Structure and functions of biological membranesusers.jyu.fi/~mmannine/BasicNanoSci/250408handoutPakkanen.pdf · biological membranes are lipid bilayers main groups of lipids in biological

Structure and functions of biological membranes

Kirsi Pakkanen

[email protected]

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


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


Structure of membranes: lipids

origin of the word in Greek: lipos, fat

Three major classes of lipids in biological membrane systems:

(glycero)phospholipidssphingolipidssterols



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



phospholipid nomenclature is based on their headgroup andfatty acidsmain categories of headgroups are

CholineEthanolamineSerineInositol


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



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



(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


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


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


Properties of membranes: curvature

head-to-tail area ratio of lipids is closely related to curvatureof membranes


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)


Functions of membranes: curvature


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


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


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


Methods of membrane studies: GUVs and flicker

diameter 5-100 µm

unilamellar


Methods of membrane studies: GUVs and flicker


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



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)


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)



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


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



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



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


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


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


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



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



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


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


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



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



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


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


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


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


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Structure and functions of biological membranesusers.jyu.fi/~mmannine/BasicNanoSci/250408handoutPakkanen.pdf · biological membranes are lipid bilayers main groups of lipids in biological