Transcript Lipidler - mustafaaltinisik.org.uk

Lipids and Membranes
Lipids
• Lipids are compounds that are
soluble in non-polar organic
solvents, but insoluble in water.
• Can be hydrophobic or amphipathic
Major Lipid Classes
• Acyl-lipids - contain fatty acid
groups as main non-polar group
• Isoprenoids – made up of 5
carbon isoprene units
Lipid Subclasses
Function of major acyllipids
•
•
•
•
Phospholipids – membrane components
Triacylglycerols – storage fats and oils
Waxes – moisture barrier
Eicosanoids – signaling molecules
(prostaglandin)
• Sphingomyelins – membrane component
(impt. in mylein sheaths)
• Glycospingolipids – cell recognition (ABO
blood group antigen)
Function of major
isoprenoid lipids
•
•
•
•
•
Steroids (sterols) – membrane component, hormones
Lipid Vitamins – Vitamin A, E, K
Carotenoids - photosynthetic accessory pigments
Chlorophyll – major light harvesting pigment
Plastoquinone/ubiquinone – lipid soluble electron
carriers
• Essential oils – menthol
Fatty acids
•
•
•
•
•
•
•
•
Amphipathic molecule
Polar carboxyl group
Non-polar hydrocarbon tail
Diverse structures (>100
different types)
Differ in chain length
Differ in degree of unsaturation
Differ in the position of double
bonds
Can contain oxygenated groups
Fatty acid nomenclature
• Short hand nomenclature describes total number
of carbons, number of double bonds and the
position of the double bond(s) in the
hydrocarbon tail.
C18:1 D9 = oleic acid, 18 carbon fatty acid with a
double bond positioned at the ninth carbon
counting from and including the carboxyl carbon
(between carbons 9 and 10)
O
C1
HO
C6
C4
C2
C3
C5
C7
C9
C11
C13
C18
C16
C14
C12
C10
C8
C15
C17
Fatty acid nomenclature
• Omega (w) notation – counts carbons from end of
hydrocarbon chain.
• Omega 3 fatty acids advertised as health promoting
• Linoleate = 18:3 D9,12,15 and 18:3w3,6,9
O
C1
HO
C3
C8
C6
C4
C2
C5
C7
C9
C11
C13
C18
C16
C14
C12
C10
C
H15
C17
Common saturated fatty
acids
common name
IUPAC name
melting point (Co)
12:0
laurate
dodeconoate
44
14:0
myristate
tetradeconoate
52
16:0
palmitate
hexadeconoate
63
18:0
stearate
octadeconoate
70
20:0
arachidate
eicosanoate
75
22:0
behenate
docosanoate
81
24:0
lignocerate
tetracosanate
84
Common unsaturated fatty
acids
common name
IUPAC name
melting
point
(Co)
16:0
palmitate
hexadeconoate
63
16:1 D9
palmitoleate
cis-D9-hexadeconoate
-0.5
18:0
stearate
octadeconoate
70
18:1 D9
oleate
cis-D9- octadeconoate
13
18:2 D9,12
linoleate
cis-D9,12- octadeconoate
-9
18:3 D9,12,15
linolenate
cis-D9,12,15- octadeconoate
-17
20:0
arachidate
eicosanoate
75
20:4 D5,8,11,14
arachindonate
cis- D5,8,11,14-eicosatetraenoate
-49
Physical Properties of Fatty acids
• Saturated chains pack
tightly and form more
rigid, organized aggregates
• Unsaturated chains bend
and pack in a less ordered
way, with greater potential
for motion
18:0
70o
18:1
13o
18:3
-17o
Melting points of fatty acids
affect properties of acyl-lipids
• Membrane fluidity determined by temperature and
the degree of fatty acid unsaturation of
phospholipids
• Certain bacteria can modulate fatty acid
unsaturation in response to temperature
• Difference between fats and oils
• Cocoa butter – perfect melt in your mouth fat
made of triacylglycerol with 18:0-18:1-18:0 fatty
acids
• Margarine is hydrogenated vegetable oil. Increase
saturation of fatty acids. Introduces trans double
bonds (thought to be harmful)
Unusual fatty acids can function
analogously to unsaturated fatty acids
Major acyl-lipids
•Phospholipids – membrane components
•Triacylglycerols – storage fats and oils
•Waxes – moisture barrier
•Eicosanoids – signaling molecules
(prostaglandin)
•Sphingomyelins – membrane component (impt.
in mylein sheaths)
•Glycospingolipids – cell recognition (ABO blood
group antigen)
Phospholipids
• Phospholipids are built on glycerol back bone.
• Two fatty acid groups are attached through
ester linkages to carbons one and two of
glycerol.
• Unsaturated fatty acid often attached to
carbon 2
• A phosphate group is attached to carbon three
• A polar head group is attached to the
phosphate (designated as X in figure)
Common membrane phospholipids
CH3
O
O
H2C
O
O
C
C
R1
R2
Phophtidate
N
NH3
CH2
CH2
H
CH2
CH2
CH2
O
O
O
O
P
O
O
CH2
O
P
O
O
O
H2C
H
C
O
O
C
C
R1
R2
CH2
O
Phophatidylethanolamine
P
O
H2C
H
C
O
O
C
C
O
O
R1
R2
Phophatidylserine
P
O
CH2
O
CH3
CH2
COO
O
O
O
H
C
H3C
NH3
O
H2C
H
C
O
O
C
C
R1
R2
CH2
O
Phophatidylcholine
O
Enzymes used to Dissect
Phospholipid Structure
X
O
phospholipase D
O
P
O
phospholipase C
O
H2C
H
C
O
O
CH2
phospholipase A1
phospholipase A2
O
C
C
R1
R2
O
Plasmalogens
NH3
CH2
•Plasmalogens have hydrocarbon at
carbon 1 attached thru vinyl ether
linkage
CH2
O
O
•Polar head group could be
ethanolamine or choline
•Important component of membranes
in central nrevous system
P
O
H2 C
H
C
O
O
CH
C
CH2
O
HC H C
2
R1
R2
O
OH
Sphingolipids
• Sphingolipids named from Sphinx due
to mysterious role
• Abundant in eukaryotic membranes,
but not found in bacteria
• Structural backbone made of
sphingosine
• Unbranched 18 carbon alcohol with a
trans double bond between C4 and C5
• Contains an amino group attached to
C2 and hydroxyl groups on C1 and C3
H
OH
C1
C2
C3
H
H
NH3 C
4
HC
C6
C7
C8
C9
C10
C11
C12
C13
C14
C13
C16
C17
C18
OH
H
Ceramides
O
OH
C1
C2
H
NH
C3
H
C4
C
HC
C2
• Sphingosine with
fatty acid attached
to carbon 2 by amide
linkage
• Metabolic precursors
to sphingolipids
C6
C3
C7
C4
C5
C8
C9
C6
C7
C10
C11
C8
C9
C14
C13
C10
C11
C14
C15
C12
C13
C14
C15
C16
C17
C18
C16
C17
C18
Sphingomyelin
• has phosphocholine group
attached to C1 of ceramide.
• Resembles phosphatidylcholine
• Major component of myelin
sheaths that surround nerve cells
CH3
H3C
N
CH3
CH2
CH2
O
O
P
O
O
H
O
OH
C1
C2
H
NH
C3
H
C4
C
HC
R1
CH2
C8
12
Cerebrosides
• contains one monosaccharide
residue attached to C-1 of
ceramide
• Glucose and galtactose are
common
• Can have up to 3 more
monosaccharide residues
attached to sugar on C1
• Abundant in nerve tissue
• Up to 15% of myelin sheath
made up of cerebrosides
H
CH2OH
OH
O

O
OH
O
OH
-D-galactose
OH
C1
C2
H
NH
C3
H
C4
C
HC
R1
CH2
C8
12
Gangliosides
• Gangliosides have oliosaccharide containing Nacetylneuraminic acid attached to C1 of ceramide
• Diverse class of sphingolipid due to variety of
olgosaccharide species attached
• Oligosaccharide moiety present on extracellular
surface of membranes
• ABO blood group antigens are gangliosides
• Impt in cell recognition, cell-cell communication
Defects in sphingolipid
metabolism lead to disease state
• Tay-Saachs disease is a genetic defect in
gangliosides degradation. Gangliosides
accumulate in spleen and brain. Leads to
retardation in development, paralysis, blindness,
and early death.
• Niemann-Pick disease is a genetic defect in
sphingomyelin degradation. Causes Sphingomyelin
accumulation in brain, spleen and liver. Causes
mental retardation. Children die by age 3 or 4.
Triacylglycerols (TAG)
H2C
H
C
CH2
O
O
O
C
O
O
C1
C2
C
C2
C2
C3
• Fats and oils
• Impt source of metabolic fuels
• Because more reduced than carbos,
oxidation of TAG yields more energy
(16 kJ/g carbo vs. 37 kJ/g TAG)
• Americans obtain between 20 and
30% of their calories from fats and
oils. 70% of these calories come
from vegetable oils
• Insulation – subcutaneous fat is an
important thermo insulator for
marine mammals
C3
C3
C4
C4
C4
C5
C5
C5
C6
C6
C6
C7
C7
C7
C8
C8
C8
C9
C9
C9
C10
C10
C10
C11
C11
C11
C12
C12
C12
C13
C13
C13
C14
C14
C14
C15
C16
C17
C18
C15
C15
C16
C17
C18
O
C16
C17
C18
Olestra
•Olestra is sucrose with fatty
acids esterified to –OH groups
•digestive enzymes cannot cleave
fatty acid groups from sucrose
backbone
•Problem with Olestra is that it
leaches fat soluble vitamins from
the body
isoprenoids
• Isoprenoids are derived from the condensation of 5
carbon isoprene units
• Can combine head to head or head to tail
• Form molecules of 2 to >20 isoprene units
• Form large array of different structures
Terprenes
Steroids
• Based on a core structure consisting of three
6-membered rings and one 5-membered ring, all
fused together
• Triterpenes – 30 carbons
• Cholesterol is the most common steroid in
animals and precursor for all other steroids in
animals
• Steroid hormones serve many functions in
animals - including salt balance, metabolic
function and sexual function
cholesterol
• Cholesterol impt membrane
component
• Only synthesized by animals
• Accumulates in lipid deposits on
walls of blood vessels – plaques
• Plaque formation linked to
cardiovascular disease
Steroids
Many steroids are derived
from cholesterol
Membranes
•
•
•
•
•
Barrier to toxic molecules
Help accumulate nutrients
Carry out energy transduction
Facilitate cell motion
Modulate signal transduction
• Mediate cell-cell interactions
The Fluid Mosaic Model
• The phospholipid bilayer is a fluid matrix
• The bilayer is a two-dimensional solvent
• Lipids and proteins can undergo
rotational and lateral movement
• Two classes of proteins:
– peripheral proteins (extrinsic proteins)
– integral proteins (intrinsic proteins)
The Fluid Mosaic Model
Motion in the bilayer
• Lipid chains can bend, tilt and rotate
• Lipids and proteins can migrate ("diffuse") in
the bilayer
• Frye and Edidin proved this (for proteins), using
fluorescent-labelled antibodies
• Lipid diffusion has been demonstrated by NMR
and EPR (electron paramagnetic resonance) and
also by fluorescence measurements
• Diffusion of lipids between lipid monolayers is
difficult.
fusion
After 40 minutes
Flippases
• Lipids can be moved from one
monolayer to the other by flippase
proteins
• Some flippases operate passively and do
not require an energy source
• Other flippases appear to operate
actively and require the energy of
hydrolysis of ATP
• Active flippases can generate membrane
asymmetries
Membranes are Asymmetric
In most cell membranes, the composition of
the outer monolayer is quite different from
that of the inner monolayer
Membrane Phase Transitions
• Below a certain transition temperature,
membrane lipids are rigid and tightly
packed
• Above the transition temperature, lipids
are more flexible and mobile
• The transition temperature is
characteristic of the lipids in the
membrane
Phase Transitions
• Only pure lipid
systems give
sharp, welldefined
transition
temperatures
• Red = pure
phospholipid
• Blue =
phopholipid +
cholesterol
Structure of Membrane
Proteins
• Integral (intrinsic) proteins
• Peripheral (extrinsic) proteins
• Lipid-anchored proteins
Peripheral Proteins
• Peripheral proteins are not strongly
bound to the membrane
• They can be dissociated with mild
detergent treatment or with high
salt concentrations
Integral Membrane
Proteins
• Integral proteins are strongly imbedded
in the bilayer
• They can only be removed from the
membrane by denaturing the membrane
(organic solvents, or strong detergents)
• Often transmembrane but not necessarily
• Glycophorin, bacteriorhodopsin are
examples
Seven membrane-spanning alpha
helices, connected by loops,
form a bundle that spans the
bilayer in bacteriorhodopsin.
The light harvesting prosthetic
group is shown in yellow.
Bacteriorhodopsin has loops at
both the inner and outer
surface of the membrane.
It displays a common membraneprotein motif in that it uses
alpha helices to span the
membrane.
Lipid-Anchored Proteins
• Four types have been found:
– Amide-linked myristoyl anchors
– Thioester-linked fatty acyl
anchors
– Thioether-linked prenyl anchors
– Glycosyl phosphatidylinositol
anchors
Amide-Linked Myristoyl
Anchors
• Always myristic acid
• Always N-terminal
• Always a Gly residue that links
Thioester/ester-linked
Acyl Anchors
• Broader specificity for lipids myristate, palmitate, stearate,
oleate all found
• Broader specificity for amino acid
links - Cys, Ser, Thr all found
Thioether-linked Prenyl
Anchors
• Prenylation refers to linking of
"isoprene"-based groups
• Always Cys of CAAX (C=Cys,
A=Aliphatic, X=any residue)
• Isoprene groups include farnesyl (15carbon, three double bond) and
geranylgeranyl (20-carbon, four double
bond) groups
Glycosyl Phosphatidylinositol
Anchors
• GPI anchors are more elaborate than
others
• Always attached to a C-terminal
residue
• Ethanolamine link to an
oligosaccharide linked in turn to
inositol of PI
Membrane transport
• Membranes are selectively permeable
barriers
• Hydrophobic uncharged small molecules
can freely diffuse across membranes.
• Membranes are impermeable to polar and
charged molecules.
• Polar and charged molecules require
transport proteins to cross membranes
(translocators, permeases, carriers)
Transport of non-polar
molecules
• Non-polar gases, lipids, drugs etc…
• Enter and leave cells through diffusion.
• Move from side with high concentration
to side of lower concentration.
• Diffusion depends on concentration
gradient.
• Diffusion down concentration gradient is
spontaneous process (-DG).
Transport of polar or charged
compounds
Involves three different types of integral
membrane proteins
1. Channels and Pores
2. Passive transporters
3. Active transporters
Transporters differ in kinetic and energy
requirements
Channels and Pores
• Have central passage that
allows molecules cross the
membrane.
• Can cross in either direction
by diffusing down
concentration gradient.
• Solutes of appropriate size
and charge can use same
pore.
• Rate of diffusion is not
saturable.
• No energy input required
Porins
• Present in bacteria plasma membrane and
outer membrane of mitochondria
• Weakly selective, act as sieves
• Permanently open
• 30-50 kD in size
• exclusion limits 600-6000
• Most arrange in membrane as trimers
Passive Transport (Facilitated Diffusion)
• Solutes only move in the
thermodynamically favored direction
• But proteins may "facilitate"
transport, increasing the rates of
transport
• Two important distinguishing features:
– solute flows only in the favored
direction
– transport displays saturation kinetics
Three types of transporters
• Uniporter – carries single molecule across
membrane
• Symport – cotransports two different
molecules in same direction across
membrane
• Antiport – cotransports two different
molecules in opposite directions across
membrane.
Saturation Kinetics of transport
•Rate of diffusion is
saturable.
•Ktr = [S] when rate of
transport is ½ maximun rate.
•Similar to M-M kinetics
•The lower the Ktr the higher
the affinity for substrate.
• Transporters undergo
conformational change
upon substrate binding
• Allows substrate to
transverse membrane
• Once substrate is
released, transported
returns to origninal
conformation.
Active Transport Systems
• Some transport occur such that
solutes flow against
thermodynamic potential
• Energy input drives transport
• Energy source and transport
machinery are "coupled"
• Like passive transport systems
active transporters are saturable
Primary active transport
• Powered by direct source of energy(ATP,
Light, concentration gradient)
Secondary active transport
• Powered by ion concentration gradient.
• Transport of solute “A” is couple with
the downhill transport of solute “B”.
• Solute “B” is concnetrated by primary
active transport.
Na+-K+ ATPase
• Maintains intracellular
Na low and K high
• Crucial for all organs,
but especially for neural
tissue and the brain
• ATP hydrolysis drives
Na out and K in
+
+
Na -K
ATPase
• Na+ & K+ concentration gradients are maintained
by Na+-K+ ATPase
• ATP driven antiportsystem.
• imports two K+ and exports three Na+ for every
ATP hydrolyzed
• Each Na+-K+ ATPase can hydrolyze 100 ATPs
per minute (~1/3 of total energy consumption of
cell)
• Na+ & K+ concentration gradients used for 2o
active transport of glucose in the intestines
1o active transport of Na+
2o active transport of
glucose
Transduction of
extracellular signals
• Cell Membranes have specific receptors
that allow cell to respond to external
chemical stimuli.
• Hormone – molecules that are active at a
distance. Produced in one cell, active in
another.
• Neurotransmitters – substances involved
in the transmission of nerve impulse at
synapses.
• Growth factors – proteins that regulate
cell proliferation and differentiation.
• External stimuli(first messenger) – (hormone,
etc…)
• Membrane receptor – binds external stimuli
• Transducer – membrane protein that passes
signal to effector enzyme
• Effector enzyme – generates an intracellular
second messenger
• Second messenger – small diffusible molecule
that carrier signal to ultimate destination
G-Proteins
• Signal transducers.
• Three subunits, (a,, g) a and g anchored to
membrane via fatty acid and prenyl group
• Catalyze hydrolysis of GTP to GDP.
• GDP bound form is inactive/GTP bound form active
• When hormone bound receptor complex interacts
with G-protein, GDP leaves and GTP binds.
• Once GTP -> GDP G-protein inactive
• GTP hydrolysis occurs slowly (kcat= 3min-1) good
timing mechanism
Epinephrine signaling pathway
• Epinephrine regulation of glycogen
degradation
• Fight or Flight response
• Ephinephrine primary messenger
• G-protein mediated response.
• G-protein activates Adenyl-cyclase to
produce cAMP
• cAMP is the second messenger
• Activates protein kinase
• Activates glycogen phosphorylase
Effect of Caffeine
• Caffeine inhibits cAMP
phosphodiesterase , prevents
breakdown of cAMP.
• Prolongs and intensifies Epinephrine
effect.
Phosphatidylinositol (PI) Signaling
Pathway
• G-protein mediated
• G-protein activates phospholipase C (PLC)
• PLC cleaves PI to form inositoltriphosphate (IP3) and diacylglycerol
(DAG) both act as 2nd messengers
• IP3 stimulates Ca2+ releases from ER
• DAG stimulates Protein kinase C