Chapter 9 (part 2)
Lipids and Membranes
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
Head
Tail
rubber
Poly-isoprene
(Greater than 80 carbons)
Rubber
(>80,000 Carbons)
amorphous
Gutta-Percha
The reason rubber is elastic and
gutta percha is plastic
Rubber forms an amorphous structure
Gutta-percha forms crystalline arrays
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