Leyland 2 Membranes Flashcards
TRANSMEMBRANE a-HELICS
The most common structural motif in lipid proteins
FEATURES
• contain mostly hydrophobic amino acids
• few charged amino acids
• approximately 20 amino acids in length
s
HYDROPHILIC
AMINO ACIDS
- Arg – 4.5 Pro - 1.6
- Lys – 3.9 Tyr – 1.3
- Asn – 3.5 Trp - 0.9
- Asp – 3.5 Ser - 0.8
- Gln - 3.5 Thr - 0.7
- Glu - 3.5 Gly - 0.4
- His - 3.2
HYDROPHOBIC AMINO ACIDS
Ile Val Leu Phe Cys Met Ala
Hydropathy plots:
Allows the position of a-helical regions in a TM protein to be predicted
Some integral membrane proteins can be formed from b-strands:
Bacterial porin
β-strands arranged in a barrel structure to form a pore
Alternating hydrophobic and hydrophilic amino acids allows
interaction with lipid or aqueous environment
Biological membranes
Proteins can be modified by
+ of carbohydrate or lipid groups
glycolipids:
only on outside of cell surface
proteins synthesized -> in ER & golgi add on post translational process.
Sterol=>cholesterol
-> modifications recognition e.g. antibodies / function of protein.
PROPERTIES of biological membranes:
x6
- all amphipathic molecules -> spontaneously form bilayers in water.
- sheets of lipids 2 molecules thick.
- lipids & proteins. Carbohydrates can be attached to these molecules.-> need to be able to move.
- held together by non-covalent interactions.
- Fluid structures. Lipids & proteins diffuse readily in plane of memb
- 2 faces of bio memb are different = asymmetric
Membrane fluidity:
1 leaflet = one side of bilayer
Membrane lipids can move in the plane of the membrane – lateral diffusion
Membrane lipids very rarely move from one leaflet to the other – flip-flop
Same applies to proteins – flip flop of a protein molecule has never been observed.-> energetically unfavourable
Not all membrane proteins are free to diffuse
- Proteins may be tethered to cytoskeletal proteins
e. g. glycophorin
Evidence for the FLUID MOSAIC MODEL:
fluorescence recovery after photobleaching
• studied using FRAP
• cell surface is labelled w/ fluorescent molecule
• laser used to ‘bleach’ molecules in small area -> too excited to fluoresce
• follow return of fluorescence to area (as a function of the mobility of labelled molecule) -> molecules removing to fill the gap to allow fluoresce
green is cell
Evidence for the FLUID MOSAIC MODEL:
CELL FUSION experiment
- 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 incubation
FACTORS AFFECTING MEMBRANE FLUIDITY:
1
- Length and degree of saturation of the fatty acid chains:
• Long FA chains interact more strongly
• Double bonds (unsaturated) interfere with FA chain packing ->kinky -> more fluid state
• bacteria alter their membrane in response to change in environment.
Membrane lipids exist in an ordered or disordered state depending on the temperature
less energy pack close => can’t move = crystalline / solid =transition temperature
FACTORS AFFECTING MEMBRANE FLUIDITY:
2
- amount of cholesterol in membrane
• cholesterol inserts between FA chains
• can affect fluidity in both ways => stopping FA’s coming together increase fluidy
hydroxyl grp can hold molecules together by interacting with phospholipid head.
Phosphatidylserine
if flips when damage to epithelial cells causes blood clotting
ROLE of membrane proteins.
Transport
• channels (work with concentration gradient but need to be selective)
• pumps (work against concentration gradient - require energy)
•
Recognition
• receptors (e.g. hormone binding)
• glycoproteins/glycolipids - carbohydrate impt in recognition
Cytoskeleton
Function:
- binding to extracellular matrix
- interaction with other cells
- maintaining/changing cell shape
Different types of transport processes
-against diffusion uses ATP
Passive transport - channels
• channel lowers activation energy for the transport across the membrane
Glucose transporters – GLUT 1-12
transport across glucose -> binds on outside to binding site
conformational change ->transferred to 2nd binding sites
T1 – glucose binding site external
T2 – glucose binding site internal
• Glucose in plasma binds T1
• Activation energy lowered – change in conformation to T2
• Glucose released to cytoplasm
• Transporter returns to T1 conformation
Ion channels – K+ channel
- K+ channel composed of 4 subunits with 2 TM sequences
- Hydrated K+ ion in cytoplasm -> specific to K+ not allowing Na+ even though smaller ion
- K+ fits precisely in pore –stabilised by interaction with protein
Active transport - pumps
- Pump exists in 2 states with ion binding sites on different sides of the membrane in each state
- ATP hydrolysis is used to convert between the 2 states
Sarcoplasmic and endoplasmic reticulum calcium pump
(SERCA)
Multi-domain protein
uses ATP hydrolysis to drive Ca2+ transport in the SR/ER
muscles cells complicated active transport
Recognition – the insulin receptor
A dimeric protein of 2 identical units
• Insulin binding on the external surface promotes cross-phosphorylation and activation of the receptor
• Phosphorylated sites act as binding sites for scaffolding molecules
• Activation of downstream kinases => cascade
Cytoskeleton – integrins
cell communication
Integral membrane proteins that allow the adhesion of cells
Link the extracellular matrix (ECM) with the intracellular cytoskeleton
-tethers parts of the cell.
