Transcript Enzyme Mechanisms - Research Centers

Lipids II;
Membranes
Andy Howard
Introductory Biochemistry
25 September 2008
Biochemistry: Lipid2/Membranes
09/25/08
What we’ll discuss
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Lipids
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Plasmalogens
Glycosphingolipids
Isoprenoids
Steroids
Other lipids
Membranes
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Membrane transport
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Passive & active
Thermodynamics
Pores & Channels
Protein-mediated
transport
Bilayers
Fluid mosaic model
Physical properties
Lipid Rafts
Membrane proteins
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iClicker quiz question 1
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What is the most common fatty acid in
soybean triglycerides?
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(a) Hexadecanoate
(b) Octadecanoate
(c) cis,cis-9,12-octadecadienoate
(d) all cis-5,8,11,14-eicosatetraeneoate
(e) None of the above
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iClicker quiz, question 2
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Which set of fatty acids would you
expect to melt on your breakfast table?
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(a) fatty acids derived from soybeans
(b) fatty acids derived from olives
(c) fatty acids derived from beef fat
(d) fatty acids derived from bacteria
(e) either (c) or (d)
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iClicker quiz question 3
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Suppose we constructed an artificial lipid
bilayer of dipalmitoyl phosphatidylcholine
(DPPC) and another artificial lipid bilayer
of dioleyl phosphatidylcholine (DOPC).
Which bilayer would be thicker?
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(a) the DPPC bilayer
(b) the DOPC bilayer
(c) neither; they would have the same
thickness
(d) DOPC and DPPC will not produce stable
bilayers
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Plasmalogens
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Another major class besides
phosphatidates
C1 linked via cis-vinyl ether linkage.
n.b. The textbook figure 8.10 is one page
later than the discussion of it
Ordinary fatty acyl esterification at C2
Phosphatidylethanolamine at C3
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Specific
plasmalogens
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Roles of phospholipids
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Most important is in membranes that
surround and actively isolate cells
and organelles
Other phospholipids are secreted and
are found as extracellular surfactants
(detergents) in places where they’re
needed, e.g. the surface of the lung
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Sphingolipids
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Second-most abundant membrane
lipids in eukaryotes
Absent in most bacteria
Backbone is sphingosine:
unbranched C18 alcohol
More hydrophobic than phospholipids
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Varieties of
sphingolipids
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Ceramides
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Sphingomyelin
Image on
steve.gb.com
sphingosine at glycerol
C3
Fatty acid linked via
amide
at glycerol C2
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Sphingomyelins
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C2 and C3 as in
ceramides
C1 has phosphocholine
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Cerebrosides
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Ceramides with one
saccharide unit
attached by glycosidic linkage at
C1 of glycerol
Galactocerebrosides
common in nervous
tissue
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Gangliosides
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Anionic derivs of cerebrosides (NeuNAc)
Provide surface markers for cell recognition
and cell-cell communication
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Isoprenoids
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Huge percentage of non-fatty-acid-based
lipids are built up from isoprene units
Biosynthesis in 5 or 15 carbon building
blocks reflects this
Steroids, vitamins, terpenes
Involved in membrane function, signaling,
feedback mechanisms, structural roles
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Steroids
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Molecules built up from ~30-carbon four-ring
isoprenoid starting structure
Generally highly hydrophobic (1-3 polar
groups in a large hydrocarbon); but can be
derivatized into emulsifying forms
Cholesterol is basis for many of the others,
both conceptually and synthetically
Cholesterol:
Yes, you need
to memorize
this structure!
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Other lipids
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Waxes
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nonpolar esters of long-chain fatty acids
and long-chain monohydroxylic alcohols,
e.g H3C(CH2)nCOO(CH2)mCH3
Waterproof, high-melting-point lipids
Image
courtesy
cyberlipid.
org
Eicosanoids
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QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
oxygenated derivatives of C20
polyunsaturated fatty acids
Involved in signaling, response to
stressors
Non-membrane isoprenoids:
vitamins, hormones, terpenes
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Images
Courtesy
QuickTime™ and a
Oregon decompr
TIFF (Uncompressed)
are needed to see this pictu
State Hort.
& Crop
Sci.
p. 15 of 41
Example
of a wax
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Oleoyl
alcohol
esterified to
stearate
(G&G, fig.
8.15)
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Isoprene units: how they’re
employed in real molecules
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Can be linked head-to-tail
… or tail-to-tail (fig. 8.16, G&G)
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Membranes
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Fundamental biological mechanism for
separating cells and organelles from one
another
Highly selective barriers
Based on phospholipid or sphingolipid
bilayers
Contain many protein molecules too
(50-75% by mass)
Often contain substantial cholesterol too:
cf. modeling studies by H.L. Scott
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Solvent
Bilayers
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Self-assembling
roughly planar
structures
Bilayer lipids
are fully
extended
Aqueous above
and below,
apolar within
Solvent
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Fluid Mosaic Model
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Membrane is dynamic
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Protein and lipids diffuse laterally;
proteins generally slower than lipids
Some components don’t move as
much as the others
Flip-flops much slower than lateral
diffusion
Membranes are asymmetric
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Salmonella
ABC
transporter
MsbA
PDB 3B60
3.7Å
2*64 kDa
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Newly synthesized components
added to inner leaflet
Slow transitions to upper leaflet
(helped by flippases)
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Fluid Mosaic Model depicted
Courtesy C.Weaver, Menlo School
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Physical properties of
membranes
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Strongly influenced by % saturated fatty
acids: lower saturation means more
fluidity at low temperatures
Cholesterol percentage matters too:
disrupts ordered packing and increases
fluidity (mostly)
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Chemical compositions of
membranes (fig. 9.10, G&G)
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Lipid Rafts
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Cholesterol tends to associate with
sphingolipids because of their long saturated
chains
Typical membrane has blob-like regions rich in
cholesterol & sphingolipids surrounded by
regions that are primarily phospholipids
The mobility of the cholesterol-rich regions leads
to the term lipid raft
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Significance of lipid rafts:
still under discussion
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May play a role as regulators
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Sphingolipid-cholesterol clusters form in the
ER or Golgi and eventually move to the
outer leaflet of the plasma membrane
There they can govern protein-protein &
protein-lipid interactions
Necessary but insufficient for trafficking
May be involved in anaesthetic
functions:
Morrow & Parton (2005), Traffic 6: 725
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Membrane Proteins
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Many proteins associate with membranes
But they do it in several ways
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Integral membrane proteins:
considerable portion of protein is embedded in
membrane
Peripheral membrane proteins:
polar attachments to integral membrane
proteins or polar groups of lipids
Lipid-anchored proteins:
protein is covalently attached via a lipid anchor
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Integral
(Transmembrane)
Proteins
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Drawings courtesy
U.Texas
Span bilayer completely
May have 1 membrane-spanning
segment or several
Often isolated with detergents
7-transmembrane helical proteins
are very typical (e.g.
bacteriorhodopsin)
Beta-barrels with pore down the
center: porins
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Peripheral
Membrane proteins
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Also called extrinsic proteins
Associate with 1 face of
membrane
Associated via H-bonds, salt
bridges to polar components of
bilayer
Easier to disrupt membrane
interaction:
salt treatment or pH
09/25/08 Biochemistry: Lipid2/Membranes
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Chloroflexus
auracyanin
PDB 1QHQ
1.55Å
15.4 kDa
p. 28 of 41
Lipid-anchored
membrane proteins
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Protein-lipid covalent bond
Often involves amide or ester bond to
phospholipid
Others: cys—S—isoprenoid (prenyl) chain
Glycosyl phosphatidylinositol with glycans
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N- Myristoylation &
S-palmitoylation
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Membrane Transport
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What goes through and what doesn’t?
Nonpolar gases (CO2, O2) diffuse
Hydrophobic molecules and small
uncharged molecules mostly pass
freely
Charged molecules blocked
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Transmembrane Traffic:
Types of Transport (Table 9.3)
Type
Protein Saturable
Carrier w/substr.
Diffusion No
No
ChannelsYes
No
& pores
Passive Yes
Yes
transport
Active
Yes
Yes
Movement
Rel.to conc.
Down
Down
Energy
Input?
No
No
Down
No
Up
Yes
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Cartoons of transport types
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From accessexcellence.org
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Thermodynamics of
passive and active transport
• If you think of the transport as a chemical
reaction Ain  Aout or Aout  Ain
• It makes sense that the free energy
equation would look like this:
• Gtransport = RTln([Ain]/[Aout])
• More complex with charges;
see eqns. 9.4 through 9.6.
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Example
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Suppose [Aout] = 145 mM, [Ain] = 10 mM,
T = body temp = 310K
Gtransport = RT ln[Ain]/[Aout]
= 8.325 J mol-1K-1 * 310 K * ln(10/145)
= -6.9 kJ mol-1
So the energies involved are moderate
compared to ATP hydrolysis
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Charged species
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Charged species give rise to a factor that
looks at charge difference as well as
chemical potential (~concentration)
difference
Most cells export cations so the inside of
the cell is usually negatively charged
relative to the outside
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Quantitative treatment of
charge differences
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Membrane potential (in volts  J/coul):
Y = Yin - Yout
Gibbs free energy associated with change in
electrical potential is
Ge = zFY
where z is the charge being transported and F is
Faraday’s constant, 96485 JV-1mol-1
Faraday’s constant is a fancy name for 1.
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Faraday’s constant
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Relating energy per mole
to energy per coulomb:
Energy per mole of charges,
e.g. 1 J mol-1, is
1 J / (6.022*1023 charges)
Energy per coulomb, e.g, 1 V = 1 J coul-1, is
1 J / (6.241*1018 charges)
1 V / (J mol-1) =
(1/(6.241*1018)) / (1/(6.022*1023) = 96485
So F = 96485 J V-1mol-1
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Total free energy change
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Typically we have both a chemical
potential difference and an electrical
potential difference so
Gtransport = RTln([Ain]/[Aout]) + zFY
Sometimes these two effects are
opposite in sign, but not always
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Pores and channels
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Transmembrane proteins with central
passage for small molecules,
possibly charged, to pass through
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Rod MacKinnon
Bacterial: pore. Usually only weakly selective
Eukaryote: channel. Highly selective.
Usually the Gtransport is negative so they don’t
require external energy sources
Gated channels:
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Passage can be switched on
Highly selective, e.g. v(K+) >> v(Na+)
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Protein-facilitated
passive transport
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All involve negative Gtransport
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Uniport: 1 solute across
Symport: 2 solutes, same direction
Antiport: 2 solutes, opposite directions
Proteins that facilitate this are like
enzymes in that they speed up
reactions that would take place slowly
anyhow
These proteins can be inhibited,
reversibly or irreversibly
09/25/08 Biochemistry: Lipid2/Membranes
Diagram courtesy
Saint-Boniface U.
p. 41 of 41