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Structure and functions of biological membranes
Kirsi Pakkanen
kiinpakk@bytl.jyu.fi
University of Jyvaskyla25.4.2008
SMBS813 Fundamentals of Nanoscience 2008
What is a membrane?
according to english dictionary: a pliable sheetlike structureacting as a boundary, lining, or partition in an organism.
the basic function of biological membranes is to act as abarrier between outside and inside
membranes and their components have variable functions
protectregulate transport in and out of cell or subcellular structurefunction as a platform for signal transductionallow cell recognitionprovide anchoring sites for cytoskeletal filaments orcomponents of the extracellular matrixcompartmentalize cellsregulate the fusion of the membrane with other membranes inthe cellprovide a passageway across the membrane for certainmolecules – platform for transport complexes
SMBS813 Fundamentals of Nanoscience 2008
What is a membrane?
biological membranes are lipid bilayers
in native biological membranes proteins are embedded into thelipid bilayer
the vast number of different lipids (and proteins) andinteractions between neighboring molecules in the bilayermake biological membranes extremely complex systems
SMBS813 Fundamentals of Nanoscience 2008
Structure of membranes: lipids
origin of the word in Greek: lipos, fat
Three major classes of lipids in biological membrane systems:
(glycero)phospholipidssphingolipidssterols
SMBS813 Fundamentals of Nanoscience 2008
Structure of membranes: lipids
phospholipid general structure
glycerol group, where attachedone phosphate group + headgroupfatty acid tails, typically two
properties of (phospho)lipids arise from their structure: bothheadgroup and fatty acid chains have effects
SMBS813 Fundamentals of Nanoscience 2008
Structure of membranes: lipids
phospholipid nomenclature is based on their headgroup andfatty acidsmain categories of headgroups are
CholineEthanolamineSerineInositol
SMBS813 Fundamentals of Nanoscience 2008
Structure of membranes: fatty acids
Fatty acids are either saturated (no double bonds) orunsaturated (one or more double bonds)saturated fatty acids have straight chains and can therefore bepacked very tightly together
saturated fatty acids have a relatively high melting point
unsaturated fatty acids have bends in their chains caused bydouble bonds
unsaturated fatty acids have typically a low melting point
SMBS813 Fundamentals of Nanoscience 2008
Structure of membranes: fatty acids
SATURATED structure Tm
Lauric CH3(CH2)10COOH +44 ◦CPalmitic CH3(CH2)14COOH +63 ◦CStearic CH3(CH2)16COOH +70 ◦CUNSATURATED structure Tm
Oleic CH3(CH2)7=CH(CH2)7COOH +16 ◦CLinoleic CH3(CH2)4=(CH(CH2))2(CH2)6COOH −5 ◦CArachidonic CH3(CH2)4=(CH(CH2))4(CH2)2COOH +70 ◦C
SMBS813 Fundamentals of Nanoscience 2008
Structure of membranes: fatty acids
(glycero)phospholipid nomenclature is based on theirheadgroup and fatty acids
POPC: P = palmitic acid, O = oleic acid, P= phosphatidylC= cholineDPPC: D= di (2), P = palmitic acid, P= phosphatidyl C=choline
SMBS813 Fundamentals of Nanoscience 2008
Structure of membranes: cholesterol
cholesterol is an important component of native biologicalmembranes
general structure features three six-membered rings attachedto a five-membered ring with a hydrophobic tail
cholesterol belongs to the family of sterol lipids
other sterol lipids include lanosterol and ergosterolthe four fused rings are shared by all steroids, includingestradiol, progesterone, corticosteroids, aldosterone,testosterone, and vitamin D
SMBS813 Fundamentals of Nanoscience 2008
Structure of membranes: building a membrane
lipids want to hide their hydrophobic tails from water
depending on the ratio of headgroup and tail region area,concentration, hydration and temperature, different structuresare formed
SMBS813 Fundamentals of Nanoscience 2008
Properties of membranes: curvature
head-to-tail area ratio of lipids is closely related to curvatureof membranes
SMBS813 Fundamentals of Nanoscience 2008
Properties of membranes: curvature
head-to-tail area ratio of lipids is closely related to curvatureof membranes
lipids with small head and large tails (cone shape) inducenegative curvature to a membrane
e.g. PE
lipids with large head and small tails (inverted cone shape)induce positive curvature to a membrane
e.g. lyso-PC (only 1 FA chain)
SMBS813 Fundamentals of Nanoscience 2008
Functions of membranes: curvature
SMBS813 Fundamentals of Nanoscience 2008
Properties of membranes: packing parameter
(critical) packing parameter describes the ratio between thehydrophobic and hydrophobic parts of the lipids
S =volume of the hydrophopic part
surface of the hydrophilic part× length of the hydrophobic part(1)
for a cone shaped lipid < 1for a cylindrical shaped lipid =1for a inverted cone shaped lipid > 1
for bilayers, S is usually between 0.5 and 1
note! dimensionless
SMBS813 Fundamentals of Nanoscience 2008
Properties of membranes: bending rigidity
despite their inherent ability to have different curvatures,membranes resist bending
free standing vesicles fluctuate spontaniously
these thermal fluctuations are due to the brownian motion ofthe water molecules around the membrane
SMBS813 Fundamentals of Nanoscience 2008
Properties of membranes: bending rigidity
The amplitudes of fluctuations of membranes are dependenton the bending elasticity
When the membrane is easy to bend, the bending elasticmodulus (κ) is low
for example in the case of a thin membrane
when the membrane is thicker it will be much more difficult tobend
κ is higher
SMBS813 Fundamentals of Nanoscience 2008
Methods of membrane studies: GUVs and flicker
diameter 5-100 µm
unilamellar
SMBS813 Fundamentals of Nanoscience 2008
Methods of membrane studies: GUVs and flicker
SMBS813 Fundamentals of Nanoscience 2008
Properties of membranes: phase behavior
membranes can exist in different physical states depending ontheir composition and surrounding environment
above their main phase transition temperature membranes areliquid
SMBS813 Fundamentals of Nanoscience 2008
Properties of membranes: phase behavior
the composition of membranes affects the Tm of themembrane
Tm of fatty acids – Tm of lipids – Tm of membranestransition of a membrane from one state (phase) to anothercan be measured conveniently using calorimetry
transition from gel phase to fluid phase needs energyendothermic
increase in ∆H, increase in excess heat capasity (Cp)
SMBS813 Fundamentals of Nanoscience 2008
Methods of membrane studies: DSC
differential scanning calorimetry measures the difference inheat required to increase the temperature of a sample vs. areferenceEnthalpy (⇒ heat capacity) changes in the sample cause adifference in its temperature
∆H =
∫Cp (2)
SMBS813 Fundamentals of Nanoscience 2008
Properties of membranes: phase behavior
In fluid phase the lipids are in constant motion
based on diffusion lipids sail in the lateral plane of themembranechains twist and turn
video by Emppu Salonen
SMBS813 Fundamentals of Nanoscience 2008
Methods of membrane studies: Molecular dynamicssimulations
Interatomic forces are derived from some classical potentialenergy functions
Classical equations of motion are solved numerically andsimultaneously for all the atoms/particles in the system
Resulting trajectory of the system provides unambiguousinformation about the positions and momenta/velocities of allthe atoms in the system studied
Typical systems at present: 104 - 106 atoms over 10-1000 ns
provides detailed atomic-scale information about the systemstudiedpossible to calculate free energy profiles in the systemclasscial (no QM effects), non-reactive (no bond formation &breaking)limitations in space, time and complexity of the system
SMBS813 Fundamentals of Nanoscience 2008
Properties of membranes: phase behavior
in complex membranes also the issue of phases is morecomplex
cholesterol, in particular, affects the phase properties ofmembranes
broadening of phase transitions (in relation to temperature)slight decrease in main phase transition temperaturesliquid ordered phase
SMBS813 Fundamentals of Nanoscience 2008
Properties of membranes: phase behavior
coexistence of two different liquid (fluid) phases inphysiological temperature is an important feature of biologicalmembranes
similar to the ld phase, the liquid ordered phase (lo) allowsrapid lateral diffusion of lipidsin lo phase the chains of lipids are ordered, whereas in ld theycan move freelymembrane in ld phase is thinner than in lo phase
SMBS813 Fundamentals of Nanoscience 2008
Properties of membranes: domain formation
based on e.g. hydrophobic mismatch, certain lipid don’t mix
they separate as distinct domains on the plane of themembrane
in giant vesicles, these domains can be visualizedmicroscopically
SMBS813 Fundamentals of Nanoscience 2008
Properties of membranes: domain formation
there are however, smaller and more transient domains onmembranes
these are too small to be seen with an optical microscope
SMBS813 Fundamentals of Nanoscience 2008
Methods of membrane studies: EPR
EPR (ESR) can detect unpaired electrons ie. transition metalsand free radicalsfor membranes we need to add ”free electrons” ie. spin labels
typically nitroxides
the magnetic field is scanned (appears as X-axis) since themicrowave frequency cannot be varied (opposite to NMR)
the 1st derivative of the absorption curve is shown as thespectrum
SMBS813 Fundamentals of Nanoscience 2008
Methods of membrane studies: EPR
EPR (ESR) can detect unpaired electrons ie. transition metalsand free radicalsfor membranes we need to add ”free electrons” ie. spin labels
typically nitroxides
the magnetic field is scanned (appears as X-axis) since themicrowave frequency cannot be varied (opposite to NMR)
the 1st derivative of the absorption curve is shown as thespectrum
SMBS813 Fundamentals of Nanoscience 2008
Methods of membrane studies: EPR
EPR gives information on
local order: order parameter, Sz
fluidity: correlation time, τcpolarity
information on local environment ⇒ can detect domains(small ones)
fast time scale < 10−10 sec
SMBS813 Fundamentals of Nanoscience 2008
Functions of membranes: domains
during the past decade membrane domains on cellularmembranes have been a hot topic in membrane science
the raft hypothesis is based on domain formation throughlateral phase separation
cholesterol, sphingomyelin and saturated PCs want to staytogether, not with everyone else
the principles of domain, ”raft”, formation are generallyagreed, yet the details of this phenomenon are still under(vigorous) discussion
SMBS813 Fundamentals of Nanoscience 2008
Functions of membranes: rafts
in literature rafts are considered to be
extractable with cold non-ionic detergents (DRM)float at a light buoyant density on sucrose gradients (LBD)enriched in phosphatidylinositol 4,5-bisphosphate (PtdIns4,5-P2)
note the controversy of this: PtdIns 4,5-P2 acyl chains highlyunsaturated
enriched in GPI-anchored proteins
SMBS813 Fundamentals of Nanoscience 2008
Functions of membranes: rafts
there are however some critical unanswered questionsregarding rafts
size: rafts most likely not big enough to bee seen with opticalmethodslifetime: static structures vs. transient clusters?about 30-50 mol-% of plasma membrane lipids is cholesterol,so the whole PM might be in lo phase
this needs to be fitted into the raft theory
SMBS813 Fundamentals of Nanoscience 2008
Functions of membranes: rafts
raft are linked with crucial cellular functions, such assignallingcaveola
misfolding leading to prion formation has also been linked torafts
rafts also play a role in pathogen entry into cells
SMBS813 Fundamentals of Nanoscience 2008
Membranes in a nutshell
biological membranes are lipid bilayers
main groups of lipids in biological membranes arephospholipids, sphingolipids and cholesterol
in a phospholipid membrane many of the properties of themembrane arise from the fatty acid composition of lipids
lipid membranes are fluid above their Tm
cholesterol is an important part of biological membranes
makes fluid membranes more ordered and solid membranesmore fluid ⇒ liquid ordered phasemakes cellular membranes less permeable ⇒ strengthen thebarrier formed by the membrane
SMBS813 Fundamentals of Nanoscience 2008
Membranes in a nutshell
the phase behavior of membranes leads in certain conditionsto coexistence of two phases
domains are formedin physiological temperature lo/ld coexistence most prominent
in addition to large µm scale domais, smaller transient lipidaggregates can be formed in membranes of certaincomposition
while the concept of lipid rafts on cellular membranes hasbeen established some 10 years ago, there is still debate onseveral points relating to the subject
one of the biggest issues under discussion are the size andlifetime of rafts
SMBS813 Fundamentals of Nanoscience 2008
Acknowledgments
For the preparation of this lecture I have received help,material and advice from:
Lars Duelund, MEMPHYS - Center for Biomembrane Physics,University of Southern Denmark, Odense, DenmarkHelene Bouvrais, MEMPHYS - Center for BiomembranePhysics, University of Southern Denmark, Odense, DenmarkJohn H. Ipsen, MEMPHYS - Center for Biomembrane Physics,University of Southern Denmark, Odense, DenmarkEmppu Salonen, Laboratory of Physics and Helsinki Instituteof Physics, Helsinki University of Technology
SMBS813 Fundamentals of Nanoscience 2008
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