Flashcards in Membrane bilayer & proteins (1) Deck (27)
What is the function of a membrane bilayer?
- Continuous, highly selective permeability barrier
- Controls enclosed chemical environment
- Recognition (signalling, adhesion, immune surveillance)
- Signal generation in response to stimuli
What are the dry mass weight proportions of a membrane's composition?
- Lipid: 40%
- Protein: 60%
- Carbohydrate: 1-10%
What proportion of a membrane bilayer's weight is water?
Outline the structure of a phospholipid.
- 2 FA chains: may have kinks
- Glycerol molecule
- Interchangeable head
What is the glycerol and FA chains together called?
- Phosphatidyl backbone
What do the heads to the phospholipid have to be? Give some examples.
What causes the kink in the FA chain?
- Cis double bond.
What are glycolipids?
- Sugar containing lipids base on sphingomyelin backbone
What are the different types of glycolipids?
- Cerebrosides: Head group sugar monomer
- Gangliosides: Oligosaccharides head group
What four types of phospholipid motion?
- Flexion (dynamic vibration)
- Lateral diffusion
- Flip-flop (rare as hydrophobic through hydrophilic environment)
What is the structure of cholesterol?
- Polar head
- Rigid planar steroid ring structure
- Non-polar hydrocarbon tail
How does cholesterol join to other phospholipid molecules?
- Via beta-OH group
What is the role of cholesterol in membranes?
- Maintains integrity to prevent crystalline islands
- Reduces phospholipid packing: increases fluidity
- Reduces phospholipid chain motion: decreases fluidity (helpful at high temps as reduces vibration so less damage)
What is the evidence for proteins in membranes?
- Facilitated diffusion
- Ion gradients
- Specificity of cell responses
- Membrane fractionation and gel electrophoresis
- Freeze fracture
What are the motions of proteins in a bilayer?
- Conformational change
- NOT flip-flop
Why can't proteins in membranes flip-flop?
- The energy requirement to move such a large hydrophobic molecule through a hydrophilic environment is too high
- Too destructive to move
What restrictions of membrane protein mobility is there?
- Aggregates (greater mass harder to move)
- Tethering (cell wall)
- Interactions with other cells
How are there restrictions on mobility for proteins in bilayer?
- Lipid mediated effects
- Proteins tend to separate out into fluid phase/cholesterol poor region
- Association with extra-membranous proteins.
What are peripheral membrane proteins?
- Bound to the surface
- Electrostatic and H bond interactions
- Removed by changes in pH/ionic strength
What are integral membrane proteins?
- Interact extensively with hydrophobic domains of lipid bilayer
- CAN'T be removed by pH/ionic strength
- Removed by non-polar interactions (detergents/organic solvents)
What are the characteristics of transmembrane domains?
- Alpha helical
- R groups of AA are hydrophobic
How can proteins be locked into place so they all face in the same direction?
- Post-translational lipid modifications.
What bands do erythropoietin proteins contain, why and where?
- Band 3
- Adhered to inside face of cellular membrane
What is hereditary spherocytosis?
- Spectrin decreased by 40-50%
- Erythrocytes become more rounded
- Become less resistant to lysis
- Cleared by spleen
What is hereditary elliptocytosis?
- Defect in Spectrin molecule
- Unable to form heterotetramers
- Fragile elliptoid cells
Outline secreted protein biosynthesis.
- SRP from docking protein recognises ribosomes with signal sequence.
- Ribosomes with signal sequence pass replicated protein strands through the membrane to the ER lumen via signal sequence receptors.
- Replicated protein joins to signal peptidase to prevent further replication.
- Protein detaches from ribosome to ER lumen
- Ribosomes separate and return to cytoplasm.