Biological Membranes

• 20% human genome codes for membrane proteins
• Important interface of cell
• Dictate how cell reacts with environment
• Important drug targets
• Faulty membrane proteins cause:
  • Cystic fibrosis
  • Malignant hypothermia

• Internal organelles bounded by membranes
• Membranes invisible to light microscopes (okay with electron)
• Two major components:
  • Lipids -small non-polar soluble molecules = permeability barrier
  • Proteins = biological properties e.g. transport, signalling, adhesion

• Glycerol-based molecules e.g. phosphatidycholine

• Sphinogosine-base molecules e.g. sphingomyelin
• Also contain sterols e.g. cholesterol 

• Ampipathic - contain polar and non-polar groups
• In an aq. environment hydrophobic effect occurs:
  • Driven by water 
  • Hydrophobic mol. interfere with water's hydrogen bonds
  • Water will squeeze lipids together to minimise interactions of hydrophobic areas to water
  • Spontaneous formation of lipid bilayers (act as permeability barriers for polar/large mols) or self-sealing vesicles in water 
• Membranes not spontaneously produced - only build from inserting lipids into new bilayers
• Enzymes move phospholipids to either side - flippases (to cytoplasmic side) and floppases (to extracellular fluid) = transverse diffusion

• Membranes have lots of lipids with acy chains containing cis-double bonds
• Cis-bonds prevent close acyl chain packing = motile
• Liquid crystaline phase = lipids can move around in bilayer (low melting temperatures)
• This is called lateral diffusion - same side
• Essential for function at low temperatures 
• Low low temperatures = gel phase and acyl chains frozen - lipids cannot move
• Membrane proteins must undergo conformational changes and interact with adjacent proteins

• Provide control for the internal cell/organelle environment
• Transporters, channels, receptors
• All can be transmembranous
• a-helical conformation (stabilised by H bonds) means they can cross hydrophobic core
• Transmembrane domain residues are hydrophobic
• End residues are polar
• R-groups project outwards = anchorage in bilyaer
• Single helix cannot form route for polar solutes as they are too big/polar
• Amphipathic helicases (~4) cluster w/ polar resiudes on inside = route

• Makes membrane selective
• Charge at beginning of channel = only opposite charge attracted
• Gate required to stop flow of ions (depending on electrochemical gradient)
• Helicases rotate to expose small polar groups and open channel
• When gate closed hydrophobic groups on inside = no polar molecules
• Ligand binds to cause helicases to rotate
• Cannot
  • Move ions/mols against concentration gradient
  • Facilitate large molecules (ions would sneak through, disrupting conc. gradients)

• 2 gates - neither open at same time = no major change in ion gradients
• Solute recognition site - solute binds causing 1st gate to shut and 2nd to open

• Transporter undergoes conformational change e.g. rocking
• Molecule binds tightly in either direction
• No net transport after equilibrium 

• Against a concentration gradient
• Molcule needs to bind tightly then let go at end
• Requires ATP to overcome attraction force = pull molecule and channel apart

• Do not readily form 3D structures = few been mapped
• Difficult to disrupt bilayer and isolate proteins
• Most models based on sequence analysis and indirect studies
• You need to form protein crystals and fire x-ray beans to be scattered = diffraction pattern
• Must trap proteins in various conformations = flip-book method

Labelling Studies

• Identify surface exposed proteins using membrane impermeant reagents - covalent modifers modifying specific residues on outside = analyse sequence
• Identify transmembrane reagents using hydrophobic labelling reagents
  • Must first break membrane to allow reagent to dissolve in bilayer 
  • Use detergent = collapse and micelles form
  • Proteins react with detergent = protected in shape
  • Can be seperated and purified

• Reveal potential transmembrane helices 
• Amphipathic helicases hard - polar amino acids break up line of non-polar ones
• Hydropathy Plots:
  • Each amino acid assigned a value for its hydrophobicity (energy required to move residue from aqueous to inside bilayer)
  • Higher value = more hydrophobic 
  • Average length of transmembrane helix 20 residues -> look for 20 consecutive hydrophobic amino acids
  • Computer plots = potential transmembrane helicases in peaks
  • No idea of orientation of proteins 
  • Combo of this and labelling studies required

• Glycophorin A etc. are glycosylated on extra cellular surface
• Precise pattern depends on battery of inherited glycosylation enzymes of an individiual
• Gives rise to ABO blood group system

• Less common
• Formed of antiparralell B-sheets -> barrel structure
• Often got alternating hydrophobic and philic residues (polar inside and non-polar outside = hydrophilic pore/porin
• Not found in plasma membrane - ion concentration gradient issues
• Found in mitochondria
  • Porous membrane preserves intermembrane space = required for mitochondria function
• Found in outer membranes of gram negative bacteria e.g. E.coli
  • Live in hostile environmnets with protases
  • Protects the inner layer which contains transporters
  • Outer layer allows mols. to enter periplamic space down conc gradient
  • Prevention of proteases entering