Biological Membranes - Biochemistry
Why are biological membranes important
25% of the genes in the human genome are for membrane proteins
They are important as they dictate how cell will interact with the environement
60% of marketed drugs target membrane proteins
Where are membranes found
All cells are enveloped by a plamsa menbrane and some internal organelle are also membrane bound like mitochondria and chloroplasts
How can you see membranes under a light microscope
They have to be stained with antiboides due to usually imvidble under a light micropscope but can be seen by electron microscopes
What does a hetergenerous structure mean
This means membranes are highly dynamic assemblies that is composed of mainly lipids and proteins
The proprotion of both is highly dependent on the function of the cell or organelle
What are the 3 major classes of lipids
Phospholipids
Cholesterol
Sphingolipids
What is the structure of a phosopholipid
Made up of a glycerol backbone withg 2 fatty acids chains bonded to it along then with a phosphate group that contains an alcohol- OH bonded to it on the end
The phosphate in the SN3 and the fatty acids on SN1 + 2 and the headgroup is attched to the phosphate
What are Sphingolipids
They are the main component of neuronal membranes and are primarily found in the plasma membrane
They can cause different behaviours in membrane due to fcorming local domains
They dont contain a glycerol backbone
Rather than an OH group bonded to a phosphate there is a N+ molecule
What are sterols
They have a hydroxyl group that CO group of a phospholipid and has a long axis that run parrallel to the membrane in order to help provide the membrane structural rigidity
What are the physicall properties of lipids
They have a hydrophillic head and hydrophobic fatty acid tails so overall are amphipathic
What happens to lipids in an aqueous solution
In an aqueous solution the lipids will form bilayers, this is due to trying minimise the delta G of the system
Hydrophillic intercation from the head intercat with the aqueous environment and the hydrophobic tails try to intercat with other hydrophobic sites, lead to the formation of a membrane biliater with the tails pointing in away from the water and the heads = facing out towards the water
This lead to formation of vesicles
What is the liquid crystalline state
Due to lipids have large acetyl chain mainly of cis-double bonds, this will prevent the close packing of acyl chain meaning then a bilayer with mobile acyl chain
In this phase the lioids move around in the plane of the bilayer at biological temperatures
What is lateral diffusion in the membrane
This allow rapid movement of molecules in the plane of the bilayer and occur rapidly with the neighbour
What is lipid flip-flop
This is unfavourable energetically and takes very long time, and is where 2 lipids that are opposite each other in the bilayer will swap sides of the bilayer that they are on, this can be done due enzymes called flippases and will preserve asymmetry in the bilayer
Why is membrane fluidity impirtant
At low temp the liquid crystalline lipid turns inot a gel phase with frozen acyl chain
Liquid crystalline is essential for fucntion of membrane proteins that need a fluid environment
Molecules need to move in the membrane to carry substrates between enzymes and start signalling events
Whar are the conformationla change in membrane proteins
Lipids based on glcyerol or sphingosine and sterols form major component of biological membranes
Others form bilayers within water due to the hydrophobic effect
All biological membranes are in liquid -crystalline phase
What are the major proteins that are in cell membranes
Protein channels
Transportters
Receptors
Structural proteins
Why are proteins important in the membrane
This is due to provide route for polar and larger molecules to cross the membrane as well as info carried by polar signalling molecules
Why do transmembranous proteins have an alpha helix shape
Channels, receptor and transporters all span the whole way across the membrane in an a-helix stucture to allow them to cross over the hydrophobic membrane core
What do transmembranous proteins do
They transmembrane helices anchor the membrane proteins
This is due to the protein sequence being hydrophobic in the transmembrane domain
The R groups project outwards to face the lipids to ensure that the protein is being anchored in the lipid bilayer
How is a path created over the whole membrane
Multiple amphipathic helices are able to cluster together in order to produce a polarv route across the bilayer
You need at least 4 amphipathic helices for this to be created
What do polar channels over the bilayer control
They are able to control the passive diffusion of ions over the membrane, this is because the channles have charged residues on the outisde of the channel so then control what type of ions can enter the channel
Also the diameter controls the ions size that can enter the channel and channel roation to bring R groups to the channle to prevent ion movement
What do transporters do
Used if need to pump the solute then use SYP hydrolysis or by a co-transportter of another ions that is being moved down a electrochemical gradient
There is no pumping used for transporter
How do ion gated channels work
Where have a + gate on the ion channel that is controlled by the R group of amino acids making up the gate so then only negtayive ions are able to pass through the channel and voce versa
How to fully close the channel
If there is a rearrangement of the helices in the channle so that there is no hole in the bilayer so then no passage of molecules due to closed channels
What are ligand gated ion channels
These are involved in nueorne transmission in the body
In the open state serine is facing towards the middle and larger leucine facing awat so a channel is created but when closed the larger leucine are projected to the middle so then no channel due to closed
What can channels not do
They cant move ion or any solute against their concentration gradient via pump or active transport
Unsuitbale for faciliatted diffusion of larger molecules like glucose or amino acids
What are the 2 types of transporters
Passive - Molecules move down the concentration gradient
Actove- Molecules move against their concentration gradient and so energy is needed from ATP hydrolysis
How do transporters work
They have 2 gates
where first 1 is opend to allow access to recognition site for the solute to be transported
This binding causes the other gate to open and the first gate to then close due to a confromational change to allow new gate to open and close the first one
What is the atomic resolution structure of the calcium ion pump
There is 10 alpha helices cross the bilayer to form the strucutre for 2Ca2+ ions to be transported across the bilayer
There is a binding site for ATP to provide energy for calcium transport
Why us calcium ion movement important
Needed in muscles to trigger contraction and to rapidly relax the muscles when Ca2+ ion removed
Also needed in synapses
Why is it hard for atomic resolution structure of membrane proteins to be made
Membrane proteins dont readily form 3D crystals and very few structures are resolved for crystallography and they are too large for liquid state NMR
What are problems caused from atomic resolution of membrane proteins
Limoted number of membrane proteins needed for function to take place, sio then limited number for crystallisation studies
Over expression of their genes doesnt work so always limited protein to study
Hard to remove 2d anchored protein without changing composition
Why is protein expression a challenge for studying proteins
For early structures easy due to high protein natural abundance their and are colourful
For smaller proteins like receptors harder due to expressed heterologously in bacteria that lack the post translational machinaery for effcient membrane protein expression so low abundnace
Why it hard to purify the and solubilize the proteins to study
Have to break down lipod bilayer by small detergent addition to remove the protein from membrane
Have to purify the protein but large amount end up denatured so hard to get large enough yield to study
Why is structural analysis of proteins hard
Cant do Xray cyrstallography due to hard to make crystals
Electron microscope have low resolution
NMR in solution need a micellar system and in soli9d state need the protein in a bilayer
How can indirect techniques be used to analyse membrane proteins
Sequence analysis can find potential transmembrane helices byt hard to fidn amphipathic helices
Can also use the location of the post translational modifications
Use labelling methods to find exposed extternal residues and hydrophobic reagent to determine transmembrane residues
What is a hydropathy plot
Each amino acid is assigned a value based on hydrophobicity
Average length or transmembrane helix is 20 a.a
Computer algorithm calcualte and plot hydrophobicity or residues to reveal the potential transmembrane helices
Can be used to determine the protein orientation due to - charge face inwards and + outwards of the bilayer
What us genomic analysis
Good tool for identyfying protein families
Families can be found due to function is reflected in the environment that the cell is in
What are B barrell structures
A less common bilayer spanning structure that form hydrophobic pores in the outer membrane of bacteria and mitochondria
In mitochondria only allow to pass molecules needed for ATP production, that are then transported across the inner membrane
In bacteria carry nutrienst like maltose and phosphate to pass through, they are too leaky for general membranes to use
What is a sucrose specific porin
Porin allows the direct diffusion of sucrose across the outer membrane of bacterium Typhimuriium and is pore is made of 16 B strands of proteins