Lipids II; Membranes

Andy HowardIntroductory Biochemistry

4 March 2008

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What we’ll discuss Lipids Phospholipids Plasmalogens Glycosphingolipids

Isoprenoids Steroids Other lipids

Membranes Bilayers Fluid mosaic model

Physical properties

Lipid Rafts

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Glycerophospholipids Also called phosphoglycerides Primary lipid constituents of membranes in most organisms

Simplest: phosphatides (3’phosphoesters)

Of greater significance: compounds in which phosphate is esterified both to glycerol and to something else with an —OH group on it

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Categories of glycerophospholipids Generally categorized first by the polar “head” group; secondarily by fatty acyl chains

Usually C-1 fatty acid is saturated

C-2 fatty acid is unsaturated

Think about structural consequences!

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Varieties of head groups

Variation on other phosphoester position

Ethanolamine (R1-4 = H) (—O—(CH2)2—NH3

+) Serine (R4 = COO-)(—O—CH2-CH-(COO-)—NH3

+) Methyl, dimethylethanolamine(—O—(CH2)2—NHm

+(CH3)2-m) Choline (R4=H, R1-3=CH3) (—O—(CH2)2—N(CH3)3

+) Glucose, glycerol . . .

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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

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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)

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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

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Plasmalogens Another major class besides phosphatidates

C1 linked via cis-vinyl ether linkage. n.b. The textbook figure 9.9 is correct; but it appears opposite the text related to sphingolipids, which is confusing.

Ordinary fatty acyl esterification at C2

Phosphatidylethanolamine at C3

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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

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Sphingolipids 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

Ceramides sphingosine at glycerol C3 Fatty acid linked via amideat glycerol C2

Sphingomyelins C2 and C3 as in ceramides C1 has phosphocholine

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Cerebrosides & gangliosides

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

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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

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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 syntheticallyCholesterol:Yes, you need to memorize this structure!

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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

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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

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Bilayers Self-assembling roughly planar structures

Bilayer lipids are fully extended

Aqueous above and below, apolar within

Solvent

Solvent

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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)

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Fluid Mosaic Model depicted

Courtesy C.Weaver, Menlo School

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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)

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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

Still somewhat controversial

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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

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Integral(Transmembrane) Proteins Span bilayer completely

May have 1 membrane-spanning segment or several

Often isolated with detergents 7-transmembrane helical proteinsare very typical (e.g. bacteriorhodopsin)

Beta-barrels with pore down the center: porins

Drawings courtesy U.Texas

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Other Membrane Proteins Peripheral membrane proteins

Associate with one face of membrane Easier to disrupt membrane interaction

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

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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

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Transmembrane Traffic:Types of Transport (Table 9.3)Type Protein Saturable Movement

EnergyCarrier w/substr. Rel.to

conc. Input?DiffusionNo No Down NoChannels Yes No Down No & poresPassive Yes Yes Down No transportActive Yes Yes Up Yes

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Cartoons of transport types

From accessexcellence.org

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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.

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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

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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

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

= in - out

(there’s an extra in eqn. 9.4) 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.

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Faraday’s constant Relating energy per moleto 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

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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

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Pores and channels

Transmembrane proteins with centralpassage 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

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Protein-facilitated passive transport All involve negative Gtransport

Uniport: one solute across Symport: two solutes, same direction Antiport: two 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.

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Kinetics of passive transport Michaelis-Menten saturation kinetics:v0 = Vmax[S]out/(Ktr + [S]out)

Vmax is velocity achieved with fully saturated transporter

Ktr is analogous to Michaelis constant:it’s the [S]out value for which half-maximal velocity is achieved.

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Primary active transport

Energy source is usually ATP or light Energy source directly contributes to overcoming concentration gradient Bacteriorhodopsin: light energy used to drive protons against concentration and charge gradient to enable ATP production

P-glycoprotein: ATP-driven active transport of many nasties out of the cell

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Secondary active transport Active transport of one solute is coupled to passive transport of another

Net energetics is (just barely) favorable

Generally involves antiport Bacterial lactose influx driven by proton efflux

Sodium gradient often used in animals

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Complex case: Na+/K+

pump Typically [Kin] = 140mM, [Kout] = 5mM,[Nain] = 10 mM, [Naout] = 145mM.

ATP-driven transporter:3 Na+ out for 2 K+ inper molecule of ATP hydrolyzed

3Na out: 3*6.9 kJmol-1,2K in: 2*8.6 kJmol-1

= 37.9 kJ mol-1 needed, ~ one ATP

Diagram courtesy

Steve Cook

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What’s this used for? Sodium gets pumped back in in symport with glucose, driving uphill glucose transport

That’s a separate passive transport protein called GluT1

Diagram courtesy

Steve Cook

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How do we transport big molecules? Proteins and other big molecules often internalized or secreted by endocytosis or exocytosis

Special types of lipid vesicles created for transport

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Receptor-mediated endocytosis Bind macromolecule to specific receptor in plasma membrane

Membrane invaginates, forming a vesicle surrounding the bound molecules (still on the outside)

Vesicle fuses with endosome and a lysozome Inside the lysozyome, the foreign material and the receptor get degraded

… or ligand or receptor or both get recycled

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Example: LDL-cholesterol

Diagram courtesyGwen Childs, U.Arkansas for Medical Sciences

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Exocytosis

Materials to be secreted are enclosed in vesicles by the Golgi apparatus

Vesicles fuse with plasma membrane

Contents released into extracellular space

Diagram courtesy LinkPublishing.com

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Transducing signals Plasma membranes contain receptors that allow the cell to respond to chemical stimuli that can’t cross the membrane

Bacteria can detect chemicals:if something useful comes along,a signal is passed from the receptor to the flagella, enabling the bacterium to swim toward the source

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Multicellular signaling

Hormones, neurotransmitters, growth factors all can travel to target cells and produce receptor signals

We’ll discuss this in detail after the midterm

Diagram courtesy Science Creative Quarterly, U. British Columbia