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BIOL1007 > Biological membranes > Flashcards

Biological membranes Flashcards

1
Q

What do biological membranes control ?

What can they also help with?

A

Biological membranes dictate how cells interact with their environment

They are also important in testing new drugs as they are a target.

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2
Q

Name some diseases which biological membranes have an important role in the disease?

A

1. Cystic fibrosis (Cl- channel)

CF transmembrane conductance regulator (CFTR) is a membrane and Cl- channel that conducts Cl- across epithelial cell membranes. Mutations of the CFTR gene affects Cl- ions leading to CF.

2. Malignant hyperthermia (Ca2+ channel)

This is a severe reaction that occurs in response to particular medications used during general anesthesia. in some individuals, the medications induce the release of stored Ca2+ ions within muscle cells, causing the muscle fibres to contract.

3. Long-QT syndrome (K+ channel)

These are inhertied cardiac disorders caused by mutations in the genes that encode Na+ or K+ transmembrane ion chennel proteins, characterised by arrhythmias which are often fatal

4. Darier disease (Ca2+ pump)

This was the first genetic skin disease caused by abnormal epidermal calcium homeostasis to be identified. SERCA2 is a Ca2+ pump of the ER transporting Ca2+ from the cytosol to the lumen of ER

5. Becker’s myotonia (Cl- channel)

Considered an ion channel disease, meaning that they result from abnormalities in the flow of certain ions across muscle cell membranes

6. Congential myasthenic syndrome (ACh receptor)

This is an inherited neuromuscular disorder caused by defects of several types at the neuromuscular juntion. mutations in the CHRNE gene, results in an ACh receptor deficiency

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3
Q

Where are cellular membranes found?

A
  • all cells are enveloped by a membrane
  • eukaryotes have internal membrane-bound organelles
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4
Q

What microscope has to be used to see biological membranes?

A

An electron microscope.

A light microscope can only use stains, which highlight proteins found in the membranes but doesn’t show the membranes themselves

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5
Q

Label the cell membrane…

A
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6
Q

What are the 3 major classes of lipids?

A
  1. phospholipids
  2. Sterols
  3. Sphingolipids
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7
Q

What lipid is this?

A

Phospholipid

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8
Q

What lipid is this?

A

cholesterol

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9
Q

What lipid is this?

A

Sphingolipid

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10
Q

Whats the basic structure of a phospholipid and how is variation achieved?

A

Variation in the phospholipid is achieved by varying fatty acid or alcohol groups being present

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11
Q

What are the 2 types of phospholipid and name some examples for each?

A

1. Neutral phospholipid (these alcohols have a +ve charge, but when they attached to the -ve phosphate group there is an overall neutral charge)

e.g. Ethanolamine, Choline

2. Negative phospholipid (these are zwitter ions or have no charge, so when attach to the -ve phosphate group there is an overall -ve charge)

e.g. Serine, glycerol, inositol

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12
Q

What is the phospholipid inositol important for?

A

It is a very important molecule in cell signalling

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13
Q

What are sphingolipids a major component of?

A

neuronal membranes

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14
Q

Where are sphingolipids primarily found?

A

in the plasma membrane

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15
Q

Phospholipids are amphiphatic molecules as they have a hydrophobic and a hydrophilic end. The lipids spontaneously form bilayers in an aqueous environment, what does the system try to minimise?

What does the hydrophilic regions interact with?

What does the hydrophobic region interact with? and why?

A

The system tries to minimise the free energy (∆G) of the system

hydrophilic sites try to make favourable interactions with the aq environment

hydrophobic regions try to interact with other hydrophobic sites, minimising entropic cost of placing lipids in water (hydrophobic effect)

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16
Q

What do water membrane lipids form?

A

vesicles

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17
Q

What do biological membranes contain large amounts of?

A

lipids with acyl choline chains (mainly C16-C18 in length) containing cis-double bonds

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18
Q

What do cis-double bonds in phospholipids prevent? What occurs in this phase/ what its called?

A

The close packing of acyl chains resulting in a bilayer where the acyl chains are mobile

In this phase- liquid crystalline phase- the lipids move around in the plane of the bilayer at biological temperatures

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19
Q

What are cell membranes described as being?

A

dynamic and interactive

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20
Q

What two movements of the phospholipids occurs in cell membranes?

A
  1. lateral diffusion
  2. flip-flop
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21
Q

Whats lateral diffusion and what does it allow?

Whats its value/ units?

A
  • Occurs very rapidly and is an exchange with their neighbour 107 x second
  • Allows the rapid movement of moelcules in the place of the bilayer

D= 10-9 cm-2 S-1

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22
Q

Whats the Flip-flop of phospholipids?

What is is enhanced by?

what does it preserve?

A
  • Energetically unfavourable
  • Occurs over very long time scales (hours/days)
  • Can be enhanced by membrane fusion or in the presence of specific enzymes (flippases)
  • Preserves membranes asymmetry
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23
Q

Why is membrane fluidity important?

A
  • At low temperatures the lipid crysalline turns into a gel phase in which the acyl chains are effectively frozen
  • The lipid crystalline phase is essential for the function of membrane proteins which require a fluid environment in which to operate
  • Molecules need to be able to move within the membrane to carry substrate between enzymes
  • cell signalling
24
Q

What types of lipids form the major lipid components of biological membranes?

A

Lipids based on glycerol and spingosine as well as sterols

25
Q

Lipids such as phosphatidylcholine form bilayers in water, what drives the assembly of the bilayer?

A

The hydrophobic effect

26
Q

What residues are found on the surface of proteins? give example

A

hydrophilic residues are found on the surface of proteins e.g. serine, aspartate, glutamate (amino acids with acidic side chains)

27
Q

Whats are the major membrane proteins that carry out many functions that are associated with cell membranes?

A
  1. channel proteins
  2. transporter proteins
  3. receptors
  4. structural proteins
28
Q

What do transmembrane helices help to do?

A

anchor the membrane proteins

29
Q

In order to cross the hydrophilic (non-polar) core of the bilayer, what conformation do most proteins typically adopt?

A

An alpha-helical conformation

30
Q

The residues found on the integral membrane proteins have what polarity of side chian and why?

A

Have hydrophobic side chains that interact with fatty acyl groups of the phospholipids.

This helps to anchor the protein to the membrane

31
Q

Why does the alpha helical conformation form?

A

polypeptides have a positive end (amine) and a negative end (carboxyl) so can’t enter the bilayer as it is energetically unfavourable.

Instead, alpha helices start to form as this maximises the hydrogen bonding between protein and the H bonding pairs up the +ve and -ve ends which neutralises the proteins

This results in a hydrophobic strucutre that cna pass through the bilayer

32
Q

A single transmembrane helix cannot form a route for the movement of polar solutes across the membrane, so instead what has to form and why?

A

amphipathic helices cluster together to produce a polar route across the bilayer.

This is able to fold to create pores for polar substances to pass through

33
Q

What do most eukaroytic membranes have? an alpha or beta motif?

A

an alpha-helical motif

34
Q

What can’t channel proteins do?

A
  • move ions or any solute against their electrochemical gradient i.e. they cannot pump
  • they are unsuitable for facilitating the diffusion of molecules such as glucose and amino acids across the plasma membrane
35
Q

What are the types of transportation across membranes and give examples?

A

1. Passive

molecules travel down conc. gradient

eg. diffusion, facilitated diffusion, osmosis

2. Active

molecules are moved against their conc. gradients. Energy is from either the electrochemical gradient or energy rich substartes e.g. ATP

e.g. active transport

36
Q

Tell me 3 facts about the atomic resolution strucutre of the calcium pump?

A
  • 10 alpha-helices across the bilayer
  • 2 Ca2+ ions being transported across the bilayer
  • binding sites for ATP, which provides the energy for the calcium transport.
37
Q

are channels in the membrane, hydrophilic or hydrophobic pores?

A

hydrophilic pores

38
Q

direction of ion flow is dictated by what?

A

the electrochemical gradeint of the ion (passive process)

39
Q

What are the 3 challenges faces when trying to work out a proteins structure?

A
  1. Protein expression
  2. solubilisation/ purification/ reconstitution (isolating the protein)
  3. Structural analysis
40
Q

What used to be used in order to would out a proteins structure?

Give examples

A
  • naturally abundant proteins (mitochondrial, bacterio-/rhodopsin, bacterial proteins)
  • Derived from bacteria (bacteriorhodopsin, PS-I/II, porins, lac permease)
41
Q

What do bacteria/ yeast lack, which makes protein expression harder?

A

The post-translational machinery for efficient membrane protein expression and membrane insertion

42
Q

What can be used to help disrupt the bilayer the proteins are in, but why is this an issue?

A

Detergents can be added to remove the proteins from the bilayer.

However, this could be an issue as non-denaturing detergents could associate with hydrophobic parts of the membrane proteins, thereby conferring miscibility to them.

43
Q

What techniques can be used for the structural analysis of proteins, tell me why each technique might have an issue?

A

1. X-ray crystallography

difficult to prepare crystals due to detergent molecule surrounding the proteins

2. Electron microscopy (2D electron diffraction)

low to medium resolution

requires formation for 2D crystals

3. NMR spectroscopy

solution state NMR techniques- structure in micellar systems

solid state NMR techniques- structure in the bilayer

44
Q

Give examples of indirect techniques to model membrane proteins?

A
  • optical spectroscopy
  • mutagenesis expts
  • modelling
45
Q

What do indirect techniques help to do?

A
  • sequence analysis to then be able to form hydropathy plots
  • location of post-translational modifications
  • labelling external residues to find out the orientation of the polypeptide
46
Q

What is a hydropathy plots and how does the labelling of the amino acids work?

A
  • each amino acids residue is assinged a value corresponding to its hydrophobicity
  • High value’s are assigned to hydrophobic amino acids
  • negative values are given to acidic amino acids
47
Q

What does this hydropathy plot show?

A
  • there is a protein which is 120 residues long
  • it has a section of 20 residues which is able to form a alpha-helical structure and is able to integrate with the cell membrane (this is known because of its positive hydrophobicity value)
48
Q

does the hydropathy plot allow you to work out the orientation of the polypeptide?

A

no

49
Q

What can be used to help work out the orientation of polypeptides and how does it do this?

A

Glycosylation is used to help work out the orientation of a polypeptide chain.

This is because proteins on the outside of the cell are the ones that are usually glycosylated.

50
Q

What does protease help to do when it comes to orientation?

A

it helps to identify what parts of the polypeptide chain are intracellular and extracellular to the cell membrane

51
Q

What does the precise glycosylation pattern depend on?

What did this pattern help to create?

A

the battery of inherited glycosylation enzymes inherited by an individual

This gives rise to the ABO blood group system

52
Q

What does this model of the polypeptide rhodopsin tell you about its hydrophobicity?

A
  • There are 7 regions that positive which means they can integrate with the cell membrane as must be hydrophobic as can form alpha-helices
53
Q

What is genomic analysis in proteins good for?

What does the families of proteins identifies help to work out?

A

identifying families of proteins

The function of families identified, reflects the environment in which the cell exists

54
Q

What are beta-barrels?

A

A type of membrane protein

55
Q

Tell me about the structure of beta-barrels

A
  • It is a beta sheet composed of tandem repeats
  • beta strands in the beta barrel are arranged in an antiparellel fashion
  • The strands contais alternating polar and non-polar amino acids- therefore, hydrophobic residues are orientated into the interior of the barrell to form a hydrophobic core
56
Q

Tell me about the surcorse specific porin found in the bacterium S. typhimurium

How does this pore specialise?

A
  • the porin is large and non-selective. this allows sucrose to diffuse across the outer membrane of the bacterium
  • the pore is made up of 16 beta strands
  • the loops can fold into the core which makes the pore more selective as it limits the size of the molecules that can get aross the bilayer via this pore.
57
Q

Why are membrane proteins difficult to crystallise?

A

There are relatively few high-resolution structures

BIOL1007 (8 decks)

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