Transcript Lipids and Membranes

Lipids and Membranes
Functions of Lipids
• Energy reserves (particularly fatty acids, lipids
with long hydrocarbon chains) - There is a large
energy yield upon oxidation of these highly
reduced hydrocarbons.
• As lipid bilayers, main components of biological
membranes.
• Intra- and intercellular signaling.
Structures of Ionized Form of Some
Representative Fatty Acids
pKa ≈ 4.5
Rigid bend (~30º) in hydrocarbon chain of oleic acid/oleate
due to presence of cis double bond.
Some Fatty Acids
•Saturated: no double bonds
•Unsaturated: one
(monounsaturated) or more
(polyunsaturated) double bonds
(almost always cis configuration)
18:2c∆9,12
18:0
18:1c∆9
18:3c∆9,12,15
Some Biologically Important Fatty Acids
Most fatty acids have an even number of carbons because they are synthesized by
concatenation of activated two-carbon units (acetyl-CoA).
Triacylglycerols: Fats
OH
HO
OH
Glycerol
•Major energy reservoir
•Very efficient way to store
metabolic energy (less
oxidized than carbohydrates or
proteins)
Soaps and Detergents
•Hydrolysis of fats with alkali such as NaOH or KOH yields soaps
(saponification), salts of ionized fatty acids.
•Synthetic detergents:
Sodium dodecyl sulfate
Triton X-100
Waxes
•Formed through esterification of fatty acid and
long-chain alcohol
•Completely water-insoluble
•Water-repellent protective coating in some
animals and plants
•Energy storage in some microorganisms
Glycerophospholipids (Phosphoglycerides):
Main Lipid Components of Biological
Membranes
Naturally occuring
glycerophospholipids have L
stereochemistry.
Glycerophospholipid Structure
Hydrophilic
“head” group
Kink or bend in
one fatty acyl
chain in this
phospholipid
because of cis
double bond
Hydrophobic hydrocarbon “tails” = fatty acidderived side chains = acyl chains
The Hydrophilic Head Groups of Major
Glycerophospholipids
Phospholipids and Membrane Structure
Micelle
Bilayer
Phospholipid Hydrolysis by
Phospholipases
Phospholipase A2 Bound to a
Phospholipid
Phosphatidylinositol-4,5-Bisphosphate (PIP2)
Hydrolysis and Signal Transduction
Along with Ca2+
(released from ER
as described below),
DG activates protein
kinase C.
IP3 binds to Ca2+
channels on ER
membrane,
causing them to
open and release
of Ca2+ into
cytoplasm.
DG, IP3, Ca2+ are examples of “second
messengers” that transmit signals inside the
cell, leading to cellular response.
The PIP2 Hydrolysis Pathway
Lipid Composition of Some Biological
Membranes
Sphingolipids: Another Major Class of
Lipids Found in Biological Membranes
Black = sphingosine
Black + red = a ceramide
Black + red + blue = a
sphingomyelin
Ceramide
Sphingosine = amino alcohol
with a long hydrocarbon chain.
Ceramide = sphingosine with a
fatty acid linked by an amide
bond to the amine to form an Nacyl chain.
Sphingomyelin = ceramides with
a phosphocholine head group.
The myelin sheath that surrounds and
electrically insulates many nerve cell axons is
rich in sphingomyelin.
Glycosphingolipids
Cerebrosides = ceramides with a
single sugar residue as head
group.
Gangliosides = ceramides with
attached oligosaccharide as head
group containing at least one sialic
acid residue.
Gangliosides
Gangliosides constitute a significant fraction (~6%) of brain lipids. The
ABO blood group antigens are also examples of gangliosides.
Cholesterol: The Third Major Class of
Lipid in Biological Membranes
Cholesterol Biosynthesis
Activated, five-carbon “isoprene units”
Cholesterol is just one of many isoprenoids (or terpenes), lipids
derived from isoprene units, which includes other steroids and
non-steroidal lipids, such as bile acids, lipid-soluble vitamins,
certain coenzymes, etc.
Cholesterol Is the Metabolic Precursor of
Steroid Hormones
Vitamins D Are Sterol Derivatives
HO
HO
Vitamin D3
Vitamin D2
An Example of Other Types of Lipids: The
Eiconasoids
Prostaglandins
Lipid Bilayers and Biological Membranes
Structure of Phospholipid Bilayer
The Gel-Liquid Crystalline Transition in a Lipid
Bilayer and Factors Affecting the Transition
Degree of Unsaturation of
Fatty Acid Side Chains
•Presence of phospholipids
with unsaturated fatty acyl
chains reduces transition
temperature, making
membrane more “fluid.”
•Bend produced by cis double
bonds prevents close packing
of side chains at lower
temperature.
Presence of Cholesterol
•Moderate concentrations of
cholesterol broaden transition,
making membrane appear more
fluid at lower temperatures yet
less fluid at higher
temperatures.
•Bulky, rigid sterol ring structure
of cholesterol prevents tight
packing of phospholipid acyl
chains at low temperatures.
•However, the rigid ring
structure also reduces mobility
of phospholipid side chains at
higher temperatures.
The Gel-Liquid Crystalline Transition in a
Lipid Bilayer and Temperature
A Model of the Effects of Cholesterol on
Plasma Membrane Structure
Experimental Demonstration of Biological
Membrane Fluidity
Diffusion of Lipids in Bilayers
Translocases or flippases:
protein catalysts that facilitate
transverse diffusion (flip-flop)
of lipids in biological
membranes.
Phospholipid Asymmetry in Plasma
Membranes
Erythrocyte membrane
New Lipids Inserted into Inner Leaflet of
Membrane
Orange = newly
synthesized,
radioactive lipids
TNBS =
trinitrobenzenesulfonic acid
(TNBS), cell-impermeant
reagent that reacts with
phosphatidylethanolamine
Flip-flop rate in biological
membrane ~100,000 faster
than in artificial lipid bilayer,
demonstrating efficiency of
translocases.
Structure of a Typical Cell Membrane
Fluid Mosaic Model (Singer
and Nicholson, 1972).
Protein, Lipid and Carbohydrate
Compositions of Some Membranes
Membrane-Bound Proteins
•Integral membrane proteins - span lipid bilayer; can only be removed from membrane with strong
treatments such as detergents or organic solvents.
•Lipid-linked proteins - interact with membrane via post-translationally attached lipid moeity.
•Peripheral membrane proteins - weakly associated with membrane; can be dissociated with mild
treatments such as high ionic strength salt solutions or pH changes.
Example of a Lipid Attachment in a LipidLinked Protein
Glycophosphosphatidylinositol
(GPI) anchor of GPI-linked
proteins
Other types of lipid-linked proteins:
•Prenylated = lipid attachment (commonly C15 or C20) built from
isoprene (C5) units
•Fatty acylated = lipid attachment is fatty acid
Protein Prenylation
Model of the Structure of the Erythrocyte
Membrane Skeleton
Major Proteins of the Human Erythrocyte
Membrane
Glycophorin A Polypeptide Has a Single
Membrane-Spanning a-Helix
Extracellular domain
Intracellular domain
Transmembrane
domain
Hydropathy/Hydrophobicity Plots
Glycophorin A
Erythrocyte
glucose
transporter
Bacteriorhodopsin
Bacteriorhodopsin: A Protein with
Multiple Membrane-Spanning a-Helices
Another Multiple-Pass Transmembrane
Protein: The Photosynthetic Reaction
Center from a Purple Bacterium
E. coli OmpF Porin: Transmembrane b Barrels
Vesicle Trafficking and Biosynthesis of
Transmembrane and Secreted Proteins in
Eukaryotes
Transport Across Membranes
Thermodynamics of Transport
• Free energy change (chemical potential difference) for transporting
1 mole of a substance from region where its concentration is C1
(e.g., Cout) to region where its concentration is C2 (e.g., Cin):
∆G = RT ln(C2/C1)
(favorable with ∆G < 0 if C2 < C1)
• Transport of ions across membrane (must consider electrical
potential in addition to concentration difference):
∆G = RT ln(C2/C1) + ZF ∆
(Z=charge of ion, F=Faraday’s constant, ∆=membrane
electrical potential in volts)
• Coupled transport (active transport):
∆G = RT ln(C2/C1) + ∆G´
(∆G´ of coupled process, such as ATP hydrolysis, may be
negative enough to compensate for unfavorable transport
concentration gradient when RT ln (C2/C1) > 0)
against
Specific Transport Processes
Modes of Transport of Substances Across
Membranes
• Diffusional transport: movement of substance
from high to low concentration across membrane
(down concentration gradient)
– Non-facilitated diffusion across lipid bilayer (slow for
most biological substances)
– Facilitated diffusion (accelerated diffusion by making
membrane more permeable to specific transported
substance, e.g., channels and carriers)
• Active transport: Actively driven (generally
directly or indirectly coupled to ATP hydrolysis)
transport against concentration gradient from low
to high concentration across membrane (e.g.,
pumps)
Types of Transport Systems
Movement of single
molecule at a time
Simultaneous transport of
two different molecules in
same direction
Simultaneous transport of
two different molecules in
opposite directions
Facilitated Diffusion
(Facilitated or Mediated
Transport)
Facilitated and Non-Facilitated Diffusional
Transport
Saturable
Non-saturable
Two Major Mechanisms for Facilitated
Diffusion
The Pore Structure of the Potassium
Channel
Scorpion toxin
K+ channel
Model for Glucose Transport
The Hemolysin Toxin from
Staphylococcus aureus:
A Channel-Forming Ionophore
Gramicidin A: Another Channel-Forming
Ionophore
Valinomycin: An Antibiotic that Acts as an
Ion Carrier (Carrier Ionophore)
ATP-Driven Active Transport
Model for a Subunit of the Na+/K+ ATPase
Schematic Model of the Functional Cycle
of the Na+/K+ ATPase
Ca+ ATPase
Ion Gradient-Driven Active Transport
Na+/Glucose Cotransport (Symport)
System
Schematic Model for the Na+/Glucose
Cotransport System
H+/Lactose Cotransport by Lactose
Permease
Electrically Excitable Membranes and
Nerve Impulse Transmission
Structure of a Typical Mammalian Motor
Neuron
Use of Squid Giant Axons for Studies of
Neural Transmission
Membrane Potential
•Nernst equation (here for MZ=ion of charge Z)
∆ = RT/ZF ln([MZ]out/[MZ]in)
(∆=membrane potential in volts, Z=charge of ion,
F=Faraday’s constant)
•Goldman equation (takes into account multiple ions and
different permeabilities of membrane to each ion)
∆ = RT/F ln((+ Pi[Mi+]out + - Pj[Xj-]in)/
(+ Pi[Mi+]in + - Pj[Xj-]out))
(+=sum of all cations involved, -= sum of all anions involved
P’s=relative permeabilities to cations and anions involved)
The Action Potential
Voltage-gated Na+
channel
The Action Potential
Transmission of the Action Potential
How an Axon Is Myelinated
Techniques for the Study of Membranes
Freeze Fracture
Freeze Fracture
Preparation of Vesicles and Bilayers
Reconstitution of the Ca2+ Pump
Preparation and Resealing of Erythrocyte
Ghosts
Differential Scanning Calorimetry
Fluorescence Photobleaching Recovery