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ESA2 - Membranes and Receptors > Lipid Bilayer, Proteins and Membrane Structure > Flashcards

Lipid Bilayer, Proteins and Membrane Structure Flashcards

1
Q

Describe the composition of the plasma membrane

A
  • 40% Lipid
  • 60% Protein
  • 1-10% Carbohydrate
  • 20% Water (hydrated)
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2
Q

What components may be associated with the hydrophilic region of a phospholipid?

A
  • Choline
  • Amines
  • Amino acids
  • Sugars
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3
Q

What is the significance of unsaturated fatty acids in phospholipid molecules?

A
  • Unsaturated fats have a DOUBLE C=C BOND which introduces a ‘kink’ in the fatty acid chain
  • This REDUCES PACKING of phospholipids in the membrane to increase fluidity
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4
Q

Describe the general structure of phospholipids

A
  • GLYCEROL backbone
  • Fatty acid chains attached to C1 and C2
  • Phosphate group attached to C3
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5
Q

What is ‘sphingomyelin’?

A
  • Only phospholipid NOT BASED ON GLYCEROL

- Resembles other phospholipids in the membrane

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6
Q

Describe the different types of glycolipids found in the membrane

A
  • CEREBROSIDES - head group contains a SINGLE sugar monomer

- GANGLIOSIDES - head group contains many sugar OLIGOSACCHARIDES

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7
Q

What is a ‘glycolipid’?

A

Lipids with a carbohydrate attached by a glycosidic bond

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8
Q

What structure(s) can a lipid form in water?

A

MICELLE or BILAYER

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9
Q

Briefly describe the formation of a lipid bilayer in water

A
  • Bilayer forms SPONTANEOUSLY in water by Van der Waals forces between the hydrophobic tails
  • Cooperative structure is stabilised by NON COVALENT electrostatic forces and H bonding between the hydrophilic moieties and the polar heads with water
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10
Q

Name 4 ways in which lipids can move in the fluid bilayer

A
  • FLEXION (intrachain motion)
  • Fast axial ROTATION
  • Fast LATERAL DIFFUSION within the plane of the bilayer
  • FLIP FLOP
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11
Q

What is meant by lipids being able to ‘flip flop’ in the membrane?

A

Movement of lipid molecules from one half of the bilayer to the other on a one-for-one exchange basis

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12
Q

Why are there proteins embedded within the membrane?

A
  • Proteins carry out a range of functions within the membrane - mainly SELECTIVE PERMEABILITY
  • Can be transporters, enzymes, pumps, ion channels, receptors or energy transducers
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13
Q

What biochemical evidence is there for the existence of membrane proteins?

A
  • Membrane fractionation and gel electrophoresis

- Freeze fracture

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14
Q

Describe the motion of membrane proteins within the membrane

A
  • Conformational change (e.g. Receptors, carrier channels)
  • Rotation
  • Lateral diffusion within the bilayer
  • NO FLIP FLOP
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15
Q

Why do membrane proteins not flip flop?

A

Contain LARGE HYDROPHILIC MOIETIES that would require a large amount of energy to pass through the hydrophobic region of the bilayer (would be THERMODYNAMICALLY UNSTABLE)

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16
Q

Describe 3 ways in which membrane protein movement is restricted

A
  • AGGREGATES of membrane proteins in cholesterol-poor regions or due to lipid mediated effects
  • INTERACTIONS WITH OTHER CELLS e.g. Synapses
  • TETHERING extra-membranous association with peripheral proteins or cytoskeleton
17
Q

What is the difference between peripheral and integral membrane proteins?

A
  • PERIPHERAL - bound non-covalently to the surface by H bonds and electrostatic attractions
  • INTEGRAL - interact extensively with hydrophobic regions of bilayer
18
Q

How can integral proteins be removed from the bilayer?

A
  • DETERGENTS or ORGANIC SOLVENTS

- Compete with proteins for the non polar interactions within the bilayer

19
Q

Why are phospholipids described as ‘amphipathic’?

A

Contain both hydrophobic AND hydrophilic regions

20
Q

How does cholesterol contribute to membrane stability at low and high temperatures?

A
  • LOW - Reduces phospholipid packing to INCREASE fluidity

- HIGH - Reduces phospholipid chain motion to DECREASE fluidity

21
Q

What are the restraints on mobility of proteins within the membrane?

A
  • LIPID MEDIATED EFFECTS where proteins separate out in fluid phase or cholesterol poor regions
  • MEMBRANE PROTEIN ASSOCIATIONS e.g. sugars
  • ASSOCIATIONS WITH EXTRA-MEMBRANOUS PROTEINS (peripheral proteins e.g. cytoskeleton)
22
Q

What is the percentage of protein in a lipid bilayer of an average cell?

A

~60% dry weight

23
Q

Give 2 ways in which peripheral proteins can be removed

A
  • Change in pH

- Change in ionic strength

24
Q

Describe how integral proteins can be removed from the bilayer

A
  • Detergents or organic solvents

- Compete for the non-polar interactions within the bilayer

25
Q

Why is the asymmetric orientation of proteins important?

A
  • RECEPTORS for hydrophilic messengers e.g. Insulin

- Recognition site must be directed towards extracellular space in order to detect signals

26
Q

The majority of proteins within the erythrocyte membrane are peripheral. How do we know that they are on the cytoplasmic face?

A

The peripheral proteins are susceptible to proteolysis only when the cytoplasmic face is accessible

27
Q

What are the only integral proteins in the erythrocyte membrane?

A
  • BAND 3

- Glycophorin A (band 7)

28
Q

What is the function of carbohydrate chains on membrane proteins?

A
  • Extracellular carbohydrates are HIGHLY HYDROPHILIC
  • Lock orientation of the protein and PREVENT FLIP-FLOP
  • Role in cellular recognition
29
Q

Describe the composition of the erythrocyte cytoskeleton

A
  • Composed of SPECTRIN and ACTIN molecules
  • Heterotetramers of spectrin are cross linked by short actin protofilaments
  • Band 4.1 and Adductin form interactions towards the end of spectrin rods
30
Q

Explain how erythrocyte membrane proteins associate with the cytoskeleton

A
  • Attached by ADAPTOR PROTEINS
  • Ankyrin (band 4.9) links spectrin to band 3 membrane protein
  • Band 4.1 links spectrin to Glycophorin A
31
Q

What two proteins link spectrin rods together in the cytoskeleton of RBC?

A

Adductin and Band 4.1

32
Q

Describe the structure of spectrin

A
  • α and β subunits wind together to form a HETERODIMER
  • Two heterodimers form a head-to-head association forming a HETEROTETRAMER of α2β2
  • Spectrin rods are cross linked by short actin protofilaments
33
Q

What is the main role of the cytoplasm in erythrocytes?

A

Allows FLEXIBILITY and DEFORMABILITY of membrane so cells can pass through small capillaries without lysis

34
Q

Describe how dominant hereditary spherocytosis can lead to haemolytic anaemia

A
  • Autosomal dominant condition whereby spectrin levels of RBC are depleted by 40-50%
  • Cells round up so cannot pass through capillaries, therefore undergo lysis in spleen
  • Insufficient production of RBC by bone marrow leads to HAEMOLYTIC ANAEMIA
35
Q

Explain the pathophysiology and treatment of hereditary elliptocytosis

A
  • Genetic defect in spectrin so that it cannot form heterotetramers
  • Results in fragile ellipsoid RBC
  • Treated with CYTOCHALASIN drugs which can alter the deformability of RBC by capping polymerising actin filaments
36
Q

Describe how membrane proteins are embedded in the ER membrane

A
  • N terminal signal sequence recognised by SRP and binds to protein translocation complex on ER membrane
  • Protein is threaded through membrane until a STOP TRANSFER SEQUENCE is reached (hydrophobic region of 18-22aa)
  • STS spans the hydrophobic region of membrane and forms transmembraneous protein
36
Q

How does the embedding of a transmembrane protein determine its orientation?

A
  • Signal sequence on N terminus remains bound to PTC as protein is threaded through membrane
  • Resulting orientation is N terminus on lumenal side and C terminus in cytoplasmic side
37
Q

Give an example of a multiple spanning transmembrane protein

A

G protein coupled receptor

38
Q

Describe how a transmembrane protein in the ER membrane is transported and embedded in the plasma membrane

A
  • Transmembrane protein travels from ER to Golgi and is processed and packaged into vesicles
  • Vesicles fuse with plasma membrane and embed protein in its correct orientation

ESA2 - Membranes and Receptors (6 decks)

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