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BL3303 > Lipids > Flashcards

Lipids Flashcards

1
Q

What are the functions of biomembranes?

A
  • Barrier between cell and its environmement
  • Protein constituents allow selective permeability and uptake
  • Communication between cell and its environment
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2
Q

What are the characteristics of the phospholipid bi-layer?

A
  • Charged hyrophilic head groups form H bonds with water
  • neutral hydrocarbon chains are hydrophobic associate via van der Waals forces (AMPHIPATHIC)
  • Energetically favorable arrangement
  • Impermeable to macromolecules and small charged ions
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3
Q

What are the two physical states in which the phospholipid bi-layer exists?

A
  • Gel-like state at low temperatures

- Liquid-crystalline state at higher temperatures (membranes MUST exist like this)

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4
Q

What is the temperature at which a membrane changes from the gel-like state to the liquid crystalline state?

A

Transition temperature

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5
Q

What are the six classes of membrane lipids?

A
  • Fatty acids
  • Triacyglycerols
  • Glycerophospholipids
  • Sphingolipids
  • Waxes
  • Isoprene-based lipids
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6
Q

What are fatty acids?

A

Alkyl chains terminated by carboxylic acid groups

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7
Q

What are the characteristics of fatty acids which affect the transition temperature?

A
  • Degree of saturation (double bonds decrease van der waals force)
  • Chain length (longer chains mean more interactions)
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8
Q

What are the two main types of phospholipids?

A

Phosphoglycerides and phosphoceramides

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9
Q

What are the characteristics of phosphoglycerides?

A

Based on glycerol backbone

- Glycerol substituted for phosphodiester group attached to an alcohol headgroup and two ester linked fatty acids

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10
Q

What is the common precursor of phospholipids?

A

Phosphatic acid

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11
Q

How are triacylglycerols used for energy?

A
  • Hydrolysed by lipase to fatty acids and glycerol and released from adipose tissue to be released to energy requiring tissues
  • Fatty acids broken down step by step to Acetyl CoA which is processed in the citric acid cycle
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12
Q

What are the six most common head groups of phospholipids?

A
  • Phosphatic acid (PA)
  • Phosphatiydyserine (PS)
  • Phosphatidylenthanolamine (PE)
  • Phosphatidylcholine (PC)
  • Phosphtidiylinositol (PI)
  • Phosphadidylglycerol (PG)
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13
Q

What can phosphoinositol further synthesize?

A

Can be phsophorylated by photolipase C to form DAG and IP3 which are both important second messengers

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14
Q

What is the structure of cardioplin/PG?

A
  • dimeric structure with four acyl groups and two negative charges
  • Found in membranes of bacteria and mitochondria, mainly on the inner layer where it interacts with a large number of mitochondrial proteins especially those related to oxidative phosphorylation
  • Cone shape allows curvatures
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15
Q

What are the characteristics of sphingolipids?

A
  • Sphingosine with acyl chain and head chain that is either ethanolaminephosphoceramide (EPC), shingomyelin (SM) or inositolphosphoceramide (IPC)
  • More stable and less prone to hydrolysis, hence found on outer thicker membranes
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16
Q

What are the a) the simplest and b) most common gylcolipids?

A

a) glucosyceramide

b) GM2 ganglioside

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17
Q

What are waxes?

A
  • esters of long chain alcohols with long chain fatty acids

- Insoluble in water and so are used to keep moisture in/out

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18
Q

What are terpenes and what is included in this classs?

A

Lipids formed by isoprene units

  • steroids are terpene derived
  • odors are often monoterpenes due to their volatility (e.g menthol)
  • retinol is a diterpene
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19
Q

What are the functions of cholesterol?

A
  • Stabilise lipid membranes by immobilising first few hydrocarbon groups of phospholipid molecules, acting as a buffer for sudden changes in temperature of the environment by preventing phase shifts/ crystallisation of hydrocarbons
  • In cell membranes regulate cell signalling
  • Precursor to vitamin D, sex hormones and corticosteroids
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20
Q

What is the structure of the lysosome membrane?

A
  • Single membrane
  • Hydrolytic enzymes degrade proteins and lipids
  • low pH maintained by membrane enzyme which pumps H+ into the lumen
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21
Q

What is the structure of the mitochondrial membrane?

A
  • Inner membrane is 76% protein
  • high proportion of cardioplin
  • outer membrane is porous
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22
Q

Describe fatty acid synthesis

A
  • Takes place in the cytoplasm
  • Intermediates liked to acyl carrier protein (ACP) and enzymes joined by single polypeptide of fatty acid synthase
  • Fatty acid chain grown through seuqential addition of 2xC units from acetyl CoA (donated by malonyl ACP) driven by the release of CO2
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23
Q

How are proteins targeted to the membrane?

A

Through covalent lipid modifications: fatty acylation, prenylation, addition of GPI anchors (present on all eukaryote outer membranes) or cholesterol groups

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24
Q

What are the 3 types of fatty acylation?

A
  • N-meristoylation
  • S-Palmitoylation
  • N-Palmitoylation
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25
Q

What are the 2 types of prenylation?

A
  • Farnesylation

- Geranylgeranylation

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26
Q

What is the strucutre of a GPI anchor?

A
  • Phosphoinositol is a phosphlipid which extends the membrane with two fatty acyl chains which extend into the bi-layer
  • Glycosylated to a variable number of sugar units and a phoethanolamine unit which creates an amide link to the COOH terminal of the amino acid
27
Q

How does a protein become anchored by GPI?

A
  • Protein undergoes transamilation reaction within the ER
  • Cleaved with part remaining in the ER and part being attached to the GPI anchor
  • This will then leave the ER through the golgi destined for the membrane and is also usually heavily glycosylated
28
Q

Which types of proteins are anchored by GPI?

A
  • No apparent pattern but more common in protozoa
  • Include enzymes, protozoan coats, adhesion molecules and receptors
  • Some can exist both GPI and transmembrane
29
Q

What is protein N-myristoylation?

A
  • Addition of myristic acid to glycine through an amide linkage
  • Protein made in the ER has initiator methionine cleaved off by methonine peptidase so that glycine may form a bond with meristate
  • This process is non-reversible but allows proteins to associate with membranes reversibly
  • Involved in HIV signal transduction pathway
30
Q

Give an example of how N-meristoylation allows a protein to be reversibly membrane bound?

A

Recoverin protein with modification

  • When Ca2+ free it is soluble as meristoyl is water soluble and small
  • When Ca2+ meristoyl group is hidden in the hydrophobic pocket and so allowed to be hydrophobic
31
Q

Describe protein palmitoylation?

A
  • Reversible post-translational modificarion
  • Addition of palmitic acid to cyteine through thioster linkage
  • Used for peripheral and integral proteins
  • Used to anchor to a specific membrane
32
Q

Which signal transduction pathways are palmitoylated?

A
  • Protein tyrosine kinases

- G-proteins involved in vesicle movement

33
Q

What is prenylation?

A
  • Addition of farnesyl (C15|) through thioster linkage to cysteine residue at C terminal
  • Irreversible
  • Associated with Ras and rab motifs (vesicle movement)
34
Q

What does the localisation of Ras isoforms in the cell depend on?

A
  • Farnesylation and palmitoylation
  • Reversible palmitoylation allows ras to cycle between cellular compartments and perform different tasks in different areas
35
Q

What are phosphoinositides?

A
  • Based on phosphoinositol: glyerophospholipid with glycerol backbone and two fatty acid chains
  • Phosphorylation at 3,4, and 5 positions alone or in combination give 7 different species with different subceullular distributions
  • Provide reversible recruitment of proteins to specific cell membranes
  • 3-phsophatase linked to cancer
36
Q

What are the functions of phosphoinositides?

A
  • Intracellular trafficking
  • Signalling
  • Cytoskeletal remodelling
37
Q

What is the distribution of the different phosphoinositides?

A
  • PI(4)P - Golgi
  • PI(3)P - Early endosomes
  • PI(4,5)P2 - plasma membranes
  • PI (3,5)P2 - multi-vesicular endosomes
38
Q

What is the basic strucutre of G-coupled protein receptors?

A
  • 7 transmembrane domains (in hydrophobic core)
  • 3 intraculular loops
  • 3 extracellular loops
  • Intracellular C terminal and extracellular N terminal
39
Q

What is the function of G-coupled receptors?

A
  • Allow extracellular ligand which cannot cross plasma membrane to provoke a response
  • activates heterodimeric G protein which switches between active and inactive forms activating membrane bound proteins which activate a signalling cascade in the cell
40
Q

What does the diversity of GCPRs show?

A
  • Number of different target tissues using different hormones to get diffferent major response with similar strucuture
  • Not sequentially related suggesting converging evolution
41
Q

What is the structure of the heteordimeric G protein?

A

Has alpha, beta and gamma subunits

  • Alpha and gamma are membrane bound, beta bound to gamma
  • Alpha binds GDP (inactive) or GTP (active) depending on state
  • When inactive all tightly associated with eachother
42
Q

How is the exchange of GTP and GDP mediated at GCPRs?

A
  • Receptor activates GEF (guanine exchange factor) which swaps GDP for GTP allowing trimer to dissociate into alpha and beta/gamma which both interact with effectors
  • RGS regulators activate GAP for hydrolysis of GTP and reassembly of the trimer
43
Q

What effects can GPCR activation have?

A
  • opening of ion channels (inhibition of cAMP or AC)
  • Increases in intracellular Ca2+
  • Differences in gene trancription for differences in differentiation, cell survival and development
44
Q

Describe GPCR activation of AC

A
  • Alpha subunit binds to AC activating it to convert ATP into cAMP
  • This activates PKA which is usually a heteromer can become a monomer
  • May then enter nucleus and bind to CREB which can change protein transcription e.g diesterases which can cleave chaperone protein of PKA resulting in further activity
45
Q

How can GPCR act through phosphodiesterase (PE)?

A
  • Phosphodiesterase will convert cGMP to GMP

- Causing closing of cGMP dependent channels

46
Q

How does the cholera subunit act through AC and GPCR?

A
  • In the presence of G protein ARF, A subunit of chloera toxin ADP-ribosylates Galpha subunit, inhibits endogenous GTPase activity so G protein remains on and inhibits endogenous GTPase activity
47
Q

What is the function of the phosphatidyl Inositol-3 Kinase pathway?

A
  • Major growth pathway
  • Phosphorylates membrane bound inositol phospholipids
  • Acts as socking sites for intracellular proteins which form signalling complexes
48
Q

What does the activation of PI-PLC on PIP2 result in?

A

The formation of 2 second messengers;

  • Diacylglycerol (DAG) which is hydrophobic and remains in plasma membrane and activates PKC
  • IP3 which is hydrophilic and brings about an increase in Ca2+
49
Q

How does IP3 increase cytosolic Ca?

A
  • Binds to IP3 receptor on ER allosterically, allowing Ca release (which can also activate PKC)
  • Depletion of Ca stores in ER is detected by specific Ca channels in the plasma membrane, allowing Ca ion entry
50
Q

By which method (other than cleavage from PIP2) can DAG be made?

A
  • Action of phospholipase D
  • This is activated by G-protein coupled receptors or PKC. This reaction produces phosphatidylcholind hydrolysis leading to phosphatic acid
51
Q

What determines the nature of response to DAG?

A
  • Levels to which activated by PLC or PLD

- Subtypes of DAG activated

52
Q

What is the structure of PKC?

A
  • Has catalytic C terminus which is active when cleaved from N terminus which is highly conserved within subtypes (regulatory domain)
  • Receptors for activated C-kinase (RACKs) only bind PKC when appropriate cofactors are present
53
Q

What are the 3 subtypes of PKC?

A 
Conventional:
- Ca dependent 
- DAG dependent 
- Phosphatidylserine dependent 
Novel 
- Ca independent 
- DAG dependent 
- phosphatidylserine independent 
Atypical 
- Ca independent 
- DAG independent 
- Phosphatidylserine dependent
54
Q

What effects can elevated Ca levels have?

A
  • Can bind to various proteins at different sites to activate them (e.g PKC)
  • Many effects mediated by calmodulin (CaM) which is a non-enzyme protein allosterically activated when binding 4 Ca ions and can act as allosteric activator
55
Q

What effect does the binding of Ca to CaM have?

A
  • Induces conformational change
  • Can then bind to inhibitory domain of protein to activate
  • Protein auto-phosphorylates to become completely active
56
Q

How can lipids be extracted to be analysed?

A

Application of solvent mixture of ethanol, chloroform and water results in the formation of two layers: the lower lipid layer and the upper aqueous layer which contains the other biomolecules (proteins)

57
Q

Describe gas chromatography of lipids

A
  • Volatalile lipids can undergo chromatography without modifaction
  • Non-volatile lipids must be converted to FAMEs: base hydrolysis, acidify and extraction, conerting to methyl esters before dissolving in DCM
  • Level of retardation shows length of chains and number of double bonds, as well as ammount in sample, but does not show head groups
58
Q

How can lipids be extracted?

A

Beligh-dyer method

  • Addition of chlorophorm and aquaeous layer
  • Lipid becomes enriched in bottom cholorophorm layer
  • Avoids hydrolysis and oxidation of lipids, preparation for tlc
59
Q

How can lipids be visualised in thin-layer chromatography?

A
  • Spraying with iodine solution (will appear as brown spots)
  • radiolabelling with H3
  • Visualises head groups
60
Q

What is a survey scan?

A

Mass spectrometry without fractionation, shows different ions that are present

61
Q

What are the different scanning techniques of a tandem mass spectrometer?

A
  • Product ion = selection of species in Q1 for detection of daughter ion in Q3
  • Precursor ion = records species from Q1 that gives pecific fragment in Q3
62
Q

What is the significance of PC and PE?

A

Compose over 50% of all membrane phospholipids

63
Q

Describe the CDP-ethanolamie pathway (Kennedy pathway)

A

3 enzymatic steps

1) Ethanolamine kinase catalyses the ATP-dependent phosphorylation of ethanolamine (to ethanolamine-P)
2) RATE LIMITING STEP, ECT (ethanolamine cytidyltransferase) uses CTP and Ethanolamine-P to form high energy donor CDP-ethanolamine with release of pyrophosphate
3) Ethanolamine phosphotransferase (EPT) catalyses transfer of Ethanolamine on to dicyglycerol to form PE and CMP as biproducts

BL3303 (15 decks)

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