Flashcards in Session 1 Deck (38):
What are the five general functions of biological membranes?
Permeability barrier, control of enclosed chemical environment, communication, recognition, signal generation
What is the general composition of a membrane?
40% lipid, 60% protein, 1-10% carbohydrate
(20% of total weight is water as membranes are hydrated structures)
Describe the general structure of membrane phosphoglycerides
Glycerol centre, 2 fatty acids (C14-24, C16 and C18 most common), phosphate; forms an amphipathic molecule with a hydrophilic head and hydrophobic tail. There may be one of a range of polar molecules attached to the phosphate head
What is Sphinogomyelin?
A phospholipid, the only one to not be based on glycerol.
How are glycolipids formed from sphingomyelin?
The phosphocoleine moeity of sphingomyelin can be replaced with a sugar
In which four ways can the phospholipids in the bilayer move?
Flexion, rotation, lateral diffusion, flip flop
What are cis double bonds and how do they affect the bilayer?
Found in unsaturated fatty acids, they are double bonds in cis orientation (same side), and in phospholipids this results in a kink in the hydrophobic tail = reduced phospholipid packing
Describe the structure of cholesterol
Rigid planar steroid ring structure, polar hydroxyl head group, non-polar hydrocarbon tail (more flexible)
What is the function of cholesterol in the lipid bilayer at lower temperatures?
At lower temperatures cholesterol interferes with fatty acid chain associations and increases fluidity; there is reduced packaging, preventing the formation of a crystalline array
What is the function of cholesterol in the lipid bilayer at higher temperatures?
At higher temperatures cholesterol tends to limit disorder and decrease fluidity; there is reduced phospholipid chain motion, due to the rigidity of cholesterol
How is the lipid bilayer affected by increasing cholesterol concentrations?
The heat tolerance of the bilayer increases
Give three examples of the functional evidence for the presence of proteins in membranes
Facilitated diffusion, ion gradients, specificity of cell responses (receptors)
What is the biochemical evidence for membrane proteins?
Freeze fracture and gel electrophoresis
What is freeze fracture?
Cell is frozen, and fractured by a knife along the lipid bilayer; this is the point of weakness. The two fracture faces produced will have different proteins and gaps, observable by electron microscopy
What are the two fracture faces called?
The P fracture face (cytosol side), and the E fracture face (extracellular side)
What are the three modes of motion of membrane proteins?
Conformational change (lipids flex/vibrate), rotational (circles), Lateral (across membrane), NO FLIP FLOP
Why is flip flop of membrane proteins not possible?
It is not energetically possible (movement through the hydrophobic region) and would cause the fracturing of the membrane
How is protein mobility restricted?
Some proteins are in aggregates, some proteins are tethered (extra or intra cellularly), and some proteins interact with other cells.
What links signalling proteins and cholesterol?
Signalling proteins are found in cholesterol rich regions; other proteins tend to prefer cholesterol poor regions
What is a peripheral protein and what bonds does it contain?
Peripheral proteins are membrane proteins bound to the surface of the lipid, by electrostatic interactions, hydrogen bonds, and disulphide bonds, which can be removed by changes in pH or ionic strength (salt concentration)
What are integral proteins?
Integral proteins interact extensively with hydrophobic domains of the lipid bilayer; they are often, but not necessarily, transmembranous
How are integral proteins removed from the lipid bilayer?
They are removed by agents that compete for non-polar interactions (e.g. detergents, organic solvents)
Describe key features of the transmembrane domain of transmembrane proteins
Often alpha-helical, around 18-22 amino acids in length, and the R groups are largely hydrophobic; usually followed by a few charged amino acids
How is hydropathy used?
Identifies sequences of hydrophobic amino acids likely to be transmembranous regions, hence giving an indication of the number of these in a protein
Which two proteins of the erythrocyte membrane are integral?
Band 3 and band 7; they both contain covalently attached carbohydrate units and are, thus, glycoproteins
Which two molecules make up the erythrocyte cytoskeleton?
Spectrin and actin
Describe the key features of spectrin
Spectrin is a long, floppy rod-like molecule, α and β subunits wind together to form an antiparallel heterodimer and then two heterodimers form a head to head association to form a head to head tetramer of α2β2
Describe the function of actin in the erythrocyte cytoskeleton
Short actin profilaments form cross links between spectrin tetramer rods
How does the attachment of integral proteins to the cytoskeleton affect membrane protein movement?
Restricts lateral movement of the attached membrane protein
What is hereditary spherocytosis?
Very rare dominant genetic disease, spectrin is depleted by 40-50%, causing erythrocytes to round up, making them less resistant to lysis; they are cleared by the spleen. Frequent transfusions are required
What is hereditary elliptocytosis?
A defect in the spectrin molecule, rendering it unable to form heterotetramers, resulting in fragile, 'rugby ball' shaped elliptoid cells.
How does membrane proteins biosynthesis differ from secretory protein synthesis?
Same pathway into ER; stop transfer signal (18-22 hydrophobic amino acids) causes the release of the protein from the translocator in the ER. Protein synthesis continues in the cytosol
What are the three main types of membrane lipid?
Phospholipids, cholesterol, gycolipids
What are the two types of membrane glycolipids? Differentiate between the two
Cerebrosides have a monomeric sugar head group, gangliosides have a oligosaccharide head group
Name the two adapter proteins in the erythrocyte cell membrane; what do they link to respectively?
Ankyrin (band 3), and band 4.1. (glcoporphyrin A)
How is the ribosome attached to the endoplasmic reticulum during membrane protein synthesis?
The ribosome is anchored to the translocation complex by riboporphyrins
Where are mitochondrial membrane proteins synthesised?