ICPP S2 Membranes And Biological Function Flashcards
Give some functional evidence for proteins being present in membranes.
- Facilitated diffusion of things that would not normally diffuse through the bilayer. Proteins must be facilitating this.
- Ion gradients are maintained by pumps.
- Specificity - certain cell responses occur when specific molecules bind to complementary proteins.
Give biochemical evidence for proteins being present in membranes.
- Membrane fractionation + gel electrophoresis - we can separate out molecules in the membrane based on their Mr.
- Freeze fracture - we can freeze a sample and cut it with a microtome and view through electron microscope. Gaps in the seperate lamellae of the bilayer will be present where integral proteins are embedded.
What are the 3 modes of motions of membrane proteins?
- Conformational change
- Rotational
- Lateral diffusion
Why do membrane proteins not ‘flip-flop’ aka transversely diffuse?
- Very large compared to phospholipids and require substantially more energy to flip flop.
- Movement of protein to opposing lamellae would be too disruptive to the bilayer.
What are 3 ways in which membrane protein mobility can be restricted.
- Aggregation
- Tethering
- Interaction with other cells.
Describe tethering.
Proteins being tethered via interactions to other molecules intra/extracellularly.
Give 3 examples of interaction between cells, in terms of restricting protein mobility.
- Desmosomes
- Gap junctions
- Tight Junctions
Give an example of aggregation.
- Lipid-mediated effect proteins tend to aggregate in fluid areas that lack cholesterol.
What are the 2 types of membrane proteins?
Peripheral and Integral
Describe the properties of peripheral proteins.
- Bound to cell surface
- Relatively weak interaction ie H bonds and electrostatic interactions.
- Easily removed by changes in pH and temperature.
Describe properties of Integral membrane proteins.
- Completely span the phospholipid bilayer.
- Extensive interaction with the hydrophobic region of the bilayer.
- Not removed by change in pH because they are held in place by stronger bond.
- Require detergents and organic solvents to complete for the non-polar interactions they form.
R groups of AA residues in the transmembrane domain are largely hydrophobic.
True or false?
True
What conformation do transmembrane domains often take?
Alpha helical.
Describe the key features of the erythrocyte skeleton.
- Heterodimers of alpha and beta spectrum from the majority of the internal cytoskeleton.
- 2 tethering sites of the spectrin to the membrane.
- One tethering site - band 3 anchored to spectrin via ankryin.
- Other tethering site - spectrin heterotetramer to the membrane. Involves actin, adducin, band 4.1 and glycoprotein
What is anaemia?
Condition in which there is a deficiency of red blood cells or Haemoglobin in the blood.
What is haemolytic anaemia?
Anaemia due to destruction of red blood cells - haemolysis.
What is hereditary spherocytosis and how is it caused?
Is the loss of biconcavity in red blood cells, resulting them to form sphere shapes.
Genetic disorder in which spectrin is depleted by about 50%.
Erythrocytes round up and are less resistant to lysis.
Why does hereditary spherocytosis lead to splenomegaly?
Abnormal, spherical erythrocytes are destroyed by spleen, increased spleen activity leads to its enlargement.
What are some symptoms of anaemia?
Weakness Shortness of breath Feeling tired Confusion Loss of consciousness Increased thirst
What is hereditary eliptocytosis and how is it caused?
Refers to the erythrocytes being shaped elliptically as opposed to biconcave.
Occurs due to defect in spectrin molecules prevent heterotetramer formation.
Resulting cells are fragile.
What are the steps involved in secreted protein biosynthesis?
- Ribosome translates proteins 5’ to 3’.
- N-terminus - contains signal sequence.
- SRP - signal recognition particle - binds to the signal arresting translation.
- SRP directs ribosome to ER membrane where the SRP binds to the SRP receptor.
- Ribosome transferred to a protein translocator complex (translocon) and at the same time SRP and its receptor are released from the complex using energy from GTP hydrolysis.
- Polypeptide translation is guided through the translocon into the ER lumen.
- Signal sequence is cleaved by signal peptidase enzyme.
What are the steps involved in membrane protein biosynthesis?
- Polypeptide - translated with an internal signal-anchor sequence.
- SRP binds to signal - SRP binds to receptor.
- SRP aids in insertion of polypeptide into the translocon.
- Internal signal sequence is uncleaved and remains inside the translocon in the transmembrane domain, whilst the growing C terminus passes into the ER lumen.
- Once protein synthesis is complete the internal signal-anchor sequence leads the protein out into the bilayer - leaving the translocon.
Why does the internal anchor sequence contain hydrophobic amino acid residues?
Because this is the region that will reside in the hydrophobic region of the bilayer.
