Membrane composition Flashcards
flippases
Move phospholipids from outer to inner leaflet of bilayer
Requires ATP
Highly specific
floppases
Move phospholipids from inner to outer leaflet of bilayer
Requires ATP
Specific
scramblases
Move phospholipids in both directions
No ATP needed
Non-specific
factors affecting membrane fluidity
Temperature
Acyl chain length
Acyl chain saturation
Effect of temperature on membrane fluidity
cold=rigid/less fluid
so at cold temperatures, an organism would have more unsaturated fatty acids
effect of acyl chain length on membrane fluidity
shorter chains increase fluidity as they have fewer van der waals forces so are less tightly packed
effect of acyl chain saturation on membrane fluidity
Unsaturated fatty acids form kinks so cant pack closely together so increase fluidity
is the membrane symmetrical or asymmetrical
asymmetrical
why do hydrophobic molecules clump together
reduce SA:V in contact with water
lipid movement within membranes
Lateral diffusion
Flip-flop (not spontaneous)
Rotation
Flexion
how is membrane fluidity measured
FRAAP (fluorescence recovery after photobleaching)
protein of interest is labelled with a fluorescent marker, and the fluorescence in a small patch of membrane is then irreversibly “bleached” by a pulse of light from a focused laser. The time it takes for fluorescence to return to the bleached membrane patch provides a measure of how rapidly unbleached, fluorescently labelled proteins diffuse through the bilayer into the area. This “recovery” is plotted on a curve that shows fluorescence over time.
noncytosolic monolayer
phosphatidylcholine
sphingomyelin
Glycolipids
cholesterol (= to cytolsolic)
cytosolic monolayer
phosphatidylserine
phosphatidylethanolamine
phosphatidylinositols
cholesterol (= to non cytosolic)
types of membrane protein
Integral
Peripheral
Lipid-anchored
Peripheral membrane proteins
Bind to surface by electrostatic interactions with the heads of lipids or other proteins
Lipid-linked membrane protein
lipid covalently attached to protein
nonpolar aliphatic R groups
glycine
alanine
valine
leucine
methionine
isoleucine
aromatic R groups
phenylalanine
tyrosine
tryptophan
polar, uncharged R groups
serine
threonine
cysteine
proline
asparagine
glutamine
charged R groups (5)
lysine
arginine
histidine
aspartate
glutamate
The polar nature of peptide bonds are not energetically favourable in the hydrophobic core of the lipid bilayer. How is this problem overcome
Hydrogen bonding between the partial negative charge in the carbonyl oxygen and the partial positive charge of the amide hydrogen in a regular pattern
Neutralises charges
Length of chain compressed due to the hydrogen bonding- alpha helix formation
This involves the backbone-not side groups
beta barrel integral membrane protein
all carbonyl oxygens and amide hydrogens are also bonded in a regular pattern, but the hydrogen bonding patterns are further away. Tend to be found in mitochondrial outer membrane and bacterial outer membranes.
helical bundle membrane proteins
the integral proteins that form alpha helices in the membrane
antiparallel beta sheet
is formed with adjacent -strands running in opposite directions. Every other side chain extends above or below the sheet
Hydrogen bonding between strands
dimensions of lipid bilayer
50 angstroms
30 of which are the hydrophobic region
extended conformation, 1 residue=
3.5 angstrom
alpha helical conformation, 1 residue=
1.5 angstrom
20 residues to span hydrophobic core
hydrophobicity scale
negative=hydrophobic amino acid
Average hydrophobicity of a stretch of amino acids is calculated, plotted on graph
When window is over a hydrophobic stretch of amino acids there is a trough
troughs indicate transmembrane regions
Hydrophobicity analysis does not predict beta-barrel membrane proteins.
how to remove integral membrane proteins from the lipid bilayer
detergents
how to remove peripheral membrane proteins that are attached to protein
high salt concentrations
protein channels structure-multipass transmembrane proteins
series of alpha helices that cross bilayer multiple times
hydrophilic side chains form an aqueous pore
