets: outer mitochondrial membrane and outer chloroplast membrane
What does a positive value of a hydropathy plot mean? And a negative value?
- Positive value: hydrophobic side chain
- Negative value: hydrophilic side chain
How long is your typical transmembrane alpha helix?
About 20-25 amino acids. So, for it to qualify as a transmembrane helix, at least 20-25 residues must be predominantly apolar.
Why do hydropathy plots not work for beta barrels?
Because the hydrophobic and hydrophilic groups alternatingly stick out into the lipid bilayer, or into the beta barrel. A hydropathy plot of a beta barrel would therefore look random.
Where is the topology of membrane proteins determined?
In the ER. ! This doesn’t change anymore in the golgi!
Where will glycosylated domains be?
On the outside
Where would you expect to find SH bonds? Where would you exppect to find disulfide (SS) bonds?
- The cytosol is a reducing environment, therefore, you’ll find SH bonds in the cytosol.
- The outside of the cell is an oxidizing environment, so here you’ll find disulfide bonds.
Many cytosolic proteins bind to specific lipid head groups on the lipid side of the membrane. What is an example of such protein and to which head group does it attach?
The enzyme protein kinase C (PKC) is activated in response to extracellular signals and binds to phosphatidylserine on the cytosolic sice. It specifically needs this negatively charged phospholipid for its activity.
Why are beta barrels more easily purified and crystallized than alpha-helices?
Because the hydrogen bonds in beta barrels bind each strand rigidly to its neighbors, making conformational changes in the wall of the barrel unlikely. (Makes them rigid)
Alpha helices, in contrast, can slide agains each other, allowing conformational changes.
In addition to being transporters, what other function can beta barrels have?
Some function as receptors or enzymes, having barrels that are completely filled by amino acid side chains. The barrel serves as an anchor that holds the protein in the membrane and organizes the cytosolic loops that form binding sites for certain intracellular molecules.
