1
Introduction
Sources
Membrane Structural Biology. M. Luckey, Cambridge University Press, 2008.
Thermal Biophysics of Membranes. Heimburg, T., Wiley-VHC, 2007. Molecular
Biology of the Cell. B. Alberts, D. Bray, J. Lewis, M. Raff, K. Roberts and J. D.
Watson, Garland Publishing, 1994. Molecular Cell Biology Lodish et al. W. H.
Freeman and Co. 2003 The Structure of Biological Membranes Editor Philip L.
Yeagle CRC Press 2012
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1.1
What Membranes Do
Functional Characterization
Structural Barrier
• Ions
• Energy Substrates
• Self and Non-self Destinction 1
• Toxic waste containment
Organization and Compartmentalization
• Complex
– Dynamic
– Composition
• Functional Stability
– Protein Composition
– Lipid Composition
1 Lewis
Thomas “Lives of a Cell”
2
• Membrane Biogenesis
– ER & Golgi
– Mitochondria
• Membrane Metabolism
– Oxidation
3
– Intracellular Storage
– Lipoprotein Export
4
1.2
Surface Chemisty
Polya’s Theorem2
Z r
N−3
|k(λ )|
0
(
∞ if N ≤ 2
dλ =
finite value if N > 2
(1)
A random walk on a lattic is “persistant” if the walker is certain to return to the
origin.
Using the Membrane Surface
Surface Binding
• Equilibrium Assumption µsur f ace = µbulk
• Assume a Lattice Model for Binding Sites θ =
X
N
• Binding Energy (Two Dimensional Surface)
Langmuir Adsorption Equation
• θ=
K[P]
1+K[P]
• Surface Coverage θ ∝ K[P]
• Membrane Integration
• Oligomerization
• Membrane Environment
2 Hughes
et al., PNAS 78:3287(1981)[1]
5
=
[X]
[P]+[PX] ,
K=
[PX]
[P][X]
• Enzymes and Transporters
– Activation, positioning, sequestration
• Receptors and Signalling
– Activation, positioning, sequestration
• Control of Reaction Environment
– Access, proximity, reversibility
• Vectoral Control of Force/Energy Transduction
– Capacitance/leak, kinetics
6
Dimensional Reduction
• Mean Capture Time ω
Two Dimensions ω ∝ ln ab
Three Dimensions ω ∝
a
b
• Membrane encounter becomes rate limiting.
Free Energy of Mixing and Surface Tension
• Mean-Field approximation
• Exchange Parameter χAB =
wAA +wBB
z
)
kT (wAB −
2
• Positive Exchange Energy (Surface Tension) χAB ∝ Kexch >0
2
Membranes
What are Biological Membranes?
Prior to Light Microscopy
• Osmotic Phenomena Required Selective Permeability
Post–Microscopy
• Selective Permeability was Connected to Individual Biological Entities, “Cells.”
• Nägeli inferred the “Plasmamembran” as a surface precipitation reaction
• Observation of cellular response to solute concentration obeyed van’t Hoff’s
law: π = RT ∑ C to develop the concept of “osmotic pressure.”
• Nernst determined that selective permeability to ionic species produced a
membrane potential:
−RT Ci
ln
Ei =
zF
Co
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2.1
Discovering Membrane Functional Properties
Basis for Selective Permeability
Overton and Meyer investigated the osmotic effects of compounds on cells and
determined that it was related to the “Partition Coefficient” of these compounds
between water and oil.
Partition Coefficient
P=
Coil
CH2 O
(2)
Overton and Meyer observed that the anesthetic dose depended on partioning into
lipid (oil).
• ED50 ∝ P−1
• Coil = constant
• Non–atomistic or continuum behavior of the membrane boundry
8
These studies led Overton to conclude that the composition of cellular boundry
was distinctive.
“the boundary was cholesterol, cholesterol ester. . .or could very possibly be lecithin or fatty oils”
• Boundry Solubility ∝ Oil Solubility
• Ionic Permeation Required Additional Pathways
2.2
Membrane Structure, A Beginning
Investigation of Membrane Structure
9
A Source of Membrane Components: Red Blood Cells
Uniform size, shape and no internal membranes.
Langmuir–Blodgett Trough
10
11
Resulting Coarse-Grained Model
• Π lateral pressure
• dF
dA T = −Π
• Temperature Dependence
• Liquid condensed–melting–liquid expanded
12
Modern Coarse-Grained Model
13
Surface Tension Contradiction
14
2.3
Membrane Composition
Membrane Composition Summary
Plasma Membrane
Mitochondria
Microsomes
Lysosomes
Nuclear
Golgi
SR–ER
Length/Unsaturations
Rat Liver
Mitochondria(OMM)
Mitochondria(IMM)
PM
Microsome(Sm)
Microsome(Rg)
Golgi
3
PC
39
40
58
40
55
50
72.7
% Phospholipid
PS
PI
PA
CL
SM
9
8
1
1
16
1
5
18
1
2
10
1
1
1
2
5
1
1
20
3
10
2
4
3
6
12
<1
1
3
1.8
8.7
<1
<1
1
Fatty Acid as Weight %
18:0
18:1
18:2
18:3
20:0
16.8
10.4
16.8
21
13.5
13.5
18.0
16.2
15.8
31.2
6.4
12.9
tr
tr
26.5
10.6
14.9
22.0
11.1
16.1
22.5
8.7
18.1
tr
tr
PE
23
35
22
14
13
20
13.5
16:0
29.7
27
27.1
36.9
28.6
22.7
34.7
PL
µ/mg
672
175
374
156
500
825
603
Chol
µ/mg
128
3
14
38
38
78
12
20:3
1.5
1
1
20:4
18.3
15.7
18.5
11.1
14
19.7
14.5
1.4
1.8
22:6
3.4
3.5
3.8
0.7
2.9
Lipid Structures
3.1
Lipid Metabolism
Lipid Definition and Types
From the Greek Lipos meaning fatty. A compound of low molecular weight
(<5000) a substantial portion of which is made up of carbon and hydrogen.
Fatty Acid
Triglycerides
15
Membrane formation requires amphipiles
Phospholipids, sphingolipids, steroids, glycolipids
Phospholipid Headgroups
16
Naming Conventions for Asymmetric Two Position
sn-stereochemical numbering
17
The R and S
configurations look similar
3.2
Alkane Chains
Common Alkane Chains
18
Chain Length
saturated
12
16
18
Monoenoic
16
18
Dienoic
18
Tetranoic
20
name
m.p.
Lauric acid 12:0
Palmitic acid 16:0
Stearic acid 18:0
44.2
63.1
69.6
Palmitoleic acid 16:19
Oleic acid 18:19
-0.5
13.4
Linoleic acid 18:29,12
-5
Arachadonic 20:45,8,11,14
-49.5
19
3.3
Steroids
Steroids Molecules modulate membrane structure and function
H3C
CH3
CH3
H
H
H
HO
20
Westover & Covey J. Memb. Biol (2004)
Stabilization
verses Detergents
4
Crystal Structures
4.1
Simple Lipids
Simple Lipid Structures
Crystals represent a low energy and presumably stable molecular state while
providing atom scale structural information.
Real...
Membranes are “fluid”
anisotropic
Membranes are hydrated Membranes are
21
Potassium Palmitate
• Periplanar configuration
• Headgroups are hydrogen bonded
• Transition across C2and3
22
4.2
Acylglycerol Structures
Acylgylcerol Crystal Structures
One acyl chain
Trans–Leaflet H-Bonding
Glycerol ⊥
23
Two acyl chains No Trans Leaflet H–bonds Glycerol k
24
Triacylglycerol
25
• Nonlamellar Lipid
• Relatively weak headgroup polarity
• Lateral packing in a single layer
• Polar region in the single layer
Jensen & Mabis (1966)
Φ = cos−1 ( nS∑ )
26
4.3
Glycerophosphate Crystals
Glycerophosphate Membrane Structure
DiLauryl-Glycerol-Phosphate
• n∑ = S
• Tail to tail packing
27
• Bilayer of 47.7 Å
• Chains parallel to bilayer normal
Lysophosphatidylethanolamine
28
• Modulation of membrane stability
• Cell Death
• Inclusion integration
• ( nS∑ ) = 0.538
3-Laurylpropanediol-1-phosphocholine
• Mixed lipid chain lengths
• ( nS∑ ) ≤ 0.5
• Structure based thickness of bilayer
• Accomodation of proteins
29
4.4
Stereochemistry of lipid structures
Membrane Leaflets
Stereochemical Leaflet Segregation
30
• Assortment of enantiomers into leaflets
• Example of head group interdigitation (dimethyl ethanolamine)
• Membrane Fusion
• Sterol modulation of function
31
32
• Alkane chains select C2–3 rotamers
• Areas give head tilt
• C2–3 affects chain stacking
• Hydrogen bonding
• C1–2 rotates
5
Bibliography
References
[1] Barry D. Hughes, Michael F. Shlesinger, and Elliott W. Montroll. Random walks with selfsimilar clusters. Proc. Natl. Acad. Sci. U.S.A., 78:3287–3291, 1981.
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