2. Fatty Acid tails
• Fatty acids are carboxylic acids with a hydrocarbon chain (C4 to C36)
• Typical membrane fatty acids range from 10 to 24 Carbons long
• Hydrocarbon portion is saturated or partially unsaturated
3. Acyl Chains
• Vary in length
• Vary in Saturation
-double bond restricts rotation
• Van der Waals forces
-double bond interrupts
6. – because of restricted rotation about a carbon-
carbon double bond, an alkene with two different
groups on each carbon of the double bond shows
cis-trans isomerism.
Cis-Trans Isomerism
7. Structural Aspects
• Double bonds in unsaturated FA in nature are in cis configuration
• Length and saturation determine melting point
• At RT, C12:0 to C24:0 are waxy, whereas unsaturated FA are oily
30° angle
9. 5.5
HH
O
A difference in the electronegativities
of the atoms in a bond creates a
polar bond.
Partial charges result from
bond polarization.
A polar covalent bond is a covalent
bond in which the electrons are not
equally shared, but rather displaced
toward the more electronegative atom.
10. Although all covalent bonds involve sharing of electron pairs, they differ in the
degree of sharing:
nonpolar covalent bond:nonpolar covalent bond: electrons are shared equally
polar covalent bond:polar covalent bond: electron sharing is not equal
11. Intermolecular Forces
The strength of attractive forces between molecules
determines whether any sample of matter is a gas, liquid,
or solid.
Near STP, the forces of attraction between molecules of most gases are so small that
they can be ignored. When T decreases and/or P increases, the forces of attraction
become important to the point that they cause condensation and ultimately solidification.
London forces/ Vander
Dipole-dipole interactions
Hydrogen bonding
13. London Dispersion Forces
– Exist between all atoms and molecules.
– They are the only forces of attraction between
nonpolar molecules.
– This force increases as the mass and number of
electrons increases.
– Even though these forces are very weak, they
contribute significantly to the attractive forces
between large molecules.
14. Figure 2.3 Differential scanning calorimetry
shows gel-liquid crystal transitions vary with
acyl chain composition.
14 C
18 C
16 C
18:1 C
15. Phase changes and packing
• Differences is in the degree of
ordering
• In the gel state, the acyl chains
are fairly ordered, with a high
trans/gauche ratio
• In the liquid state more
rotational freedom allows for
configuration of the
hydrocarbon chain
• Double bonds disrupt packing
19. Hydrogenation
• Hardening: reduction of some or all of the
carbon-carbon double bonds of an
unsaturated triglyceride using H2/metal
catalyst.
– In practice, the degree of hardening is carefully
controlled to produce fats of a desired consistency.
– The resulting fats are sold for kitchen use (Crisco,
Spry, Dexo, and others).
– Margarine and other butter substitutes are
produced by partial hydrogenation of
polyunsaturated oils derived from corn,
cottonseed, peanut, and soybean oils.
20. Hydrogenation of Unsaturated
Fats
• Oils can be converted to semi-solids through
hydrogenation that converts the double bonds to
single bonds
• In the process, some double bonds are converted
to trans form
22. 22
fatty acid nomenclature
• Saturated – no C-C double bonds
• Unsaturated – C-C double bonds
• Chain length is the # of carbons
• A naming convention is:
Chain length: # double bonds
16:0 = palmitic
18:2 (∆9,12
) = linoleic
• Common pattern to location of double bonds
• Even numbers of carbons are more common
23.
24. 24
physical properties: length & saturation
C skeleton name source mp
12:0 lauric acid coconut
palm kernel
44 °C
20:0 arachidic
acid
peanut 77 °C
18:2
(Δ9,12)
linoleic acid safflower 1-5 °C
18:3
(Δ9,12,15)
linolenic acid vegetable oils -11 °C
20:4
(Δ5,8,11,14)
arachidonic acid phospholipids -50 °C
20:5
(Δ5,8,11,14, 17)
eicosapentanoic
acid
marine fish ? °C
25.
26. Membrane lipids
• Over 500 different species of fatty acids
have been identified
• Over 100 species of lipids in a typical
biomembrane
• Acyl chains usually range from 10-24
carbons long
• Free fatty acids make up only 1% of total
• Most are in complex lipids
• If the fatty acid is part of a complex lipid,
ending changes to -oyl
27. Biological Membranes
• Bilayers composed of phospholipids
(glycerophospholipids) and spingolipids and sterols
• Membranes are flexible, self-sealing, selectively permeable
• 5 to 8 nm thick
29. Membrane lipids
• Phospholipids and spingolipids are main components
(amphipathic)
• Glycolipids could be considered a separate class
• Polar end and a hydrophobic end (spontaneous formation of
bilayers)
• Polar end = ethanolamine, choline, serine, inositol-phosphate
• Spingolipids contain spingosine (amino alcohol)
30. 30
Glycerophospholipids
• Most abundant lipid in
biological membranes
• Polar or charged
group is attached to
the third carbon of
glycerol
– Basis for
nomenclature
• Acyl group on C1
often saturated and
16-18 carbons,
• Acyl on C2 is often
unsaturated.
32. Nomenclature
• The abbreviations of the phospholipids are
coupled with the common names of the
acyl chains
• Sooo… dioleoylphosphatidylcholine is
DOPC
• When a phospholipid loses an acyl chain it
becomes a lysophospholipid with
increased water solubility and detergent
activity
33. 33
Sphingolipids
• One fatty acid joined to
sphingosine in an amide
linkage
• Acyl chain is typically
saturated between 16-24
carbons long
• Sphingomyelin, cerebroside,
gangliosides
• Sphingomyelin - animal cells,
especially myelin
• Some involved in signal
transduction & cell surface
recognition
glycosphingolipids
36. Sterols and linear isopenoids
• Linear isoprenyl groups are
used to anchor certain
proteins to the bilayer
• Derived from five carbon unit isoprene
• Cholesterol content
in cells varies from
0-25%
• Eukaryotic cells
have up to 90%
cholesterol in
plasma membranes
37. Close packing of cholesterol with lipids
• Rigid sterol imposes
conformational order
on neighboring lipids
• Larger headgroups of
phospholipids form an
“umbrella”
• Can align better next
to saturated fatty acids
• Makes membrane
thicker
• Decreases
translational diffusion
38. Diffusion of bilayer lipids
• Rotational – spinning of a single molecule
on its axis
• Lateral – neighboring molecules exchange
positions via Brownian motion
• Transverse – exchange of lipid molecules
between leaflets, aka “flip-flop”
39. 39
Lateral Diffusion
FRAP – Fluorescence recovery after
photobleaching
•Diffusion rates are typically around
10-7
to 10-8
cm2
/sec in model
membrane systems
40. Hop diffusion from tracking single
labeled lipid
• In biological membranes
diffusion rates are slightly
slower over the whole
surface but comparable
within compartments
• Hop diffusion technique
follows the path of single
gold-labeled DOPE
molecule on the surface of
the cell.
41. Transverse diffusion
• Is the exchange of lipid molecules
between leaflets and is commonly
called “flip-flop”
• Energetically unfavored
42. Asymmetric distribution of lipids
Figure 2.14 erythrocyte
• Outer membrane is 6-
fold higher in
sphingolipids
• Interdigitation decreases
membrane thickness
• Cholesterol stabilizes
acyl chains in most
extended form
43. Polymorphic phases of phospholipids
• Bilayer is only one of the
possible lipid aggregates
that form spontaneouly
S = Cross-section area of
polar headgroup x lipid
length/ lipid volume
• For cylindrical S = 1,
conical S > 1, wedge-
shaped S < 1
• Important for fusion,
endocytosis, crystallization
47. Lipid Rafts
• Lipid ordered phase =
Lo
• A Eukaryote cell have
1 million rafts
• Evidence of rafts in
intracellular
membranes
• Outer and inner
leaflet?
Colocolization?
• High in cholesterol,
saturated FA, proteins
• Treatment with β-
cyclodextrin
• Size range from 10
nm to 200 nm
• Detergent resistant?
• Used by the golgi for
protein trafficking
• Some contain
caveolae
• Important in signaling
50. • Some lipid rafts contain the protein caveolin
• Each caveolin monomer has a central
hydrophobic domain and three long-chain
acyl groups (red)
• Several caveolin dimers cause a curvature in
the bilayer, forming a caveola.
• May function in protein trafficking and signal
transduction
• Evidence suggests similar process happened
for viral envelop formation
cholesterol
51. NT TN
F
Sterol X
A B C
• nSMase (green), lipid raft (red), colocalization of nSMase (yellow)
• (A) Control, (B) TNFα (200 ng/ml), (C) Compound X isolated from AKBB (5 μg/ml)
and TNFα
Disruption of lipid rafts
52. •nSMase activity stimulates a rapid accumulation of ceramide through
cleavage of sphingomyelin
•sphingolipid metabolism is altered in adipose tissue and plasma in
obese animal models
nSMase
Phospholipid membrane
SMSM
FANFAN
nSMasenSMase
CeramideCeramide
TNFαTNFα
TNFαR
53. Disruption of lipid rafts by compounds in AKBB
β-sitosterol
Cholesterol
Ursolic acid
54. Sources of Lipids
• Biosynthesis from acetyl-CoA
• Dietary intake
• Limited ability to synthesize
unsaturated fatty acids (humans)
55. Membrane biosynthesis
- Mainly occurs on surface of smooth ER
and the mitochondrial inner membrane
1.Synthesis of backbone molecule (glycerol
or sphingosine)
2.Attachment of fatty acids to the backbone
through an ester or amide linkage
3.Addition of a hydrophilic head group
4.in some cases, alteration or exchange of
the head group
56. First step
• Shared with formation of
triglycerides
• Majority of Glycerol 3-
phosphate is obtained by
reduction of
dihydroxyacetone
phosphate
• Acylation of 2 free hydroxyl
groups of L-glycerol 3-
phosphate by 2 molecules
of fatty acyl CO-A to yeild
phosphatidic acid
aka diacylglycerol 3-phophate
58. 2 ways to attach polar head group
• Attached with a phosphodiester
bond
• One of the hydroxyls is first
activated by attachment of a
nucleotide, cytidine diphosphate
1) The CDP-diacylglycerol is
attached either to the
diacylglycerol, forming the
activated phophatidic acid, CDP-
diacylglyerol or
2) The hydroxyl of the head group.
59. • Phosphatidic acid is the precursor of both triacylglycerols and
glycerophospholipids.
60. Summary of the pathways for synthesis
of major phospholipids.
•The pathways vary in different classes of
organisms.
•pathways used by mammals are highlighted
in yellow; those used by bacteria and yeast
are highlighted in pink. Orange highlighting
shows where the paths overlap.
•In mammals, phosphatidylethanolamine and
phosphatidylcholine are synthesized by a
pathway employing diacylglycerol and the
CDP-derivative of the appropriate head
group.
•Conversion of phosphatidylethanolamine to
phosphatidylcholine in mammals takes place
only in the liver.
61. Biosynthesis of
Sphingolipids
1. Synthesis of the 18
carbon amine
sphingamine from
palmitoyl-CoA and
serine
2. Attachment of a fatty
acid in an amide
linkage
3. Desaturation of the
sphingamine moiety to
form N-
acylsphingosine
(ceramide)
4. Attachment of a head
group to produce a
FIGURE 10-2cd The packing of fatty acids into stable aggregates. The extent of packing depends on the degree of saturation. (c) Fully saturated fatty acids in the extended form pack into nearly crystalline arrays, stabilized by many hydrophobic interactions. (d) The presence of one or more fatty acids with cis double bonds interferes with this tight packing and results in less stable aggregates.
COO- in neutral pH
Phosphatidylcholines. Phospholipids with choline as the head group.
2-methacryloyloxyethyl phosphorylcholine = PMPC
Measures heat consumption as the temperature is increased. Peaks correspond to melting points
High temperature bacterial membranes re enriched in saturated and longer chained fatty acids
Systematic names rarely used.
FIGURE 11-3 Fluid mosaic model for membrane structure. The fatty acyl chains in the interior of the membrane form a fluid, hydrophobic region. Integral proteins float in this sea of lipid, held by hydrophobic interactions with their nonpolar amino acid side chains. Both proteins and lipids are free to move laterally in the plane of the bilayer, but movement of either from one leaflet of the bilayer to the other is restricted. The carbohydrate moieties attached to some proteins and lipids of the plasma membrane are exposed on the extracellular surface of the membrane.
FIGURE 10-7 Some common types of storage and membrane lipids. All the lipid types shown here have either glycerol or sphingosine as the backbone (pink screen), to which are attached one or more long-chain alkyl groups (yellow) and a polar head group (blue). In triacylglycerols, glycerophospholipids, galactolipids, and sulfolipids, the alkyl groups are fatty acids in ester linkage. Sphingolipids contain a single fatty acid, in amide linkage to the sphingosine backbone. The membrane lipids of archaea are variable; that shown here has two very long, branched alkyl chains, each end in ether linkage with a glycerol moiety. In phospholipids the polar head group is joined through a phosphodiester, whereas glycolipids have a direct glycosidic linkage between the head-group sugar and the backbone glycerol.
Glycolipids are phospholipids or sphingolipids with oligosaccharide headgroups
Ester linkages
glycerol-based phospholipids
Derivatives of phosphatidic acid (parent compound)
Anionic PL have a net negative charge
Zwitterion are neutral (PE and PC)
PE and PS can participate in hydrogen bonding
Membrane glycerolipids are a category of amphiphilic molecules having a 3-carbon glycerol scaffold (each carbon is numbered following the stereospecific numbering nomenclature sn-1, sn-2, sn-3), harboring one or two hydrophobic acyl chains esterified at positions sn-1 and sn-2, and a hydrophilic polar head at position sn-3.
Loses as chain by the activity of phospholipase
Biological role as enigmatic as a sphinx
Over 60 different sphingolipids in humans
Phosphodiester linkage in some cases and glycosidic linkage in others
Sphingosine instead of glycerol (long chain amino alcohol)
Differ in the polar head group attached to C-1
Ceramide is the parent molecule
Sphingomyelin classified as a phospholipid because of phosphocholine side chain
Myelin is membranous sheath surrounds and insulates axons of some neurons
Glycosphingolipids have head groups with one or more sugar… subclasses of this are:
Cerebrosides – sugar linked to ceramide, if galactose it is heavy in neural tissue, glucose prominent in non-neural tissues
Globoside have 2 sugar or more sugars
Gangliosides have oligosaccharides as polar group and a sialic acid giving it a negative charge
Cerebrosides and globosides are collectively known as glycosphingolipids.
Associated with multiple sclerosis
Johann Thudichum discovered sphingolipids in 1874 named after the sphinx
Cerebrosides and globosides are collectively known as glycosphingolipids.
Carbohydrate moieties of sphingolipids define blood type
Cholesterol s
is the random motion of particles suspended in a fluid
FIGURE 11-17 Measurement of lateral diffusion rates of lipids by fluorescence recovery after photobleaching (FRAP). Lipids in the outer leaflet of the plasma membrane are labeled by reaction with a membrane-impermeant fluorescent probe (red), so the surface is uniformly labeled when viewed with a fluorescence microscope. A small area is bleached by irradiation with an intense laser beam and becomes nonfluorescent. With the passage of time, labeled lipid molecules diffuse into the bleached region, and it again becomes fluorescent. Researchers can track the time course of fluorescence return and determine a diffusion coefficient for the labeled lipid. The diffusion rates are typically high; a lipid moving at this speed could circumnavigate an E. coli cell in one second. (The FRAP method can also be used to measure lateral diffusion of membrane proteins.)
This is in model systems: biological membranes are complex mixture
the size of the polar head by comparison of the hydrophobic acyl-glycerol backbone affects lipid behavior in aqueous dispersions
large negative curvature lipids favor HII, positive curvature favor HI, small curvature favor lamellar
HII and cubic have been observed in ER of different cells
Other phases have been observed in highly curved membranes, like tubules and the inner mitochondrial membrane
HII forming lipids are able to switch from a HII phase to a lamellar phase sometimes through an intermediate cubic phase by lowering the temperature
In in vitro systems, aqueous dispersions of lipids are however able to form a large variety of other phases such as non-liquid crystalline and non-bilayer phases. These “solid phases” (also called “gel phases”) are favored by low temperature and long and saturated fatty acid chains. They were mainly characterized on model membranes of saturated phosphatidylcholine (PC; for example, see Mason, 1998). However, natural lipids are mostly unsaturated and organ- isms adapt their fatty acid composition to the environment to prevent the formation of gel phases. Moreover, even though gel phase domains were detected in biological membranes in very spe- cific cases such as the myelin sheath (Ruocco and Shipley, 1984) or in the stratum corneum (Norlen, 2001b), most biological mem- branes are organized in liquid phase.
FIGURE 11-4 Amphipathic lipid aggregates that form in water. (a) In micelles, the hydrophobic chains of the fatty acids are sequestered at the core of the sphere. There is virtually no water in the hydrophobic interior. (b) In an open bilayer, all acyl side chains except those at the edges of the sheet are protected from interaction with water. (c) When a two-dimensional bilayer folds on itself, it forms a closed bilayer, a three-dimensional hollow vesicle (liposome) enclosing an aqueous cavity.
Membrane fusion between phospholipid bilayers can be induced by the HII lipids, PA, and PS, in conjunction with Ca2+ (for a review see Papahadjopoulos et al., 1990) or by dehydration, that drives bilayers into very close contact
Stalk formation is promoted by an HII forming lipid like PE
for the membrane fusion process share at least one intermediate structure called the fusion stalk
Evidence that exist in both leaflets, may be colocalization between inner and outer membrane. Perhaps by interdigitation of lipids from outer leaflet into inner. Outer leaflet likely forms first and induces lower leaflet. Proteins can span the membrane.
Clustering of proteins likely induce lipid raft formation or at least increase size.
High in sphingolipids
Lipid ordered- fatty acids in extended anti configuration, tightly packed.
Cholesterol tightens packing and lends to order
Detergents like triton x – 100 produce insoluble membrane fraction
Lower Temperature at which rafts are extracted might influence Liquid order state and type of detergent used as well
FIGURE 11-20a Membrane microdomains (rafts). (a) Stable associations of sphingolipids and cholesterol in the outer leaflet produce a microdomain, slightly thicker than other membrane regions, that is enriched with specific types of membrane proteins. GPI-linked proteins are common in the outer leaflet of these rafts, and proteins with one or several covalently attached long-chain acyl groups are common in the inner leaflet. Caveolin is especially common in inwardly curved rafts called caveolae (see Figure 11-21). Proteins with attached prenyl groups (such as Ras; see Box 12-2) tend to be excluded from rafts.
Small rafts around 50 nm have lifetimes less than 1 msec
A lot of small rafts will often merge to reduce line tension, favors entropy.
FIGURE 11-20b Membrane microdomains (rafts). (b) In this artificial membrane—reconstituted (on a mica surface) from cholesterol, synthetic phospholipid (dioleoylphosphatidylcholine), and the GPI-linked protein placental alkaline phosphatase—the greater thickness of raft regions is visualized by atomic force microscopy (see Box 11-1). The rafts protrude from a lipid bilayer ocean (the black surface is the top of the upper monolayer); sharp peaks represent GPI-linked proteins. Note that these peaks are found almost exclusively in the rafts.
FIGURE 11-21 Caveolin forces inward curvature of a membrane. Caveolae are small invaginations in the plasma membrane, as seen in (a) an electron micrograph of an adipocyte surface-labeled with an electron-dense marker. (b) Each caveolin monomer has a central hydrophobic domain and three long-chain acyl groups (red), which hold the molecule to the inside of the plasma membrane. When several caveolin dimers are concentrated in a small region (a raft), they force a curvature in the lipid bilayer, forming a caveola. Cholesterol molecules in the bilayer are shown in orange.
All cells were treated with 200 ng/ml of TNFα except for Control
Alexa Fluor
594 conjugate of cholera toxin subunit B (CT-B) (Table 1). This
CT-B conjugate binds to the pentasaccharide chain of plasma
membrane ganglioside GM1, which selectively partitions into lipid
rafts.12,34
Ceramide= signaling molecule involved programmed cell death and apoptosis
such as TNF receptor I with nSMase.
The subsequent accumulation of ceramide through nSMase activation serves to coalesce lipid rafts into larger platforms involved in the signaling cascade leading to apoptosis
a phosphorylation cascade induced by insulin binding to its receptor activates the phosphoinositide 3-kinases and the G protein, TC10. The later binds and recruits an exocyst protein complex (APS-CAP-Cbl) integral to the docking of Glut4 at lipid raft domains. assembly is necessary for Glut4 translocation and glucose uptake (Inoue et al., 2006). Fujimura et al. (2006) found a receptor associated with lipid rafts that bind with Epigallocatechin-3-O-gallate (EGCG), a potent flavonoid in green tea.
APS is a Cbl-binding protein that is tyrosine phosphorylated by the insulin receptor kinase
FIGURE 21-18 Phosphatidic acid in lipid biosynthesis. Phosphatidic acid is the precursor of both triacylglycerols and glycerophospholipids. The mechanisms for head-group attachment in phospholipid synthesis are described later in this section.
FIGURE 21-29 Summary of the pathways for synthesis of major phospholipids. The pathways vary in different classes of organisms. In this diagram, pathways used by mammals are highlighted in yellow; those used by bacteria and yeast are highlighted in pink. Orange highlighting shows where the paths overlap. In mammals, phosphatidylethanolamine and phosphatidylcholine are synthesized by a pathway employing diacylglycerol and the CDP-derivative of the appropriate head group. Conversion of phosphatidylethanolamine to phosphatidylcholine in mammals takes place only in the liver. The pathways for phosphatidylserine synthesis for various classes of organisms are detailed in Figures 21-27 and 21-28.
Enzyme capable of hydrolyzing a specific bond
Tay-Sachs
Niemann-Pick defect in sphingomyelinase and build of sphingomyelin retardation
Many result in neurological disorders and mental retardation because sphingolipids heavy concentration in brain
BOX 10-2 FIGURE 1 Pathways for the breakdown of GM1, globoside, and sphingomyelin to ceramide. A defect in the enzyme hydrolyzing a particular step is indicated by a red circled &quot;×&quot;; the disease that results from accumulation of the partial breakdown product is noted.
