Chapter 5 of Yeagle Structure of Biological Membranes

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• Chapter 5 of Yeagle Structure of Biological
  Membranes
• Non-lamellar phases
• Spontaneous curvature
• Actual curvature

What is the physical basis of non-lamellar phase
structures? Can we understand the competing
forces that stabilize a lamellar versus a
non-lamellar phase?
In cells, primarily lamellar structures are
found, yet lipids extracted from cells will form
non-lamellar phases in vitro.
Bilayer thickness Surface charge Dielectric
constant Lipid composition
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• POLYMORPHISM
• MESOMORPHISM

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General Motivation (1) by studying structural
polymorphism/mesomorphism, one gaines an
understanding of the forces that are locked-up
in biomembranes that can affect the organization
and function of membrane proteins (2) Generally
extended to surfactant/detergent chemistry
This chapter is interested in phase changes that
change the CURVATURE of the lipid-water
interface. These phases occur at temperatures
above the gel-liquid transition, but below the
transition temperature to an isotropic liquid.
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Why do we care about CURVATURE?

• Cell division
• Endocytosis
• Membrane fusion
• Structure
• organelles

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X-ray diffraction and NMR
EPR, UV-VIS, IR, Calorimetry, Neutron diffraction
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GOAL INTUITIVE UNDERSTANDING OF THE FORCESTHAT
DRIVE THE FORMATION OF CURVATURE ALTERING PHASE
TRANSITIONS

• TERMINOLOGY
• NON-BILAYER PHASE really means non-lamellar, but
  still a bilayer of phospholipids
• INVERTED/WATER-IN-OIL PHASE HII phase, possess a
  net concave curvature when viewed from the water
  domain.
• NONINVERTED/OIL-IN-WATER PHASE HI phase,
  possesses a net convex curvature when viewed from
  the water domain

OIL-WATER SURFACTANT MICELLES
What happens when you have detergents and a small
amount of oil in water? What happens when the oil
is the majority constituent?
THERMOTROPISM and LYOTROPISM
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Phenomenological Approach like Hookes Law the
force required to stretch an elastic object is
linearly proportional to displacement from
equilibrium position. This disregards the
molecular forces at play
For lipid bilayers the fundamental unit of all
lipid mesomorphs is the lipid monolayer, and that
this monolayer may be endowed with a spontaneous
tendency to curl.
Co spontaneous curvature Ro radius of
spontaneous curvature
Co 1/Ro
Rigidity of object
DE 0 when R R0
Parabolic dependence
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Perform X-ray crystallography and obtain
d-spacing and reflections
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By adding 16(w) tetradecane, it is possible to
minimize the unfavorable packing of the
acylchains, thus lowering the phase transition
temperature to the hex phase
As you raise the Temperature, the tube radius
becomes smaller
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Inverted Hex phase is a cylinder
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• Bending membranes using
• Lipids themselves
• Molecular motors
• Protein binding

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Bending by proteins Scaffolding Mechanism The
protein coats that cover budding membrane
surfaces function as scaffolds to curve the
membrane. Constraint is that protein must be
curved and rigid for the membrane to follow.
There must also be a tight binding examples
dynamin and BAR-domain proteins Local
Spontaneous Curvature Mechanism. Spontaneous
curvature is generated by the penetration of a
protein into the membrane. Example is the ENTH
domain of epsin. It is involved in clathrin
mediated endocytosis. The ENTH domain binds to
PIPs (PI-4,5 biphosphate) Amphiphysin is another
protein example. Both have amphipathic helices
that insert into the bilayer which may cause a
local curvature strain.
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Sensing by proteins
A new concept in cell trafficing and membrane
curvature is that proteins can sense
curvature. Proteins have been discovered that
have binding affinities that are dependent upon
the radius of membrane curvature.
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