09/25/08Biochemistry: Lipid2/Membranes Lipids II; Membranes Andy Howard Introductory Biochemistry 25...

09/25/08Biochemistry: Lipid2/Membranes

Lipids II; Membranes

Andy HowardIntroductory Biochemistry

25 September 2008

09/25/08 Biochemistry: Lipid2/Membranes p. 2 of 41

What we’ll discuss Lipids

Plasmalogens Glycosphingolipids Isoprenoids Steroids Other lipids

Membranes Bilayers Fluid mosaic model Physical properties Lipid Rafts Membrane proteins

Membrane transport Passive & active Thermodynamics Pores & Channels Protein-mediated

transport


iClicker quiz question 1

What is the most common fatty acid in soybean triglycerides? (a) Hexadecanoate (b) Octadecanoate (c) cis,cis-9,12-octadecadienoate (d) all cis-5,8,11,14-eicosatetraeneoate (e) None of the above


iClicker quiz, question 2 Which set of fatty acids would you

expect to melt on your breakfast table? (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)


iClicker quiz question 3 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? (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


Plasmalogens 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


Specific plasmalogens


Roles of phospholipids 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


Sphingolipids Second-most abundant membrane

lipids in eukaryotes Absent in most bacteria Backbone is sphingosine:

unbranched C18 alcohol More hydrophobic than phospholipids


Varieties of sphingolipids

Ceramides sphingosine at glycerol

C3 Fatty acid linked via

amideat glycerol C2

Sphingomyelins C2 and C3 as in

ceramides C1 has phosphocholine

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

SphingomyelinImage on steve.gb.com


Cerebrosides Ceramides with one

saccharide unit attached by -glycosidic linkage at C1 of glycerol

Galactocerebrosides common in nervous tissue


Gangliosides Anionic derivs of cerebrosides (NeuNAc) Provide surface markers for cell recognition

and cell-cell communication


Isoprenoids

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


Steroids 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!


Other lipids Waxes

nonpolar esters of long-chain fatty acids and long-chain monohydroxylic alcohols, e.g H3C(CH2)nCOO(CH2)mCH3

Waterproof, high-melting-point lipids Eicosanoids

oxygenated derivatives of C20

polyunsaturated fatty acids Involved in signaling, response to

stressors Non-membrane isoprenoids:

vitamins, hormones, terpenes



Image courtesy cyberlipid.org



Images Courtesy Oregon State Hort. & Crop Sci.


Example of a wax Oleoyl

alcohol esterified to stearate (G&G, fig. 8.15)


Isoprene units: how they’re employed in real molecules

Can be linked head-to-tail … or tail-to-tail (fig. 8.16, G&G)


Membranes 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


Bilayers Self-assembling

roughly planar structures

Bilayer lipids are fully extended

Aqueous above and below, apolar within

Solvent

Solvent


Fluid Mosaic Model Membrane is dynamic

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 Newly synthesized components

added to inner leaflet Slow transitions to upper leaflet

(helped by flippases)



Salmonella ABC transporter MsbAPDB 3B603.7Å2*64 kDa


Fluid Mosaic Model depicted

Courtesy C.Weaver, Menlo School


Physical properties of membranes 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)


Chemical compositions of membranes (fig. 9.10, G&G)


Lipid Rafts

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


Significance of lipid rafts:still under discussion

May play a role as regulators 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


Membrane Proteins Many proteins associate with membranes But they do it in several ways

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


Integral(Transmembrane) Proteins

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

Drawings courtesy U.Texas


Peripheral Membrane proteins 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



Chloroflexus auracyaninPDB 1QHQ1.55Å15.4 kDa


Lipid-anchored membrane proteins

Protein-lipid covalent bond Often involves amide or ester bond to

phospholipid Others: cys—S—isoprenoid (prenyl) chain Glycosyl phosphatidylinositol with glycans


N- Myristoylation & S-palmitoylation


Membrane Transport

What goes through and what doesn’t? Nonpolar gases (CO2, O2) diffuse Hydrophobic molecules and small

uncharged molecules mostly pass freely

Charged molecules blocked


Transmembrane Traffic:Types of Transport (Table 9.3)

Type Protein Saturable Movement Energy

Carrier w/substr. Rel.to conc. Input?

Diffusion No No Down NoChannels Yes No Down No & poresPassive Yes Yes Down No transport

Active Yes Yes Up Yes


Cartoons of transport types

From accessexcellence.org


Thermodynamics ofpassive 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.


Example 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


Charged species

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


Quantitative treatment of charge differences Membrane potential (in volts J/coul):

= in - out

Gibbs free energy associated with change in electrical potential isGe = zFwhere 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.


Faraday’s constant Relating energy per mole

to energy per coulomb: Energy per mole of charges,

e.g. 1 J mol-1, is1 J / (6.022*1023 charges)

Energy per coulomb, e.g, 1 V = 1 J coul-1, is1 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


Total free energy change

Typically we have both a chemical potential difference and an electrical potential difference so

Gtransport = RTln([Ain]/[Aout]) + zF Sometimes these two effects are

opposite in sign, but not always


Pores and channels Transmembrane proteins with central

passage for small molecules,possibly charged, to pass through 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: Passage can be switched on Highly selective, e.g. v(K+) >> v(Na+)

Rod MacKinnon


Protein-facilitated passive transport

All involve negative Gtransport

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

Diagram courtesySaint-Boniface U.

09/25/08Biochemistry: Lipid2/Membranes Lipids II; Membranes Andy Howard Introductory Biochemistry 25...

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