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
What is distinguishable about Phosphatidylinositol (PI)?
Pi is an unusual phospholipid that contains stearic acid on carbon 1 and arachidonic acid on carbon 2 of the glycerol. Therefore, PI serves as a reservoir of arachidonic acid in membranes and thus provides the substrate for prostaglandin (PG) synthesis when required
Describe how the phosphorylation of Phosphatidylinositol (PI) allows it to be used as a signal transduction across membranes
1) Phosphorylation of membrane-bound PI produces polyphosphoinositides such as phosphatidylinositol 4,5-biphosphate (PIP2)
2) Cleavage of PIP2 by phospholipase C occurs in response to the binding of various neurotransmitters, hormones and growth factors to G protein-coupled receptors (GPCRs)
3) Products of this cleavage (inositol 1,4,5-trisphosphate (IP3) and DAG) mediate the mobilisation of intracellular calcium and the activation of protein kinase C which acts synergistically to evoke specific cellular responses
Lipoprotein lipase, an enzyme that degrades TAG in lipoprotein particles is attached to capillary endothelial cells by what?
A glycosyl phosphatidylinositol (GPI) anchor
How can specific proteins be covalently attached to membrane bound PI?
through a carbohydrate bridge
What advantage does being attached to a membrane lipid (rather than being an integral part of the membrane) give GPI anchored proteins?
Increases lateral mobility on the extracellular surface of the plasma membrane
How can proteins be cleaved from their GPI-anchor?
Phospholipase C
Describe the haemolytic disease ‘Paroxysmal nocturnal hemoglobinuria’
- Deficiency in the synthesis of GPI in hematopoietic cells
- Some of the GPI-anchored proteins protect blood cells from the immune system (recognises foreign substances e.g viruses & bacteria)
- Lack of GPI-anchored proteins on the red blood cells are no longer recognised as “self” and are susceptible to destruction by complement-mediated lysis
Where are phospholipids and sphingolipids degraded?
Lysosomes
For each hydrolysable bond in a __________________, there is a specific hydrolytic enzyme in the lysosome
glycerophospholipid
Describe the hydrolysis of phospholipids
- Phospholipases of the A type remove one of the two fatty acids, producing a lysophospholipid
- Lysophospholipases remove the remaining fatty acid
- Phospholipases C and D each split one of the phosphodiester bonds in the head group
Which ester bonds do phospholipases A1 and A2 hydrolyse respectively?
Phospholipases A1 and A2 hydrolyse the ester bonds of intact glycerophospholipids at C1 and C2 of glycerol respectively
Where can Phospholipase A1 be found?
In many mammalian tissues
What is distinguishable about Phospholipase A2?
- present in many mammalian tissues, pancreatic juice and snake and bee venoms
- pancreatic secretions are rich in PHOSPHOLIPASE A2 PROENZYME which is activated by TRYPSIN and requires BILE SALTS for activity
- phospholipase A2 acting on phosphatidylinositol releases ARACHIDONIC ACID (precursor of the Eicosanoids)
- inhibited by GLUCOCORTICOIDS (e.g cortisol)
What is distinguishable about Phospholipase D?
involved in signal transduction, generating phosphatidic acid (PA) and choline from phosphatidylcholine and diacylglycerol from PA
What is distinguishable about Phospholipase C?
- found in liver lysosomes and the alpha toxin of clostridia and other bacilli
- membrane-bound phospholipase C activates the PIP2 system thus plays a role in producing second messengers
The polar lipids of membranes undergo constant metabolic turnover, what does this mean?
The rate of their synthesis normally is counterbalanced by the rate of breakdown
The breakdown of lipids is promoted by ______________________ in lysosomes with each enzyme capable of hydrolysing a specific bond
hydrolytic enzymes
How does sphingolipid degradation cause serious disease?
When sphingolipid degradation is impaired by a defect in one of these enzymes, partial breakdown products accumulate in the tissues, causing serious disease
How many distinct lysosomal storage diseases have been discovered and how did they arise?
More than 50
Resulting of a single gene mutation in one of the genes for a lysosomal protein
What are the properties of glycosphingolipids?
- occur largely in the outer surface of plasma membranes
- head groups with one or more sugars connected directly to the -OH at C1 of the ceramide moiety
- do not contain phosphate
- polar head function is provided by a monosaccharide or oligosaccharide attached directly to the ceramide by an O-glycosidic bond
What are Cerebrosides?
- simplest neutral glycosphingolipid
- have a single sugar linked to ceramide
- ceramide monosaccharides that contain either a molecule of galactose (forming “ceramide-galactose/galactocerebroside”) or glucose (forming “ceramide-glucose/glucocerebroside”)
- ceramide-galactose or galactocerebroside is the most common cerebroside found in myelin
- ceramide-glucose or glucocerebroside is an intermediate in the synthesis and degradation of the more complex glycosphingolipids
Where are Cerebrosides found in the body?
- Those with galactose are found in the plasma membranes of cells in neutral tissue
- Those with glucose are found in the plasma membranes of cells in non-neutral tissues
What are Globosides?
- glycosphingolipids with two or more sugars
- usually D-glucose, D-galactose or N-acetyl-Dgalactosamine
Cerebrosides and Globosides are sometimes called _________________ as they have no charge at pH 7
neutral glycolipids
How are ceramide oligosaccharides (GLOBOSIDES) produced?
by attaching additional monosaccharides to a glucocerebroside
(the additional monosaccharides can include substituted sugars)
Where does the negative charge come from in acidic glycosphingolipids?
The negative charge is provided by N-acetylneuraminic acid (NANA) which is a sialic in gangliosides or by sulphate groups in sulfatides
What is the composition of Gangliosides?
- most complex sphingolipids
- oligosaccharides as their polar head groups
- one or more residues of N-acetylneuraminic acid (NANA) (a sialic acid) at the termini
Why are gangliosides of medical interest?
Several lipid storage disorders involve the accumulation of NANA-containing glycosphingolipids in cells
What are Sulfatides?
- Sulfoglycosphingolipids that are sulphated galactocerebrosides that are negatively charged at physiologic pH
- Found predominately in the brain and kidneys
What is the link between glycosphingolipids and human blood groups?
Human blood groups (O, A & B) are determined by the oligosaccharide head groups of these glycosphingolipids. The same three oligosaccharides are found attached to certain blood proteins of individuals of blood types O, A & B respectively (antigen O, A & B)
Describe the synthesis of glycosphingolipids
- occurs primarily in the Golgi apparatus
- sequential addition of GLYCOSYL monomers transferred from UDP-sugar donors to the acceptor molecule
- enzymes involved are GLYCOSYLTRANSFERASES (specific for the type and location of the glycosidic bond formed)
Describe how sulfatidegalactocerebroside 3-sulfate is formed
A sulfate group from the sulfate carrier 3′-phosphoadenosine-5′- phosphosulfate is added by a sulfotransferase to the 3′-hydroxyl group of the galactose in a galactocerebroside, forming the sulfatidegalactocerebroside 3-sulfate and steroid hormone catabolism
Describe what happens in glycosphingolipid degradation
- Glycosphingolipids are internalised by phagocytosis since all enzymes in the degradative process are present in lysosomes which fuse with the phagosomes
- lysosomal enzymes hydrolytically and irreversibly cleave specific bonds in the glycosphingolipid
- a sequential process following the rule “last on, first off” in which the last group added during synthesis is the first group removed in degradation
What results in lysosomal storage diseases?
Defects in the degradation of the polysaccharide chains in glycoconjugates
What are Sphingolipidoses and how is it caused?
Sphingolipidoses are lysosomal lipid storage diseases caused by deficiencies in sphingolipid synthesis and degradation
Synthesis and degradation of glycosphingolipids are balanced so that the amount of these compounds present in membranes is constant. If a specific lysosomal acid hydrolase required for degradation is partially or totally missing, a sphingolipid accumulates
What are the characteristics of Sphingolipidoses?
- a specific lysosomal hydrolytic enzyme is deficient in the classic form of each disorder so only a single sphingolipid (the substrate for the deficient enzyme) accumulates in the involved organs in each disease
- disorders are progressive and vary
- incidence is low in most populations except for Gaucher and Tay-Sachs disease which show a high frequency in the Ashkenazi Jewish population
How is Sphingolipidosis diagnosed?
- measuring enzyme activity in patient cultured fibroblasts or peripheral leukocytes, or by analysing patient DNA
- histologic examination of the affected tissue
(shell-like inclusion bodies in Tay-Sachs & crumpled tissue paper appearance of the cytosol in Gaucher disease) - Prenatal diagnosis using cultured amniocytes or chorionic villi
What is Gaucher disease and how is it treated?
Gaucher disease is when macrophages become engorged with glucocerebroside since Glucocerebrosidase cannot be formed
- heart, kidneys and skin are treated by recombinant human enzyme replacement therapy (high cost!)
- bone marrow transplantation (macrophages are derived from hematopoeitic stem cells)
- use of miglustat which reduces the substrate (glucosylceramide) for the deficient enzyme (substrate reduction therapy)
What is Fabry disease and what are the symptoms?
Globosides accumulate in the vascular endothelial lysosomes of the brain since alpha-galactoside A cannot hydrolyse ceramide trihexoside
SYMPTOMS:
- episodic peripheral neuropathy
- angiokeratomas
- hypohidrosis
- progressive renal failure & cardiovascular disease (late)
What are the two diseases that are treated by recombinant human enzyme replacement therapy?
Gaucher disease & Fabry disease
What is Niemann-Pick disease?
Niemann-Pick disease (types A and B) is an autosomal-recessive disorder caused by the inability to degrade sphingomyelin due to a deficiency of sphingomyelinase (a type of phospholipase C). Sphingomyelin accumulates in the brain, spleen and liver (hepatosplenomegaly)
What are the effects of Niemann-Pick disease?
- disease becomes evident in infants and causes intellectual disability and early death
- macrophages of the reticuloendothelial system become engorged with sphingomyelin (foamy histologic appearance)
- rapid and progressive neurodegeneration as a result of deposition of sphingomyelin in the CNS
- cherry-red spot in the macula of the eye develops due to lipid deposition and edema in the retinal ganglion cells
What is the less severe variant of Niemann-Pick disease and how is it different from the more severe form?
Type B
- later age of onset
- longer survival time since causes little to no damage to neural tissue
- lungs, spleen, liver and bone marrow are affected
What are the symptoms of Gaucher disease?
- Hepatosplenomegaly (swollen liver and spleen)
- Pancytopenia (lower amount of white & red blood cells and platelets in blood)
- Osteoporosis
- Avascular necrosis of femur
- Bone crises
- Gaucher cells (lipid-laden macrophages resembling crumpled tissue paper)
What is Tay-Sachs disease and what are the symptoms?
Tay-Sachs disease is a rare genetic disorder passed from parents to child. It’s caused by the absence of Hexosaminidase A that helps break down ganglioside (fatty substance). These fatty substances, called gangliosides, build up to toxic levels in the brain and spinal cord and affect the function of the nerve cells.
SYMPTOMS:
- progressive neurodegenration
- development delay
- hyperreflexia
- hyperacusis
- “cherry-red” spot on macula
- lysosomes with onion skin
- no hepatosplenomegaly
What is Metachromatic Leukodystrophy and what are the symptoms?
Arylsulfatase A cannot hydrolyse Cerebroside sulphate
SYMPTOMS: central and peripheral demyelination with ataxia and dementia
What is Krabbe disease and what are the symptoms?
Galactocerebrosidase cannot be formed to hydrolyse Galactocerebroside & Psychosis
SYMPTOMS:
- peripheral neuropathy
- destruction of oligodendrocytes
- development delay
- optic atrophy
- globoid cells
What is Hurler syndrome and what are the symptoms?
Children with Hurler syndrome have an abnormal accumulation of complex sugars in their cells, which affects many of the systems in their bodies. Hurler syndrome is associated with Mucopolysaccharidoses and is when alpha-L-iduronidase cannot hydrolyse Heparin sulphate and Dermatan sulphate
SYMPTOMS:
- development delay
- skeletal abnormalities
- airway obstruction
- corneal clouding
- hepatosplenomegaly
What are the three types of Eicosanoids?
- Prostaglandins
- Thrombaoxanes
- Leukotrienes
What do prostaglandins, thromboxanes and leukotrienes have in common?
- originate from polyunsaturated fatty acids with 20 carbons
- extremely potent compounds that elicit a wide range of responses, both physiologic (inflammatory response) and pathologic (hypersensitivity)
- not stored
- extremely short half-life
- rapidly metabolised to inactive products
What is the pathway to form prostagladins and thromboxanes from lipids (cell membranes)?
1) Phospholipase A2 cleaves lipids out of the cell membrane to form arachidonic acid
2) Cyclooxygenase converts arachidonic acid to either prostaglandins or thromboxanes
What is the pathway to form leukotrienes from lipids (cell membranes)?
1) Phospholipase A2 cleaves lipids out of the cell membrane to form arachidonic acid
2) Lipoxygenase converts arachidonic acid to Leukotrienes
What is the pathway for Sphingomyelin degradation?
Sphingomyelin degradation is catalysed by Sphingomyelinase. The deficiency leads to Niemann-Pick (types A and B) which is characterised by sphingomyelin accumulation in liver and spleen and neurodegeneration (type A)
What is the pathway for Glycero-phospholipid degradation?
Glycero-phospholipid degradation is catalysed by phospholipases producing Glycerol, Fatty acids, Phosphate, Alcohols
