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Chapter 9 (part 2)

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Americans obtain between 20 and 30% of their calories from fats and oils. ... Always Cys of CAAX (C=Cys, A=Aliphatic, X=any residue) ... – PowerPoint PPT presentation

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Title: Chapter 9 (part 2)


1
Chapter 9 (part 2)
  • Lipids and Membranes

2
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

3
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

4
Head
Tail
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rubber
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8
The reason rubber is elastic and gutta percha is
plastic
Rubber forms an amorphous structure
Gutta-percha forms crystalline arrays
9
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

10
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

11
Steroids
12
Many steroids are derived from cholesterol
13
Membranes
  • Barrier to toxic molecules
  • Help accumulate nutrients
  • Carry out energy transduction
  • Facilitate cell motion
  • Modulate signal transduction
  • Mediate cell-cell interactions

14
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)

15
The Fluid Mosaic Model
16
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.

17
fusion
After 40 minutes
18
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
21
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

22
Phase Transitions
  • Only pure lipid systems give sharp, well-defined
    transition temperatures
  • Red pure phospholipid
  • Blue phopholipid cholesterol

23
Structure of Membrane Proteins
  • Integral (intrinsic) proteins
  • Peripheral (extrinsic) proteins
  • Lipid-anchored proteins

24
Peripheral Proteins
  • Peripheral proteins are not strongly bound to the
    membrane
  • They can be dissociated with mild detergent
    treatment or with high salt concentrations

25
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

26
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.
27
Lipid-Anchored Proteins
  • Four types have been found
  • Amide-linked myristoyl anchors
  • Thioester-linked fatty acyl anchors
  • Thioether-linked prenyl anchors
  • Glycosyl phosphatidylinositol anchors

28
Amide-Linked Myristoyl Anchors
  • Always myristic acid
  • Always N-terminal
  • Always a Gly residue that links

29
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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