WEEK 5 & 6 (Phospholipids, Glycosphingolipids & Eicosanoids) Flashcards
What are the four major types of membrane lipids?
- Phospholipids
- Sphingolipids
- Glycolipids
- Cholesterol
Describe the construction of Phospholipids
Membrane lipids that are amphipathic with the hydrophobic region consisting of two fatty acids joined to glycerol/sphingosine via an ester bond and the hydrophilic end consisting of a phosphate end attached to glycerol/sphingosine via a phosphodiester bond
Describe the construction of Glycolipids
Lipids that contain a simple sugar or a complex oligosaccharide at the polar ends
What are sterols?
Compounds characterised by a rigid system of four fused hydrocarbon rings
What are the two types of phospholipids?
- Phospholipids with a glycerol backbone (from glucose)
- Phospholipids with a sphingosine backbone (from serine and palmitate)
What is a glycerophospholipid/phosphoglyceride?
Membrane lipids in which two fatty acids are attached in ester linkage to the first and second carbons of glycerol and a highly polar or charged group is attached through a phosphodiester linkage to the third carbon
What makes Phosphatidylserine, Phosphatidylethanolamine, Phosphatidylcholine (lecithin), phosphatidylinositol and Phosphatidylglycerol?
Serine + PA = Phosphatidylserine (PS)
Ethanolamine + PA = Phosphatidylethanolamine (PE)
Choline + PA = Phosphatidylcholine (PC)
Inositol + PA = Phosphatidylinositol
Glycerol + PA = Phosphatidylglycerol
What is Cardiolipin?
A two-tailed glycerophospholipid in which two phosphatidic acid moieties share the same glycerol via their head group
What is the function of Cardiolipins?
- Cardiolipins are virtually exclusive to the inner mitochondrial membrane where it maintains the structure and function of certain respiratory complexes of the electron transport chain
- Antigenic (antibody or immune response triggered by the antigens on a particular virus)
Describe an example of Cardiolipin being antigenic
A patient infected with Treponema Pallidum (bacterium that causes Syphilis) develops antibodies against Cardiolipin. The Wasserman test for Syphilis detects antibodies raised against T.Pallidum by subjecting the patient’s serum to cardiolipin as an antigen.
All cells except which cells can synthesise phospholipids?
Mature erythrocytes
Where does Triacylglyceride synthesis only occur?
- Liver
- Adipose tissue
- Lactating mammary glands
- Intestinal mucosal cells
What are ‘ether lipids’?
Lipids in which one of the two acyl chains is attached to glycerol in ether linkage
What is the difference between an ester and an ether?
An ester group requires two oxygen atoms and two carbon atoms to complete its characteristic structure, while an ether group only needs one oxygen atom and two carbon atoms for its structure.
(the acyl group that forms an ETHER linkage is UNSATURATED whereas the alkyl group that forms an ESTER linkage is SATURATED)
What is an example of a saturated ether-linked chain?
Alkyl ether lipids
What is an example of an unsaturated ether-linked chain?
Plasmalogens
In plasmalogens a double bond is found between C-1 and C-2
Describe the formation of a Plasmalogen
When the fatty acid at carbon 1 of a glycerophospholipid is replaced by an unsaturated alkyl group attached by an ether linkage to the core glycerol molecule
What type of tissue is enriched in ether lipids
Vertebrate heart tissue
About half of the heart phospholipids are plasmalogens
What is the functional significance of ether lipids in heart membranes?
Unknown but ether lipids have resistance to the phospholipases that cleave ester-linked fatty acids from membrane lipids which could be important
What is an example of an ether glycerophospholipid and what is its structure?
Platelet-activating factor (PAF)
- a saturated alkyl group linked to carbon 1 via ether linkage
- acetyl residue at carbon 2
- glycerol backbone
- Hydrophilic phosphate head with ethanol amine attached
How is platelet-activating factor synthesised and what is its function?
Platelet activating factor is synthesised and released by a variety of cell types
FUNCTION:
- binds to surface receptors which triggers potent thrombotic and acute inflammatory events
- activates inflammatory cells
- mediates hypersensitivity, acute inflammatory and anaphylactic reactions
- causes platelets to aggregate and activate neutrophils and alveolar macrophages to generate superoxide radicals to kill bacteria
- lowers blood pressure
What is the composition of sphingomyelin?
- Backbone is sphingosine (an amino alcohol) rather than glycerol
- long-chain fatty acid is attached to the amino group of sphingosine through an amide linkage (this produces a CERAMIDE which can also serve as a precursor of glycolipids)
- the alcohol group at carbon 1 of sphingosine is esterified to phosphorylcholine which produces SPHINGOMYELIN (the only significant sphjingophospholipid in humans)
What is the importance of Sphingomyelin in the human body?
Sphingomyelin is an important part of myelin sheath of nerve fibres and is essential for myelin integrity and function
What are the two ways that Glycerophospholipid can be synthesised?
- Donation of Phosphatidic acid from cytidinediphosphate (CDP)-DAG to an alcohol
- Donation of the phosphomonoester of the alcohol from CDP-alcohol to diacylglycerol(DAG)
in both cases, the CDP-bound structure is considered an activated intermediate and cytidine monophosphate (CMP) is released as a side product
What is the key concept of glycerophospholipid synthesis?
Activation of either DAG or the alcohol to be added by linkage with CDP
_______________ is the precursor of other glycerophospholipids
Phosphatidic acid
Where are most phospholipids synthesised?
In the smooth endoplasmic reticulum (SER)
Where does ether lipid synthesis from dihydroxyacetone phosphate begin?
Peroxisomes
What are the most abundant phospholipids in most eukaryotic cells?
Phosphatidylcholine & Phosphatidylethanolamine
Describe the synthesis of phosphatidylcholine and phosphatidylethanolamine from pre-existing choline and ethanolamine
1) Choline or ethanolamine are phosphorylated by kinases
2) choline or ethanolamine are converted to the activated form (CDP-choline or CDP-ethanolamine)
3) choline phosphate or ethanolamine phosphate is transferred from the nucleotide (leaving CMP) to a molecule of DAG
What is Dipalmitoylphosphatidylcholine (DPPC or Dipalmitoyl lecithin)?
A major lipid component of lung surfactant (extracellular fluid layer lining the alveoli) that is secreted by type II pneumocytes
What is the purpose of lung surfactant?
Surfactant serves to decrease the surface tension of this fluid layer which reduces the pressure needed to re-inflate alveoli, thereby preventing alveolar collapse (ATELECTASIS)
How can foetal lung maturity be determined?
By determining the lecithin/sphingomyelin (L/S) ratio in amniotic fluid
A value >2 is evidence of maturity, because it reflects the shift from sphingomyelin to DPPC synthesis that occurs in pneumocytes at around 32 weeks gestation
What is Respiratory distress syndrome (RDS)?
Respiratory distress syndrome (RDS) in preterm infants is associated with insufficient surfactant production and/or secretion and is a significant cause of all neonatal deaths in Western countries
How can Lung maturation be accelerated?
- by giving the mother glucocorticoids shortly before delivery to induce expression of specific genes
- postnatal administration of natural or synthetic surfactant (by intratracheal instillation)
What is Phosphatidylinositol (PI) synthesised from?
Free inositol and CDP-DAG