Post on 15-Mar-2018
Biomembranes,Subcellular Organization, and Membrane Trafficking
Biomembranes
Fundamental structure and function of all cell membranes depends on lipids (phospholipids, steroid derivatives)
Specific function of each membrane depends on Lipid compositionMembrane proteins, present in that specific membrane
Membrane lipids and proteins may be glycosylated
Phospholipid structureExamples of the most frequent types
FA chains can be long, short, and various degrees of branched. Branching makes the sidechains more bulky.
Phospholipid structure
Due to the amphipathic nature of phospholipids, these molecules spontaneously assemble to form closed bilayers
Bilayer structure of biomembranes
In aquous solution phospholipids spontaneously form organized polar structures driven by hydrophobic effect and van der Waals interactions between the FA tails
Electron micrograph
Membrane lipids - sterols
Eucaryotic membranes also contain sterols.
Sterols align with and stabilize the FA sidechains giving membranes ehanced thermo-stability
Cholesterols are precursors for bile acids, steroid hormones, vitamin D, and function as signaling molecules
Fluid mosaic model of biomembranes
3 classes of membrane interacting proteins:
Integral lipid-anchored peripheral
Measuring the dynamics of membranes
Flourescence recovery after photo-bleaching (FRAP) can quantify the lateral movement of proteins and lipids within the plasma membrane
Mobility (diffusion) of a given membrane component depends on:
the size of the moleculeits interactions with other moleculestemperaturelipid composition (tails, cholesterol)
Each closed compartment has two faces
Some organelles are enclosed by double bilayers (=2 membranes)
The two faces of a membrane are asymmetric in terms of lipid and protein composition
Cytosolic=cytoplasmic
Animal cell structure
Functions of the plasma membrane
Regulate transport of nutrients into the cellRegulate transport of waste out of the cellMaintain “proper” chemical conditions in the cell eg pHProvide a site for chemical reactions not likely to occur in an aqueous environmentDetect signals in the extracellular environment Interact with other cells or the extracellular matrix (in multicellular organisms)
Organelles of the eukaryotic cell
LysosomesPeroxisomesMitochondriaChloroplasts (plants)the Endoplasmic Reticulum (ER) the Golgi complexthe Nucleusthe Cytosol
Lysosomes-acidic organelles containing a battery of degradative enzymes
Responsible for degrading certain cell components material internalized from the extracellular environment
Key Featuressingle membranepH of lumen ≅ 5acid hydrolases carry out degradation reactions
Several hundred lysosomes in a single cell
Location of lysosomes and mitochondria in living cell
Tay-sachs disease is caused by a defect in a lysosomal enzyme -> ganglioside glycolipids can not be broken down -> nerve cell dysfunction -> dementia + blindness (infant death)Confocal deconvoluted image of mitochondria (green flourescence stain) and lysosomes (red flourescence stain)
Peroxisomes
Responsible for degrading fatty acids toxic compounds (alcohol ☺)
Key Featuressingle membrane contain oxidases and catalase
FA or ETOH + O2 acetyl groups + H2O2 (ΔG = negative)
2H2O2 2H2O + O2
catalase
Mitochondria
Site of ATP production via aerobic metabolism
Key Featuresouter membrane (very porous)intermembrane spaceinner membrane MatrixHas its own genome!!
The endoplasmic reticulum (ER)
Responsible formost lipid synthesismost membrane protein synthesisCa++ ion storageDetoxification
Key Featuresnetwork of interconnected closed membrane tubules and vesicles composed of smooth and rough regions
The Golgi complex
Modifies and sorts most ER products
Key Featuresseries of flattened compartments & vesiclescomposed of 3 regions: cis (entry), medial, trans (exit)each region contains different set of modifying enzymes
Secretory proteins are synthesized in the ER and pass through the Golgi on the way to the extracellular
environment
mRNA coding secreted proteins contain a signal that target it to the outer ER membrane. Ribosomes then translate the protein directly into the ER
The nucleus
Separates DNA from cytosol transcription from translation
Key Featuresouter membrane inner membranenuclear poresNucleolusCarries the main genome of cells
The cytosol
The portion of the cell enclosed by the plasma membrane but not part of any organelle Key Features
the cytoskeleton polyribosomesmetabolic enzymes
Transmembrane Transport of Ions and Small Molecules
Where are my zebra‐stripes ?
Sorry Mate, you seem to have a
transport problem...
Outline
Overview of membrane transportersClassificationExamples:
Uniport Transport (example: glucose)ATP-powered pumps & intracellular ionic environment (V-class, P-class, and ABC family)Nongated ion channels and the resting membrane potentialCotransport by symporters and antiporters
Multiple transport proteins cooperate
Pure phospholipid bilayers have only limited permeability
Requirements for facilitated flow (transport)
Diffusion rate of a molecule across a lipid bilayer is determined by the molecule’s:
concentration gradient across the lipid bilayerhydrophobicitySizeRelation to the electrical potential across the bilayer (only for charged molecules)
If low diffusion rate transporters required (= facilitated flow)
ΔGFlow < 0 transport will proceed with facilitated bilayer crossing
ΔGFlow > 0 transport must be coupled to energetically favorable process:cotransport with another molecule (the sum of the ΔGFlow for all transported molecules must be
negative)hydrolysis of the energy-rich terminal phosphoanhydride bond of ATP
Overview of membrane transport proteinsTransport proteins are a unique sub-class of integral membrane proteins capable of assisting flow of molecules to which the membrane is non-permissive
Primary active transportPassive transport
Secondary active transport
The 3 main classes of membrane transport proteins!
Transporters, Uniporters
Glucose uniporters; a simple glucose transport mechanism
There are 12 known mammalian glucose transporters with different glucose affinities and tissue specific expressions transporting glucose into mammalian cells
Transporters, UniportersGlucose uniporters; a simple glucose transport mechanism
ATP-powered pumps
The four classes of ATP-powered transport proteins
ATP-powered pumps
Active transport of ions against their concentration gradients and electric potential, driven by specific ATP-powered pumps
Ionic gradients and an electric potential is maintained across the plasma membrane
Represents stored energy that can be used for transmitting electrical signals (nerve cells), facilitating other transports or membrane located molecule synthesis
ATP-powered pumps
P-type: the Calcium ATPase in the Sarcoplasmatic reticulum of muscle cells
A high concentration of Ca2+ is stored in the muscle ER
Ca2+ influx in muscle cell cytosol induces muscle contraction. Muscle relaxation requires an active pump
ATP-powered pumps
P-type: the Na+/K+ ATPase maintains the intracellular Na+ and K+ concentrations in animal cells
1997 jens Skou recieved the Nobel price for discovering the function of this transporter, thereby opening for an understanding and investigation of membrane potentials
Special importance: nerve cells and cardiac electrical impulse (heart beat initiation)
ATP-powered pumps
V-type: the H+ ATPase maintains the acidity of lysosomes and vacuoles
To obtain acidity the electrogenic effect of the H+ transport must be overcome.
This is achieved by concurrent import of negative ions, or export of another positive ion.
ATP-powered pumps
ABC family: Bacterial permeases import nutrients from the environment against concentration gradients.
ATP-powered pumps
ABC family: Transport of a wide variety of substances in eucaryotic cells.
Approximately 50 different mammalian ABC transporters are known
ATP-powered pumps
ABC family: Certain ABC proteins “flip” lipid soluble substrates from one half of the bilayer to the other
Flippase model of transport by the multidrug resistance protein (MDR1)
Ion channels
Can be gated (regulated) or non-gated (always “relaxed”)
Mammalian plasma membranes contain many K+ and a few Na+ channels the membrane has capacitor function with the inside being negative.
Na+/K+ pumps maintain a conc gradient that gives a flow of K+ back through the ion channels resulting in an equilibrium net membrane potential (resting membrane potential = 70mV))
In a few cell types (nerve, muscle) charge fluctuations (impulse) occur when gated ion channels are suddenly opened
Ion channels
Ion flux through individual channels can be calculated from patch clamp tracings
Two patches of a muscle plasma membrane were clamped
The patch electrode contained NaCl
When the Na+ channel was open a flux of 10 million Na+ ions per second was measured.
The electrical depolarization “travelled” the surface of the membrane, reaching the second clamp after 10ms
Ion channels
Novel ion channels can be investigated by a combination of oocyte expression and patch clamping
Transporters, Symporters
Na+ linked symporters import amino acids and glucose into animalcells against high concentration gradients
The favorable negative free energy of Na+ transport down its
concentration gradient “compensates” the symport of glucose
Transporters, Symporters
The 2-Na+/one-glucose symporter mediates glucose uptake from the intestine
Transporters, AntiportersPutative Na+/Ca+ antiporter determines skin color!
When the gene SLC24A5 is mutated the zebrafish looses its stripes and becomes “golden”!
This gene appears to encode a Na+/Ca+ antiporter....
The human parallel is very prevalent in africans, and mutated in europeans...
The antiporter influences the accumulation of melanin in the skin
Further investigation pending!
The human genomes contains hundreds of putative transporters, the function of most of these is unknown.....