Chapter 9 (part 2)

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Chapter 9 (part 2)

• Lipids and Membranes

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Triacylglycerols (TAG)

• 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

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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

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Head
Tail
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rubber
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The reason rubber is elastic and gutta percha is
plastic
Rubber forms an amorphous structure
Gutta-percha forms crystalline arrays
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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

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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

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Steroids
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Many steroids are derived from cholesterol
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Membranes

• Barrier to toxic molecules
• Help accumulate nutrients
• Carry out energy transduction
• Facilitate cell motion
• Modulate signal transduction
• Mediate cell-cell interactions

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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)

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The Fluid Mosaic Model
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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.

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fusion
After 40 minutes
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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

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Membranes are Asymmetric
In most cell membranes, the composition of the
outer monolayer is quite different from that of
the inner monolayer
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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

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Phase Transitions

• Only pure lipid systems give sharp, well-defined
  transition temperatures
• Red pure phospholipid
• Blue phopholipid cholesterol

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Structure of Membrane Proteins

• Integral (intrinsic) proteins
• Peripheral (extrinsic) proteins
• Lipid-anchored proteins

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Peripheral Proteins

• Peripheral proteins are not strongly bound to the
  membrane
• They can be dissociated with mild detergent
  treatment or with high salt concentrations

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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

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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. Bacteriorhodo
psin has loops at both the inner and outer
surface of the membrane. It displays a common
membrane-protein motif in that it uses alpha
helices to span the membrane.
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Lipid-Anchored Proteins

• Four types have been found
• Amide-linked myristoyl anchors
• Thioester-linked fatty acyl anchors
• Thioether-linked prenyl anchors
• Glycosyl phosphatidylinositol anchors

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Amide-Linked Myristoyl Anchors

• Always myristic acid
• Always N-terminal
• Always a Gly residue that links

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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

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Thioether-linked Prenyl Anchors

• Prenylation refers to linking of "isoprene"-based
  groups
• Always Cys of CAAX (CCys, AAliphatic, Xany
  residue)
• Isoprene groups include farnesyl (15-carbon,
  three double bond) and geranylgeranyl (20-carbon,
  four double bond) groups

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