Leyland 2 Membranes

Question 1

TRANSMEMBRANE a-HELICS

Answer 1

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

Question 2

HYDROPHILIC

AMINO ACIDS

Answer 2

• 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

Question 3

HYDROPHOBIC AMINO ACIDS

Answer 3

Ile
Val
Leu
Phe
Cys
Met
Ala

Question 4

Hydropathy plots:

Answer 4

Allows the position of a-helical regions in a TM protein to be predicted

Question 5

Some integral membrane proteins can be formed from b-strands:

Bacterial porin

Answer 5

β-strands arranged in a barrel structure to form a pore

Question 6

Alternating hydrophobic and hydrophilic amino acids allows

Answer 6

interaction with lipid or aqueous environment

Question 7

Biological membranes

Proteins can be modified by

Answer 7

+ of carbohydrate or lipid groups

Question 8

glycolipids:

Answer 8

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.

Question 9

PROPERTIES of biological membranes:

x6

Answer 9

1. all amphipathic molecules -> spontaneously form bilayers in water.
2. sheets of lipids 2 molecules thick.
3. lipids & proteins. Carbohydrates can be attached to these molecules.-> need to be able to move.
4. held together by non-covalent interactions.
5. Fluid structures. Lipids & proteins diffuse readily in plane of memb
6. 2 faces of bio memb are different = asymmetric

Question 10

Membrane fluidity:

Answer 10

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

Question 11

Not all membrane proteins are free to diffuse

Answer 11

• Proteins may be tethered to cytoskeletal proteins

e. g. glycophorin

Question 12

Evidence for the FLUID MOSAIC MODEL:

fluorescence recovery after photobleaching

Answer 12

• 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

Question 13

Evidence for the FLUID MOSAIC MODEL:

CELL FUSION experiment

Answer 13

1. Mouse and human cells labelled with coloured antibodies against cell surface proteins
2. Cells fused to produce heterokaryon – no mixing of labelled proteins initially
3. Proteins mixed after several hours incubation

Question 14

FACTORS AFFECTING MEMBRANE FLUIDITY:

1

Answer 14

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.

Question 15

Membrane lipids exist in an ordered or disordered state depending on the temperature

Answer 15

less energy pack close => can’t move = crystalline / solid =transition temperature

Question 16

FACTORS AFFECTING MEMBRANE FLUIDITY:

2

Answer 16

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.

Question 17

Phosphatidylserine

Answer 17

if flips when damage to epithelial cells causes blood clotting

Question 18

ROLE of membrane proteins.

Answer 18

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

Question 19

Cytoskeleton

Function:

Answer 19

• binding to extracellular matrix
• interaction with other cells
• maintaining/changing cell shape

Question 20

Different types of transport processes

Answer 20

-against diffusion uses ATP

Question 21

Passive transport - channels

Answer 21

• channel lowers activation energy for the transport across the membrane

Question 22

Glucose transporters – GLUT 1-12

Answer 22

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

Question 23

Ion channels – K+ channel

Answer 23

• 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

Question 24

Active transport - pumps

Answer 24

• 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

Question 25

Sarcoplasmic and endoplasmic reticulum calcium pump

Answer 25

(SERCA)
Multi-domain protein
uses ATP hydrolysis to drive Ca2+ transport in the SR/ER
muscles cells complicated active transport

Question 26

Recognition – the insulin receptor

Answer 26

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

Question 27

Cytoskeleton – integrins

Answer 27

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.