Phospholipids and Membrane
Phospholipids
• Phospholipids
– Are composed of
• Glycerol backbone
• 2 fatty acids
• Phosphate
– Are soluble in water
– Are manufactured in our bodies so they are not
required in our diet
Phospholipids
O
Each glycerophospholipid
includes
Š a polar region:
glycerol, carbonyl O
of fatty acids, Pi, & the
polar head group (X)
Š non-polar
hydrocarbon tails of
fatty acids (R1, R2).
H2C
O
R1
C
O
O
CH
H2C
C
R2
O
O
glycerophospholipid
P
O−
O
X
Sphingolipids
Sphingolipids (SPLs) also have a
polar head group and two non-polar
tails but do not contain glycerol
• Instead, the backbone is
sphingosine, a long-chain amino
alcohol
• Some derivatives are Ceramide,
Sphingomyelin, and
Glycosphingolipids
Sphingosine
is a fatty amine, a
glycerol molecule
is never seen!
O
CH3
H3C
Sphingomyelin has a
phosphocholine or
phosphoethanolamine
head group.
+
N
H2
C
H2
C
CH3
O
P
−
O
OH
O
phosphocholine
H2C
sphingosine
Sphingomyelins are
common constituent of
plasma membranes
Sphingomyelin
H
C
CH
NH
CH
O
C
fatty acid
R
HC
(CH2 )12
CH3
Sphingomyelin, with a phosphocholine head group, is similar in size
and shape to the glycerophospholipid phosphatidyl choline.
Glycosylated sphingolipids
A cerebroside is a
sphingolipid (ceramide)
with a
monosaccharide such
as glucose or galactose
as polar head group.
A ganglioside is a ceramide with a polar head group that is a complex
oligosaccharide, including the acidic sugar derivative sialic acid.
Cerebrosides and gangliosides, collectively called glycosphingolipids, are
commonly found in the outer leaflet of the plasma membrane bilayer, with their
sugar chains extending out from the cell surface.
Sterols
• Compounds containing 4 carbon ring structure
with any of a variety of side chains
• Many important body compounds are sterols
– cell membranes, bile acids, sex & adrenal hormones,
vit D & cholesterol.
• Sterols are found in plant & animal foods
– Manufactured in bodies so non-essential
Sterols
•
•
•
Compounds containing 4 carbon
ring structure with any of a
variety of side chains
Many important body compounds
are sterols
– cell membranes, bile acids,
sex & adrenal hormones, vit D
& cholesterol.
Sterols are found in plant &
animal foods
– Manufactured in bodies so
non-essential
Cholesterol
Sterols
• Metabolic precursors of steroid
hormones
–
–
–
–
Regulate physiological functions
Androgens (testosterone)
Estrogens (β-estradiol)
Glucocorticoids (cortisol)
• Insoluble in water
• Bind to proteins for transport to
target tissue
Steroid Hormones Derived from Cholesterol
Bile Acid
Fat-Soluble Vitamins
• A vitamin is a compound that is essential to the health of
humans but cannot be synthesized internally and must be
obtained through diet.
• Vitamins A and D are precursors of hormones:
– Vitamin D is a cholesterol derivative that is converted into
a hormone regulating calcium metabolism in the intestine,
kidney, and bone (deficiency leads to ricketts)
– Vitamin A (aka. retinol) derivatives can absorb light
(retinal + opsin = rhodopsin, a vision thing), and control
epithelial tissue development
Fat-Soluble Vitamins
• Vitamin K – a cofactor essential for activating prothrombin,
needed to promote blood clotting
• Vitamin E – group of lipids called tocopherols, which as
antioxidants, protect unsaturated fatty acids, and scavenge
damaging free radicals
• Warfarin is a synthetic compound that inhibits the formation
of active prothrombin (excellent rat poison or human
anticoagulant!)
• Ubiquinone (aka Coenzyme Q) and Plastiquinone funcation
as lipophilic electron carriers in the redox reactions that drive
ATP synthesis
Several vitamins are lipids.
Vitamins.
Definition - Organic compound that are required in small amounts.
Say a few words about the lipid vitamins (the fat soluble vitamins).
Lipid vitamins
Water soluble vitamins:
Vitamin A
Vitamin D
Vitamin E
Vitamin K
B1, B2, B3, B5, B6, B7, B9, B12
(we will discuss water soluble vitamins later in the course)
Vitamin A
•
•
•
Collective term for retinol, retinal, retinoic acid
Formed from oxidative cleavage of β-carotene in
liver
Function
– Aldehyde: visual cycle/process, component of
rhodopsin (visual pigment)
– Alcohol, carb acid: growth, reproduction
•
Deficiency
– Night blindness
– Xerophthalmia
•
•
•
•
Dryness in eyes
No tear production
Damage to cornea
Leads to blindness
Vitamin A - Retinol
Retinol (vitamin A)
Sources in diet - Many plants, also meat, especially liver. Vitamin A is fat
soluble. You can get too much, or too little if absorption is a problem.
Some uses of vitamin A:
Vision (11-cis-retinol bound to rhodopsin detects light in our eyes).
Regulating gene transcription (retinoic acid receptors on cell nuclei are part
of a system for regulating transcription of mRNAs for a number of genes).
Vitamin D refers to a group of similar lipid-soluble molecules (major
forms are D2 and D3, also D1, D4, D5).
Vitamin D3 (cholecalciferol)
Vitamin D2 (ergocalciferol)
Vitamin D can be obtained in the diet, or derived from cholesterol in a
reaction that requires UV light.
Vitamin D binds to a “vitamin D binding protein” (VDP) for transport to target
organs. Why is VDP needed?
Vitamin D is not active itself; it is modified to yield biologically active forms,
such as calcitriol.
Calcitriol (derived from vitamin D) is a transcription factor, influencing
expression of proteins involved in calcium absorption and transport.
Deficiency causes bone loss. Vitamin D is also important for immune
system function.
Vitamin D
•
Sterol derivatives
– Open B rings
•
Function
– Regulate Ca and P absorption
during bone growth
•
Sources
– Diet: D2 (milk additive, plant
sources) and D3 (animal sources)
– Precursor: intermediate in
cholesterol synthesis
– Formed in skin non-enzymatically
from 7-dehydrocholesterol
•
Deficiency
– Soft bones, impaired growth and
skeletal deformities in children
Inactive form
Vitamin D production requires UV light
(sunlight)
Sometime after the first humans migrated north out of Africa about
50,000 years ago, mutations appeared that reduced melanin (pigment)
production in the skin, permitting vitamin D production with less sunlight.
Disadvantages of less melanin production are skin that is easily
damaged by the sun, skin cancer risk, and loss of folic acid due to UV
damage.
The melanin-reducing mutations helped early humans make vitamin D
in northern europe in winter.
Vitamin E
α-tocopherol
•
Function
– Antioxidant: prevents cell damage from oxidation of polyunsaturated FAs in
membranes by O2 and free radicals
•
Deficiency
– Associated with defective lipid transport/absorption
Vitamin K
•
•
Phylloquinone or menaquinone
Function
O
CH3
– Synthesis of blood clotting proteins
•
Sources
– K1 = plants; K2 = animals
– Bacteria in intestine
•
O
CH3
O
CH3
Toxicity
– Jaundice from large doses of vit. K, toxic
effects on membrane of red blood cells, cells
die, lead to increased levels of yellow
bilirubin (formed from heme)
CH3
Vitamin K1 (phylloquinone)
Deficiency
– Unlikely due to synthesis and wide
distribution in food
– Injection for infants
– Hemolytic anemia = destruction of red blood
cells
•
CH3
CH3
O
CH3
CH3
Vitamin K2 (menaquinone)
Other Lipids
• Classified on basis of physical properties
– Solubility
– Hydrophobicity
– Amphiphilicity
• Fat-soluble vitamins
– Vitamins A, E, K (and D)
– Isoprenoids
• Eicosanoids
– Prostaglandins
– Thromboxanes
– Leukotrienes
Terpenes
• Simple lipids, but lack fatty acid component
• Formed by the combination of 2 or more
molecules of 2-methyl-1,3-butadiene (isoprene)
• Monoterpene (C-10) – made up of 2 isoprene units
• Sesquiterpene (C-15) – made up of 3 isoprene
units
• Diterpene (C-20) – made up of 4 isoprene units
Monoterpenes
CHO
OH
limonene
citronellal
menthol
camphene
Monoterpenes are readily recognized by their characterisitic
flavors and odors ( limonene in lemons, citronellal in roses and
geraniums, pinene in turpentine, and menthol from
peppermint)
Lipid Derivatives
Active roles for lipids derivatives
• Lipids as Signals, Cofactors & Pigments
– Intracellular signaling – Phosphatidylinositol
– Paracrine hormones – Eicosanoids
– Steroid hormones –polar cholesterol derivatives
– Vitamins – A, D, E, and K
• Provide crucial parameters of membrane fluidity
Eicosanoids
• Hormones involved in production of pain, fever, inflammatory
reactions
– Prostaglandins
– Thromboxanes
– Leukotrienes
• Metabolites of arachidonic acid (a polyunsatruated FA)
• Synthesis inhibited by NSAIDs
– e.g. acetylsalicylic acid (aspirin)
– Acylate Ser residue, preventing access to active site
Eicosanoids
• Eicosanoids are paracrine hormones that act on cells near their point
of synthesis to affect inflammation, blood clotting, gastric acid secretion,
etc.
• They are derived from arachidonic acid (20:4(Δ5, 8, 11, 14)) liberated
from particular membrane phospholipids by phospholipase A2
• This group includes prostaglandins, thromboxanes and leukotrienes
Pathways for Eicosanoids Synthesis
Diet
Body
LTB4
LTC4
LA
AA
LNA
EPA
DHA
LA
AA
LNA
EPA
DHA
AA
E,D
E,D
LTB5
LTC5
L
L
E,D
EPA
DHA
AA
+
EPA, DHA
C
C
PGI2
TXA2
PGI3
TXA3
Mechanisms of action of EPA and DHA
L
C
LTB4, LTC4 <-------------- AA -----------------> PGI2, TXA2
LTB5, LTC5 <---------- EPA, DHA -----------> PGI3, TXA3
* LTB4, LTC4: increase adhesion of Leucocytes to endothelium and
inflamatory effects
* LTB5, LTC5: less effective than LTB4, LTC4
*PGI2: vasodilator, inhibit platelet aggregration
*PGI3: help effect of PGI2
*TXA2: increase platelets aggregation, vasoconstriction
*TXA3: less potent than TXA2
N3 fatty acids compete with N6 for elongase, desaturase, cyclooxyenase
and lipoxigenase
Leukotrienes
Leukotrienes are derived from arachidonic acid via the enzyme
5-lipoxygenase which converts arachidonic acid to 5-HPETE
(5-hydroperoxyeicosatetranoic acid) and subsequently by
dehydration to LTA4
OH
OH
COOH
COOH
H
C5H11
H
S
C5H11
Cys
gGlu
LEUKOTRIENE F4 (LTF4)
Peptidoleukotrienes
S
Cys
Gly
gGlu
LEUKOTRIENE C4 (LTC4)
Biological activities of leukotrienes
1. LTB4
2. LTC4
3. LTD4
4. LTE4
- potent chemoattractent
- mediator of hyperalgesia
- growth factor for keratinocytes
- constricts lung smooth muscle
- promotes capillary leakage
1000 X histamine
- constricts smooth muscle; lung
- airway hyperactivity
- vasoconstriction
- 1000 x less potent than LTD4
(except in asthmatics)
Lipids are the major component of membranes.
Membranes separate cells from their environment.
Also, membranes separate intercellular regions
(such as mitochondria, vesicles, nucleus).
Lipid bilayer:
Phospholipids
Lipid bilayer forms sphere in aqueous solution
Forms barrier defining inside and outside spaces
“Fluid” means the membrane interior is liquid-like (the
lipids are mobile).
“Mosaic” means the membrane is a mosaic of different
types of components (phosphate groups, carboxylic acids,
proteins, sugars).
Cell Membrane-more complex
Contains a variety of lipids, proteins, and carbohydrates
Outer leaflet
Lipids
Lipid bilayer
5 nm
Multiple types of lipids
are found in membranes
Inner
leaflet
Protein
Cytosol (inside)
Three Types of Membrane Lipid Molecules
all amphipathic
Phospholipids
serine
Sterols
(cholesterol)
Glycolipids
(sugar lipid)
ECB 11-7
phosphatidylserine
galactocerebroside
Type B blood - terminal sugar is galactose.
Type A blood - terminal sugar is N-acetylated galactose.
Type O blood - terminal sugar is neither of above.
Fuzzy stuff on the surface is the sugar part of the glycolipids.
Membrane Fluidity (viscosity)
Describes the physical state of the membrane
Pure lipid bilayer - two states
Liquid state
Hydrophobic tails free to move
Gel state
Movement is greatly restricted
(crystalline gel)
Transition
Liquid at temperatures
Above the transition temp. temperature
Crystalline gel at
temperatures below the
transition temp
Living cells require a fluid membrane, but not too fluid:
Membrane fluidity is regulated by the cell
Membrane fluidity is governed by FA length and
saturation
1. Fatty acid length - shorter the FA, the lower the transition
temperature (melting point), favors liquid state
2. Fatty acid saturation - the more saturated, the higher the
transition temperature, favors gel state
Melting points of 18-carbon Fatty Acids
Fatty Acid
Stearic acid
Oleic acid
α-Linoleic acid
Linolenic acid
Double bonds
0
1
2
3
Melting point (˚C)
70
13
-9
-17
3. Presence of cholesterol - broadens the temperature over which
transition occurs.
Membrane fluidity:
The interior of a lipid bilayer is
normally highly fluid.
liquid crystal
crystal
In the liquid crystal state, hydrocarbon chains of phospholipids are
disordered and in constant motion.
At lower temperature, a membrane containing a single
phospholipid type undergoes transition to a crystalline state in
which fatty acid tails are fully extended, packing is highly ordered,
& van der Waals interactions between adjacent chains are maximal.
Kinks in fatty acid chains, due to cis double bonds, interfere with
packing in the crystalline state, and lower the phase transition
temperature.
Lipid composition varies in inner and outer leaflet
Glycolipids in
outer leaflet
Phospholipids
Influence of FA saturation on lipid bilayer
order
ordered
Saturated straight
hydrocarbon chains
(no double bonds)
less ordered
Unsaturated
hydrocarbon chains
(with double bonds)
Less ordered state increases membrane fluidity
Polar head
group
Rigid
Planar
Steroid
ring
Nonpolar
hydrocarbon
tail
ECB 11-16
Mainly in animal cells,
Not in plants
Cholesterol
stiffens lipid
bilayers
Polar head
group
Stiffened
region
Fluid
region
HO
Cholesterol
Cholesterol
in membrane
Cholesterol inserts into bilayer membranes with its hydroxyl
group oriented toward the aqueous phase & its hydrophobic ring
system adjacent to fatty acid chains of phospholipids.
The OH group of cholesterol forms hydrogen bonds with polar
phospholipid head groups.
Interaction with the relatively rigid
cholesterol decreases the mobility of
hydrocarbon tails of phospholipids.
Cholesterol
in membrane
But the presence of cholesterol in a phospholipid membrane
interferes with close packing of fatty acid tails in the crystalline
state, and thus inhibits transition to the crystal state.
Phospholipid membranes with a high concentration of cholesterol
have a fluidity intermediate between the liquid crystal and
crystal states.
Lipids in membranes are mobile.
1. Spin (fast)
2. Lateral
movement
(less fast)
3. Flip-flop
Almost never
Two strategies by which phase changes of membrane
lipids are avoided:
Š Cholesterol is abundant in membranes, such as plasma
membranes, that include many lipids with long-chain saturated
fatty acids.
In the absence of cholesterol, such membranes would
crystallize at physiological temperatures.
Š The inner mitochondrial membrane lacks cholesterol, but
includes many phospholipids whose fatty acids have one or
more double bonds, which lower the melting point to below
physiological temperature.
Membranes are permeable to small non-polar uncharged molecules
(includes O2 and N2 and CO2).
Impermeable to ions and most other water-soluble molecules; these
need specific transporters to get across the membrane.
Lipid Bilayer
Permeability
Small hydrophobic
Molecules
O2, CO2, N2, benzene
Small Uncharged
polar molecules
H2O, glycerol, ethanol
Large, uncharged
Polar molecules
Amino acids, glucose,
nucleotides
IONS
H+, Na+, HCO3-,
K+, Ca2+, Cl-, Mg2+
ECB Fig.12-2
Ions such as Na+
and K+ can NOT
pass through lipid
membranes
(except through
special pores)
The red blood cell glucose transporter (only lets glucose through,
passive transport).
The red blood cell glucose transporter (only lets glucose through,
passive transport).
Uniport - specific for one type of molecule.
Symport - transports 2 molecules together.
Antiport - transports 2 molecules in opposite directions.
Active transport requires energy (ATP hydrolysis).
Can work against a concentration gradient.
Example of active transport:
Na+/K+ pump (Na+ conc is higher outside cells).
3 Na+ ions bind to transporter protein inside cell.
ATP phosphorylates protein, causes conformational change.
The 3 Na+ ions are released outside cell ; 2 K+ ions bound.
Triggers dephosphorylation of protein.
Protein goes back to original state ; K+ released inside cell.
This is an “antiport” ; two ions moving in opposite directions through
the some transporter.
A good animation of the Na+/K+ pump:
google: “animation sodium potassium pump”
Porins - Relatively simple transporters located in bacterial outer
membranes, mitochondria and chloroplasts.
Porin proteins are trimeric, a group of 3 beta-barrels.
Core of barrel has narrow aqueous channel. Small molecules with
MW less than about 600 can pass through.
OmpF trimer &
view of one
beta-barrel
(from text)
The E. coli OmpF protein is a porin. OmpF stands for “outer
membrane protein F”.
Ion channels - Found in neurons and other eukaryotic proteins, as
well as bacteria.
A well-known ion channel is the potassium channel (bacterial). Allows
potassium to pass, but not sodium.
The core of the potassium ion channel has an arrangement of
backbone carbonyl atoms that has the correct geometry to coordinate
potassium ions, but not sodium.
Membrane potential Many cells have a charge imbalance across the membrane, typically
excess negative charge inside the cell, and excess positive charge
outside (due to different Na+ and K+ inside and outside cell).
This charge imbalance results in a voltage across the membrane.
Typical animal cells have a membrane potential of about 70 mVolts,
due to difference in Na+ and K+ inside and outside cells.
When an axon of a nerve cell is stimulated, Na+ channels in the
membrane open, allowing Na+ to cross, and altering the membrane
potential, typically from -70 mV to +50 mV, within a millisecond.
This triggers the opening of a nearby K+ channel, which returns the
potential to -70 mV, and stimulates other Na+ channels farther along
the neuron.
This propagating voltage change
is called an “action potential”.
Action potentials propagate
rapidly along an axon (in
milliseconds).