Lecture 13 - Molecular architecture of membranes

Question 1

What are the three types of amphipathic lipids?

Answer 1

Phospholipids, glycolipids, and sterols

Question 2

How are lipid bilayers stabilised?

Answer 2

Hydrophobic effect
Polar interactions between the head groups and water
Non covalent interactions between the lipid molecules

Question 3

How are membranes of the Archaea structured?

Answer 3

Made from ether lipids and the non-polar hydrocarbon chains are branched

In some species non polar chain spans the entire membrane to give an even more resistant structure

Question 4

What else do membranes contain?

Answer 4

Proteins

Question 5

The lipid bilayer is an effective barrier to what?

Answer 5

The free diffusion of most water soluble compounds - ions and charged molecules

Question 6

How do membrane potentials arise?

Answer 6

Membrane potentials arise from the unequal distribution of charge - across semi permeable membranes

Question 7

What contributes to the imbalance between the number of positive and negative charges in a membrane bound compartment?

Answer 7

Every ion transport process

Question 8

The negative membrane potential across the plasma membrane is important for three things:

Answer 8

• Ion transport
• Energy transduction
• Nerve function

Question 9

Membrane fluidity is determined by what three factors?

Answer 9

• Temperature
• Fatty acid composition
• Cholesterol content

Question 10

How does temperature affect membrane fluidity

Answer 10

As temp falls molecular motions decreases so phospholipids pack together more closely.

Question 11

What happens below the transition temperature (Tm)

Answer 11

The membrane changes from a fluid liquid crystalline state into semi rigid gel state.

Question 12

What happens as temperature rises?

Answer 12

Molecular motion increases and the liquid crystalline state disperses and the membrane melts

Question 13

What happens as you increase the length of the fatty acid chain

Answer 13

Reduces the fluidity by increasing the favourable interactions between the closely packed chains

Question 14

What happens if cis double bonds are added?

Answer 14

Cis double bonds in the fatty acid chain increases fluidity by disrupting close packing of the bilayer - produces a bend in the hydrocarbon chain

Question 15

What does cholesterol do

Answer 15

It smoothes out the transition between the liquid crystalline and gel states

increases fluidity below Tm and decreases it above Tm

(see graph in lecture handout - shows point clearly)

Question 16

Two examples of peripheral membrane proteins? And where are they located?

Answer 16

Cytochrome c - electron transport protein at the outer surface of the inner mitochondrial membrane

Spectrin - cytoskeletal protein attached to the inner surface of the plasma membrane of erythrocytes

Question 17

How is anchorage of peripheral proteins achieved?

Answer 17

By non covalent interactions with polar and non polar regions of membrane surfaces

Question 18

Why do membranes contain different repertoires or amounts of proteins

Answer 18

Membranes perform different functions e.g. membranes of mitochondria and chloroplasts have 75% protein content compared to myelin (18%) - relatively pure lipids are well suited for insulation.

Question 19

How are peripheral membrane proteins bound to the membrane?

Answer 19

By electrostatic and hydrogen-bond interactions with the head groups of lipids. Others are attached by a covalently attached hydrophobic chain (e.g. a fatty acid )

Question 20

Describe one example of attachment of a peripheral protein by covalent modification

Answer 20

Many peripheral proteins are covalently modified by adding a non polar chain that can be inserted into the bilayer. e.g. a palmitoyl group attached to a specific cysteine residue by a thioester bond

NB modifications are used to attach cytosolic proteins to membrane surfaces

Question 21

Glycosyl phosphatidyl inositol (GPI) anchors. Describe what they do.

Answer 21

A GPI anchor binds the protein to he noncytosolic surface of the ER

Question 22

The temperature at which phase transition occurs is lowered by…? (membrane becomes more difficult to freeze)

Answer 22

Short hydrocarbon chains or presence of double bonds

Question 23

A shorter chain length reduces the tendency to what?

Answer 23

Reduces the tendency of the hydrocarbon tails to interact with each other

Question 24

How does cholesterol modulate properties of lipid bilayers

Answer 24

• Cholesterol inserts into the bilayer with hydroxyl group close to the phospholipid heads so that the rigid steroid rings immobilise regions of the hydrocarbon chains closest to the polar head groups
• Decreases the permeability of the bilayer to small water soluble molecules
• At high conc. prevents crystallisation

Question 25

What two ways can cytosolic proteins be attached to the cytosolic monolayer of the lipid bilayer?

Answer 25

1. Amphiphilic alpha helix exposed on the surface of the protein 2. One or more covalently attached lipid chains

Question 26

What are integral membrane proteins?

Answer 26

These proteins span the whole membrane and interactions with the hydrophobic region of the bilayer

Question 27

Examples of integral membrane proteins?

Answer 27

ATPases Glycophorin A - glycoprotein that coats erythrocytes in carbohydrate and provides an anchor for some of the peripheral proteins of the cytoskeleton

Question 28

What do the structural motifs alpha helices and B barrel allow?

Answer 28

Allow the polar polypeptide backbone to go across the lipid bilayer provided the side chains in contact with the fatty acid chains are predominantly nonpolar

Question 29

Transmembrane alpha helices Describe why alpha helices form

Answer 29

Transmembrane proteins have cytosolic and non cytosolic domains that are separated by membrane spanning segments of the polypeptide chain. Because the peptide bonds are polar - in the non polar lipid region of the bilayer, the peptide bonds form hydrogen bonds with one another . Forming a regular alpha helix maximises the hydrogen bonding when it crosses the bilayer

Question 30

Describe the B barrel Multipass transmembrane proteins

Answer 30

An alternative way for peptide bonds to form H bonds with each other. Multiple transmembrane strands to be arranged as a B sheet that is rolled up into a cylinder. Seen in porin proteins

Question 31

Porin : The B barrel transmembrane motif Describe the structure

Answer 31

The transmembrane region of porin (pore forming protein from the outer region of bacteria) is made from antiparallel B strands The pore is filled with water and amino acid side chains that face the pore are hydrophilic Outer surface of porin is embedded in the membrane and the side chains of amino acids are mostly non polar NB smaller channels with gating properties can be made from clusters of amphipathic alpha-helices.

Question 32

What is the function of Bacteriorhodopsin protein?

Answer 32

A light activated H+ pump that transfers H+ out of the archaeal cell

Question 33

Structure of bacteriorhodopsin

Answer 33

Each molecule is folded into 7 transmembrane alpha helices and contains a single light absorbing group

Question 34

What is the hydropathy index

Answer 34

The hydropathy index is the free energy change when a 20 amino acid segment is transferred from a non polar region to water

Question 35

List the five functions of membrane proteins

Answer 35

``` Structural support and protection Energy transduction Signal detection and transmission Catalysis Transmembrane transport ```

Question 36

Why is membrane fluidity essential?

Answer 36

Diffusion and conformational flexibility are often functionally important

Question 37

How do bacteria regulate the fluidity of their membranes?

Answer 37

Vary the number of double bonds and length of fatty acid chains

Question 38

Describe the structure of cholesterol and how it inserts itself into the membrane

Answer 38

Contains a steroid nucleus with a hydroxyl group at one end and a flexible hydrocarbon tail at the other end. Inserts with its long axis perpendicular to the plane of the membrane. the hydroxyl group forms a hydrogen bond with the carbonyl oxygen atom of a phospholipid head group. The hydrocarbon tail of the cholesterol is located in the non polar core of the bilayer.