4. Biological Membranes II

Question 1

Protein constituents of the cell membrane

Answer 1

Phospholipid bilayer:

• Integral protein
• Oligosaccharide
• Glycoprotein
• Glycoprotein
• Glycolipid
• peripheral protein
  => Hydrophobic core

Phospholipid:

=> Cytosol

• integral protein
• peripheral proteins
• hydrophilic polar head
• fatty acyl tails

Question 2

Membrane proteins

Answer 2

 Fluid Mosaic Model: plasma membrane is a fluid combination of phospholipids, cholesterol and proteins (Singer and Nicolson, 1972)
 Protein composition in membranes between 20%(nerve cell axons) and 75% (mitochondria/chloroplasts)
 Membrane proteins perform membrane’s specific tasks
 Can be integral (transmembrane) or peripheral (associated with membrane)

Question 3

Different types of membrane proteins

Answer 3

Can be attached to the membrane in many ways - reflecting their function

Hydrophobic transmembrane regions (alpha - helices, beta - sheets):
=> Only transmembrane proteins function on both sides of membrane

Covalent bound lipid:
=> Function restricted to one side (membrane asymmetry)

Question 4

Study of membrane protein - Detergent

Answer 4

membrane protein in lipid bilayer + [hydrophobic tail, hydrophilic head, detergent monomer detergent micelles] => water-soluble protein lipid detergent complex + soluble mixed lipid detergent micelles

=> sodium dodecyl sulphate (SDS)
=> Triton X - 100

Question 5

Hydropathy plots of membrane proteins

Answer 5

To predict potential hydrophobic membrane spanning segments, such as alpha-helices from the amino acid sequence

Question 6

Beta - barrel Trans - membrane proteins

Answer 6

multiple hydrophobic beta - sheets

1. ) 8 - stranded OmpA -> Receptor for bacterial virus
2. ) 12 - stranded OMPLA -> enzyme that hydrolysis phospholipids

Membrane channels:

3. ) 16 - stranded porin -> bacterial porin
4. ) 22 - stranded FepA -> bacterial iron transporter

Question 7

Peripheral - bound membrane proteins

Answer 7

 Covalent bound fatty acids used to attach proteins to membrane
 Weak attachment (sometimes more multiple hydrocarbon chains)

A) amide linkage between terminal amino group + myristic acid
B) thioester linkage between cysteine + palmitic group
c) thioester linkage between cysteine + phenyl group

D) myristoyl anchor -> myristic acid (14 C)
E) palmitoyl anchor -> palmitic acid (16 C)
F) farnesyl anchor -> isoprenyl groups

Question 8

membrane proteins + glycosylation

Answer 8

 Many membrane proteins in animals are glycosylated
 Sugar residues added in ER and Golgi lumen
 Membrane asymmetry: glycosylation only found on non - cytosolic side of membrane

Carbohydrate layer: (with sugar residue)

• transmembrane glycoprotein
• adsorbed glycoprotein
• transmembrane proteoglycan

lipid bilayer:

• glycoprotein
  => Cytosol

Question 9

Glycosylation

Answer 9

• > Yeast
• > Insect
• > Animal
• > Plants
• > Human

Question 10

Membrane proteins + glycosylation

Answer 10

 Glycosylated proteins provide cell coat or glycocalyx (carbohydrate layer)
 Protection against chemical and mechanical damage
 Keeps cells at distance preventing unwanted protein - protein interaction

Question 11

Membrane proteins + disulfide - bridge formation

Answer 11

 Cytosol is rather reduced compared to the outside (oxidized) of the cell
 membrane asymmetry: Disulfide bridge formation is
predominantly extracellular
 Keeps protein in specific conformation

Oxidation Reduction => Redox reaction

Oxidation -> gain of oxygen
Reduction -> loss of oxygen

Question 12

membrane proteins can form large complexes

Answer 12

• > Photosynthetic reaction centre

- > Mitochondrial ATP synthase

Question 13

Self - assembly into aggregates

Answer 13

Bacteriorhodopsin in the purple membrane of Halobacterium salinarum

Question 14

Lateral motility of membrane proteins

Answer 14

Many proteins can laterally diffuse in plane of the membrane
(1) Fluorescence Recovery After Photobleaching (FRAP)

FRAP -> (Bleach with laser beam) -> bleached area -> recovery

Fluorescence in bleached area
Bleach (peak - down drop)
Recovery curve (fluorescent labeled proein)
Time ->

Question 15

membrane proteins interact with macromolecules inside the cell

Answer 15

Red Blood Cell:
 No nucleus, no organelles 
 RBC has biconcave shape 
 interaction between membrane proteins and 
underlying cortical cytoskeleton

Question 16

Cortical Cytoskeleton

Answer 16

A deformable meshwork of a filamentous protein called spectrin

Actin -> Adductin -> spectrin -> tropomyosin -> band 4.1

Junctional complex -> spectrin dimer -> Actin -> spectrin -> ankyrin -> band 3 -> glycophorin -> band 4.1

106 amino acid long repeating domain -> alpha chain -> flexible link -> beta chain => H2N HOOC => COOH NH2

 Analogous networks beneath plasma membranes of most cells (mostly actin filaments)

(1) Protects against mechanical forces of trip through narrow capillaries

(2) restricts diffusion of membrane proteins (formation of corrals
 Confinement depends on interaction with other proteins and size of cytoplasmic domain
 Aids building of large protein complexes
 concentration of protein complexes increases speed and efficiency
 Similar to what Membrane Rafts (microdomains) do

Corrals:
membrane ‘skeleton’ + associated proteins -> plasma membrane -> transmembrane proteins -> membrane domains

High speed single particle tracking

• > requires high speed + high sensitivity to achieve time resolution in milliseconds
• > count + measure size of molecules or vesicles in solution

Question 17

the cell membrane is a ‘semi - selective’ barrier

Answer 17

Permeability coefficient:
small or unchanged -> high permeability
large or charged -> low permeability

Hydrophobic molecules (O2, CO2, N2, steroid hormones) => diffusion permeability

small uncharged polar molecules (H2O, urea, glycerol) => permeability dependent on size + polarity

large uncharged polar molecules (glucose, sucrose) => lipid bilayers highly impermeable to large molecules + ions

Ions (H+, Na+, HCO-3, K+, Ca2-, Cl-, Mg2+) => diffusion (permeation) difficult / impossible

=> diffusion (permeation) alone is not sufficient -> Transporters