Characteristics of Membranes

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Characteristics of Membranes

• Composed of lipids, proteins and carbohydrates
• Membrane lipids are amphipathic
• Membranes form as a result of noncovalent
  interactions between membrane components
• Membranes are asymmetric
• Lipids (and occasionally proteins) move freely
  within membranes
• Specialized functions of membranes are due mainly
  to membrane proteins

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Fatty Acids The building blocks of membrane
lipids
Definition- A long chain hydrocarbon that
terminates in a carboxylic acid Palmitate
(un-ionized palmitic acid), 160, hexadecanoate
(un-ionized hexadecanoic acid) CH3 (CH2)14 COO-
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Numbering rules for fatty acid carbons
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IUPAC Nomenclature

• Number of carbons 12dodec, 14tetradec,
  16hexadec, 18octadec, 20eicos
• B. Number of double bonds-named according to
  parent hydrocarbon with the ending oic
• example C18 fatty acid with
• no double bonds octadecanoate
• one double bond octadecenoate
• two double bonds octadecadienoate
• three double bonds octadecatrienoate
• four double bonds octadecatetraenoate
• C. Double bond location count from carboxylic
  acid to
• carbon prior to double bond
• D. Cis vs trans double bonds

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Common fatty acids

• Common name palmitate
• Hexadecanoate
• Cis-D9-octadecenoate
• Trans-D9-octadecenoate

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Some naturally occurring fatty acids in animals
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Major Types of Membrane Lipids

• Phospholipids
• Glycolipids
• Cholesterol

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Structure of a phospholipid
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A Simple Phospholipid
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Cholesterol is a membrane lipid composed of 4
hydrocarbon rings
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Lipids form micelles and bimolecular sheets in
aqueous solution
Phospholipids are amphipathic polar head groups
and non-polar tails General Properties of Lipid
Bilayers Self-assembling Membrane stability
favors large surface area (up to 1 mm2)
Resistant to edge formation self sealing
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Liposomes Can Be Used to Deliver Non-Lipid
Soluble Compounds to Cells
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Preparation of a liposomeLiposomes are being
used to deliver DNA into human cells for gene
therapy
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Integral and peripheral membrane proteins
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The Fluid Mosaic Model
Membranes are a two dimensional solution of
lipids and proteins that can move laterally but
tend to maintain a fixed orientation relative to
the two surfaces of the membrane
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Spontaneous Transverse Movements Flip-Flops of
Membrane Lipids Are Infrequent
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How do we know? Fluorescence Recovery After
Photobleaching (FRAP)
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Membranes Have Phase-Transitions ThatAre
Determined by Lipid Composition
P.S. What about nucleic acids?
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Double Bonds Decrease the Melting Point of a
Fatty Acid

• Saturated no double bonds
• Monounsaturated one double bond
• Polyunsaturated multiple double bonds
• The melting point of a fatty acid is inversely
• proportional to its degree of saturation and
  directly proportional to carbon chain length

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Oleic acid C18
Stearic acid C18
Unsaturated or polyunsaturated Kinky structures
Saturated Flexible structures
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3 molecules of stearic acid (saturated)
2 molecules of stearic acid 1 molecule of oleic
acid (unsaturated)
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Cholesterol can also affect the fluidity of
membranes
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Why is lipid fluidity important?
Membrane fluidity is dependent on lipid
fluidity Lipid fluidity is dependent on both C-C
chain length and the number of CC bonds. Lipid
fluidity is also dependent on cholesterol. It is
an important biological feature of membranes
because it allows interaction of proteins
embedded within the bilayer.
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Flippases Facilitate the Flipping ofMembrane
Phospholipids
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Transverse Asymmetry is Evident in
thePhospholipid Composition of the TwoHalves of
the Lipid Bilayer
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Studying membrane proteins

• Peripheral membrane proteins can be removed from
  the membrane by changing pH and salt (Why?)
• Integral membrane proteins are released from
  membranes by adding detergents

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Important properties of a detergent

• Should dissociate the protein from the other
  membrane components
• Should not unfold the protein
• Should be readily removable after purification
• Balancing these 3 properties is not easy!

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Sodium dodecyl sulphate (SDS)
CH3
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
O
O
O
S
Na
_
O
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The Not-So-Fluid Mosaic Model Restricting
proteins to specific cellular domains
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Summary of lipid properties
1. Membrane lipids are amphipathic the property
which contributes to the selective permeability
of membranes
2. Membrane lipids are amphipathic the property
which drives the self-assembly of lipid bilayers
3. Membrane phospholipids have a characteristic
transition temperature which is determined by FA
chain length and degree of saturation this
property determines the fluidity of the lipid
bilayer
4. Membrane phospholipids are mobile frequently
laterally but rarely transversely which
contributes to the fluidity of the bilayer and
preserves the asymmetry