Chapter 12- Lipids and Biological Membranes

Lipids

Water-insoluble biomolecules that are highly soluble in organic solvents

• most lipids are hydrophobic due to fatty acids

Fatty acids

Long hydrocarbon chains that terminate with carboxylic acid groups

• vary in length and degree of saturation

• Often referred to in their carboxylate form because they are ionized at physiological pH

Fatty acid naming

Derived from the parent hydrocarbon by substitution of oic for the final e

First number is the number of carbon atoms in the chain, and the second number is the number of double bonds

Fatty acid numbering can be done two ways:

Carbons can be numbered starting at the carboxyl terminal carbon atom.

• Carbon atoms 2 and 3 are often referred to as α and β, respectively.

• Position of a double bond can be represented by the symbol ∆ followed by a superscript number (examples:
  cis-∆9, trans-∆2).

• The methyl carbon atom at the distal end of the chain is called the omega (ω) carbon.

• Position of a double bond can be represented by counting from the distal end.

Chain Length and Degree of Saturation in fatty acid properties

Fatty acids in biological systems contain:

• an even number of carbon atoms between 14 and 24 (16 and 18 are most common).

• an unbranched hydrocarbon chain in animals.

• a saturated or unsaturated (with double bonds in cis configuration) alkyl chain.

Short chain length and the unsaturation enhance the fluidity of fatty acids and their derivatives.

• Unsaturated fatty acids have lower melting points than saturated fatty acids of the same length

• shorter chain length = lower melting point

Lipids function as:

• fuel molecules.

• highly concentrated energy stores.

• signal molecules and messengers in signal-transduction pathways.

• the essential component of biological membranes

Principal lipids in eukaryotic membranes are
phospholipids, glycolipids, and cholesterol.

Common attributes of membranes

• are sheetlike structures, two molecules thick, that form closed boundaries.

• lipids, small molecules with hydrophobic and hydrophilic moieties that form lipid bilayers.

• contain proteins embedded in lipid bilayers with distinct functions.

  • serve as pumps, channels, receptors, energy transducers, enzymes

• are asymmetric, noncovalent assemblies.

  • outer and inner sections of the membrane are very different from each other

• are fluid structures.

  • things can move around in the membrane

• tend to be electrically polarized.

  • sum of charges on the inside of a cell is different than the sum of charges on the outside of a cell
    

Phospholipid composition

• one or more fatty acids.

  • provides a hydrophobic barrier

• a platform to which the fatty acids are attached (examples: glycerol, sphingosine).

• a phosphate.

• an alcohol attached to the phosphate.

Sphingomyelin

Common membrane phospholipid with a sphingosine backbone instead of glycerol

Sphingosine

an amino alcohol that contains a long, unsaturated hydrocarbon chain

Glycolipids

Lipids containing a sphingosine backbone with 1+ sugars attached to the primary -OH group

• sugar residues are always on the extracellular side of the membrane

  • asymmetric

Cerebroside

Glycolipid containing a single glucose or galactose residue

Simplest glycolipid

Cholesterol

Lipid based on a steroid nucleus

• contains a linked hydrocarbon tail at one end and an -OH group at other end

• Oriented parallel to fatty acid chains of phospholipid in membranes

• -OH group interacts with phospholipid head groups

  • helps with fluidity of the membrane

Amphipathic (amphiphilic) molecules

Molecules that contain both a hydrophobic and hydrophilic moiety

Membrane lipids are amphipathic molecules

• hydrophobic moiety: fatty acid tails

• hydrophilic moiety: phosphorylcholine (polar head group)

How do phospholipids and glycoproteins form bimolecular sheets (membrane) in water?

Membrane formation is due to the amphipathic nature of the molecule

• micelle will form

Micelle

A globular structure with the polar head groups on the outside surface and hydrocarbon tails sequestered inside

Lipid bilayers (bimolecular sheet)

Two lipid sheets

• hydrophobic tails of each sheet interacting with one another, forming a permeability barrier

• Hydrophilic head groups interact with the aqueous medium.

True or false: lipid bilayer formation is spontaneous

True

• Phospholipids and glycolipids, because of the space taken up by their two tails, do not form small micelles the way single-tailed salts of fatty acids do.

• Phospholipids and glycolipids spontaneously form lipid bilayers in aqueous solutions, stabilized by:

  • hydrophobic interactions.

  • van der Waals interactions between hydrocarbon tails.

  • electrostatic and hydrogen-bonding attractions between polar head groups and water molecules.

  • Langmuir-Blodgett trough

Are bimolecular sheet or micelle formation favored?

Bimolecular sheets

Biological Consequences of hydrophobic interactions

• Lipid bilayers have an inherent tendency to be extensive.

• Lipid bilayers will tend to close on themselves so that there are no edges with exposed hydrocarbon chains.

  • forms compartments

• Lipid bilayers are self-sealing.

  • A hole in a bilayer is energetically unfavorable.

Lipid vesicles (liposomes)

• Aqueous compartments enclosed by a lipid bilayer

  • used to study membrane permeability or to deliver chemicals to cells

• Can be formed from phospholipids

• Liposomes containing trapped ions/molecules can be synthesized

  • Formed by suspending a lipid in aqueous medium and sonicating (agitating by high frequency sound waves).

  • Ions or molecules can be trapped in the aqueous compartments by forming the vesicles in their presence.

  • Specific membrane proteins can be embedded by solubilizing the proteins in the presence of detergents and then adding them to the phospholipids

Lipid bilayer permeability

• Lipid bilayer membranes have a very low permeability for ions and most polar molecules.

• Permeability of small molecules is correlated with their solubility in a nonpolar solvent relative to their solubility in water.

• Water is an exception due to its:

  • low molecular weight.

  • high concentration.

  • lack of complete charge.

Membrane proteins

• Carry out most membrane processes

• Allow transport of molecules and information across a membrane

  • establish compartments

• Membranes vary in protein content

  • Ranges from <20% to as much as 75%

• The types of membrane proteins in a cell are a reflection of the biochemistry occurring inside the cell

• Can be visualized by SDS-polyacrylamide gel electrophoresis

  • shows that membranes performing different functions contain different types of proteins

Integral membrane proteins

interact extensively with the hydrocarbon chains of membrane lipids

• released by agents that compete for these nonpolar
  interactions

• most span the lipid bilayer

Peripheral membrane proteins

bound to membranes primarily by electrostatic and hydrogen-bond interactions with the head groups of lipids

• disrupted by adding salts or by changing the pH

• often bound to the surfaces of integral proteins

• may be anchored to the lipid bilayer by a covalently attached hydrophobic chain

What are most common structural motifs in membrane proteins?

• Membrane-spanning α helices

  • Bacteriorhodopsin = integral membrane protein & light-powered proton pump in archaea

    • contain predominantly nonpolar amino acids in contact with the hydrocarbon core of the membrane or with one another

    • polar and charged residues tend to be found in the cytoplasmic and extracellular regions

• Beta strands

  • forms a channel protein

  • porin = protein from the outer membranes of bacteria

    • formed from a single anitparallel beta sheet curled up to form a pore or channel

    • outer surface is nonpolar

    • inner surface is hydrophilic and filled with water

    • contains alternating hydrophobic and hydrophilic amino acids along each Beta strand

Prostaglandin H₂ synthase-1

ER membrane-bound enzyme that promotes inflammation and modulates gastric acid secretion

• homodimer

• lies along the outer surface of the membrane

• bound by a set of α helices that extend from the bottom of the protein into the membrane

  • classified as an integral membrane protein even though it does not span the membrane because detergent is required to release the protein

    • linkage is strong enough that only the action of detergents can release the protein from the membrane

• catalyzes the formation of prostaglandin H₂

  • arachidonic acid = a hydrophobic molecule generated by the hydrolysis of membrane lipids

    • reached the Prostaglandin H₂ Synthase-1 Active Site Through a Hydrophobic channel

• Aspirin inhibits the cyclooxygenase activity of prostaglandin H₂ Synthase-1

  • Aspirin inhibits cyclooxygenase activity by transferring its acetyl group to a Ser 530 in prostaglandin H₂ synthase-1

    • Ser 530 lies along the path to the active site

    • Asprin blocks the channel

Lipid and many membrane protein diffusion

• Diffuse rapidly in the plane of the membrane

• Biological membranes are not rigid, static structures.

• lateral diffusion = process by which lipids and many membrane proteins are constantly in lateral motion

• can be visualized for proteins via the technique of fluorescence recovery after photobleaching (FRAP)

Fluorescence Recovery After Photobleaching (FRAP)

• A cell-surface component is labeled with a fluorescent chromophore and visualized via fluorescence microscopy.

• Fluorescent molecules in a region are destroyed (bleached) by an intense light pulse from a laser.

• Fluorescence is subsequently monitored as a function of time using a low light level.

  • prevents further bleaching
    

FRAP Recovery

a process resulting in an increase in the fluorescence intensity

• occurs if the labeled component is mobile because bleached molecules leave and unbleached molecules enter

• measures lateral diffusion

• Rate of recovery depends on the lateral mobility of the fluorescence-labeled component.

  • Average distance S traversed in time t depends on a diffusion coefficient, D according to the expression S = (4Dt)^1/2

Fluid mosaic model

Describes the biological membrane organization as 2-D solutions of oriented lipids and globular proteins

• lipid bilayer is both a solvent and permeability barrier

• Lipids rapidly diffuse laterally in membranes

• Transverse diffusion (flip-flopping is very slow)

• Proposed by Singer and Nicholson in 1972

Allows for lateral movement but not rotation through the membrane

Is lateral or transverse diffusion of lipids faster?

Lateral

Flip-flop (transverse diffusion) of a protein molecule has not been observed

• preserves membrane asymmetry

What is membrane fluidity controlled by?

Fatty acid composition and cholesterol content

• Many membrane processes depend on the fluidity of the membrane

  • depends on properties of fatty acid chains

• the transition from rigid to fluid state takes place above the melting temperature (T_m)

  • As temperature increases, membrane changes from a packed ordered state to a more random one

What does melting temperature of a fluid membrane depend on?

Degree of unsaturation

• Straight hydrocarbon chains of saturated fatty acid residues interact favorably with one another.

  • favors the rigid state

• A cis double bond produces a bend in the hydrocarbon
  chain interferes with a highly ordered packing of fatty acid chains.

  • lowers the Tm

Chain length

• Long hydrocarbon chains interact more strongly than short ones

  • each additional CH₂ group contributes about -2 kH mol to the free energy of interaction of two adjacent hydrocarbon chains

What disrupts the highly ordered packing of a fatty acid chain in a membrane?

Presence of cis double bonds

Cholesterol

• the bulky steroid nulceus of cholesterol disrupts the regular interactions between fatty acid chains

  • helps maintain proper membrane fluidity in membranes in animals

Lipid Rafts

Membrane domains containing such complexes that exhibit reduced fluidity

• may induce conformational changes in membrane proteins to regulating their functional activities

• may function in signal transduction by providing a
  favorable environment for specific protein–protein
  interactions

Highly dynamic complexes formed between cholesterol and specific lipids

• Cholesterol can form complexes with lipids that contain the sphingosine backbone and with lipid-anchored membrane proteins

True or false: all biological membrane are asymmetric

True

• The outer and inner surfaces of all biological membranes have different components and different enzymatic activities.

  • example: the Na+–K+ pump in the plasma membrane

  • ATP must be on the inside of the cell to drive the pump

    
    

What can bacteria be classified into based on their usage of biological membranes

• Gram positive = have a single membrane surrounded by a thick cell wall

• Gram negative = have two membranes separated by periplasm (which contains the cell wall)

Gram staining

Technique for classifying bacteria and distinguishing bacterial membranes

• crystal violet strain (purple) is added to bacteria

• iodine is added to trap the stain in the cell

• alcohol is added to wash out the stain

• a secondary stain (pink) is added to stain cells

• Gram positive cells stain purple because their thick cell wall retains the crystal violet stain

• Gram negative cells stain pink because their thin cell wall does not retain the crystal violet stain

Receptor-mediated endocytosis

process by which cells take up molecules from their environment

• Receptor binding induces membrane invagination by the action of intracellular clathrin and dynamin.

membrane budding and fusion are highly controlled processes

• Vesicle fusion to the plasma membrane is critical for neurotransmitter release from a neuron into the synaptic cleft

RME of transferrin receptor mediates cellular uptake of iron

• Free iron is toxic to cells and tightly bound to transferrin in the bloodstream.

• Complex formation between the transferrin receptor and iron-bound transferrin initiates receptor-mediated endocytosis.

• leads to internalization of these these complexes within vesicles (endosomes)

  
  

SNARE Complexes

Guide membrane fusion to ensure specificity

• Draw appropriate lipid bilayers together through the formation of tightly coiled four-helical bundles

• largely determine the compartment with which a vesicle will fuse

Initiate membrane fusion