Phospholipids-complex lipids

Classified into two chemically different groups based on whether the alcohol is (a) glycerol or (b) sphingosine

GlycerophospholipidsSphingophospholipids

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Glycerophospholipids

1. Diacylglycerophospholipids Phosphatidic acid (PHOSPHATE ATTACHED TO GLYCEROL?) Phosphatidylcholine (lecithin) Phosphatidylethanolamine Phosphatidylglycerol Phosphatidylinositol Phosphatidylserine

2. Monoacyl or lyso-glycerophospholipids

3. Cardiolipin (diphosphatidylglycerol)

4. Ether phospholipids and plasmalogens

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Biosynthesis of glycerophospholipids

Cells can synthesize PL in the smooth endoplasmic reticulum, which are then sorted through the Golgi apparatus

Glycerophospholipid synthesis follows the same pathway as TAG synthesis as far as PA and 1,2-DAG.

Thereafter, energy in the form of CTP is needed to attach the polar head group

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After the formation of 1,2-DAG, the next stage requires an input of energy in the form of CTP

Two mechanisms are possible

Activation of 1,2-DAG by CTP

Activation of alcohol by CTP

Formation of phospholipids

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Synthesis of phosphatidylcholine, phosphatidylethanolamine, and phosphatidylcholine

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Synthesis of cardiolipin and phosphatidyl inositol

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Synthesis of ether glycerolipids Synthesized from glycolytic intermediate DHAP

is found in cardiac muscle

is found in myelin

Produced in the macrophage peroxisomes.

It cause platelets aggregation, edema, and hypotension.

Zellweger syndrome compromises PAF synthesis 8

Degradation of phospholipids Glycerophospholipids

Degraded by many different phospholipases

Sphingomyelins

Unique enzyme, sphingomyelinase-a lysosomal phospholipase

Phospholipases are found in all tissues & pancreatic juice

Also present in several toxins, venoms & several pathogenic bacteria which dissolve cell membranes & allow spread of infection

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4 main types of phospholipases

Phospholipases are found in all tissues at the level of the cell membranes, in lysosomes & pancreatic juice

Phospholipases allow fatty acid composition of phospholipids to be remodeled

Also present in several toxins, venoms & several pathogenic bacteria which dissolve cell membranes & allow spread of infection

Lysophospholipase removes remaining FA from lysoPL (also called phospholipase B, not shown)

Phospholipase A2 provides major repair mechanism for membrane lipids damaged by

oxidative free-radical reactions. It recognizes the distortion of membrane structure caused by partially degraded fatty acid & removes it

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Lysophospholipase

Removes remaining fatty acids from lysophospholipid

Also called phospholipase B

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Phospholipases allow fatty acid composition of phospholipids to be remodeled

Lung surfactantLung surfactant

Phospholipase A2 provides major repair mechanism for membrane lipids damaged by

oxidative free-radical reactions. It recognizes the distortion of membrane structure caused by partially degraded fatty acid & removes it

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Functions of phospholipids

Major component of cell and organelle membranes

Contribute charge to the membrane

Surface coat of lipoprotein particles & component of bile

PIG or GPI – membrane anchor for specific proteins

PAF (platelet activating factor)

Signal transduction

Pulmonary surfactant

Source of PUFA-arachidonic acid for eicosanoid synthesis

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Phospholipids in membranes

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Phospholipids in the surface coat of lipoproteins

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Membrane anchorsPhosphatidylinositol is an integral component of the glycosylphosphatidylinositol(GPI)

structure that anchors various proteins to the plasma membrane

In contrast to other membrane phospholipids, GPI has a glycan chain containing glucosamine & mannose attached to the inositol.

Ethanolamine connects the GPI-glycan to the carboxy terminus of the protein.

Many membrane proteins in eukaryotic cells are anchored by a GPI structure, which are released from the cell surface by phospholipase C in response to regulatory processes

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Paroxysmal nocturnal hemoglobinuria (PNH)

Mutations in the PIGA gene cause paroxysmal nocturnal hemoglobinuria. The PIGA gene provides instructions for making a protein called phosphatidylinositol glycan class A. This protein takes part in a series of steps that produce a molecule called GPI anchor. GPI anchor attaches many different proteins to the cell membrane, thereby ensuring that these proteins are available when needed at the surface of the cell.

People with paroxysmal nocturnal hemoglobinuria have sudden, recurring episodes of symptoms (paroxysmal symptoms), which may be triggered by stresses on the body, such as infections or physical exertion. During these episodes, red blood cells are prematurely destroyed (hemolysis)

Abnormal platelets associated with paroxysmal nocturnal hemoglobinuria can cause problems in the blood clotting process. As a result, people with this disorder may experience abnormal blood clotting (thrombosis), especially in large abdominal veins; or, less often, episodes of severe bleeding (hemorrhage).

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Resembles phosphatidylcholine with an acetyl group at C2 of glycerol & a saturated C18 at C1 linked by alkyl ether group to the –

OH

Platelet-activating factor, also known as a PAF, PAF-acether or AGEPC (acetyl-glyceryl-ether-phosphorylcholine) is a potent phospholipid activator and mediator of many leukocyte functions, including platelet aggregation, inflammation, and anaphylaxis.

Causes CV & pulmonary changes, edema & hypotension

Ag-IgE complexes on the surface of inflammatory cells initiate the synthesis & release of PAF thereby causing platelet aggregation

Platelet activating factor (PAF)

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Phospholipids in signal transduction

1. Activation of phospholipase C in response to receptor-mediated binding of neurotransmitters, hormones and growth factors leads to subsequent degradation of PIP2 & release of 2 second messengers

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Lithium and inositol metabolism

Lithium is prescribed for depression and bipolar disorder

Lithium, a primary treatment for bipolar disorder, may exert its therapeutic effects by depleting neuronal myo-inositol (mI) concentrations through the inhibition of inositol monophosphatase (IMPase) resulting in the downregulation of the PI signaling pathway and dampening overactive neurotransmission

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Phospholipids as pulmonary surfactant

Surfactant serves to decrease the surface tension of fluid lining the alveolarsurface thus preventing alveolar collapse

Without surfactant, surface tension forces in the alveoli would:

1. Make breathing very labor-intensive

2. Cause lungs to collapse (atelectasis)

3. Fill lungs with transudate

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Phospholipids as pulmonary surfactantSurfactant serves to decrease the surface tension of fluid lining the alveolar surface thus preventing alveolar collapse

Without surfactant, surface tension forces in the alveoli would:

1. Make breathing very labor-intensive

2. Cause lungs to collapse (atelectasis)

3. Fill lungs with transudate

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Infant respiratory distress syndrome (IRDS) of a premature infant

hyaline membrane disease due to lack of surfactant

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IRDS

Cause of 15-20% of neonatal deaths in western countries

Affects premature infants & incidence directly related to the degree of prematurity

Synthesis of DPPC starts only at 32wks of gestation from type II (granular) pneumocytes

Neonates born with immature lungs (insufficient surfactant) will have labored breathing, acidosis, low oxygen levels in blood and ‘ground glass’ appearance of lungs on X-ray

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IRDS risk factors

Premature birth

Diabetic mother – insulin delays lung maturity and raised insulin in the fetus occurs when maternal blood sugar rises

Male sex – male sex hormones delay lung maturity, female sex hormones speed it up

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Prevention of IRDS

Ensuring full term delivery

Premature labor should be stopped if possible following amniotic fluid analysis to establish fetal lung maturity (L/S ratio-2 or above is evident of lung maturity)

If premature birth is unavoidable, corticosteroids given to mother shortly before delivery speed up fetal lung maturity [by increasing synthesis of phosphatidic acid phosphatase, enzyme which links CDP-choline and diacylglycerol (CDP-choline: diacylglycerol phosphorylcholine transferase) of surfactant]

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Lab tests to establish fetal lung maturity

Lecithin/Sphingomyelin ratio in amniotic fluid (>2 indicates mature lung)

Foam stability index

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Treatment Until recently IRDS was a serious problem even in the western world and infant deaths from IRDS were common

Treatment was pediatric intensive care, oxygen and often, mechanical ventilation

Surfactant replacement therapy (Animal or synthetic) now used- intratracheal instillation

Synthetic pulmonary surfactants

• Exosurf

• Pumactant

• KL-4

• Venticute

Animal derived surfactants

• Alveofact

• Curosurf

• Infasurf

• Survanta

• Exosurf, Curosurf, Infasurf, and Survanta are the surfactants currently FDA approved for use in the U.S.

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Hughes syndrome (antiphospholipid syndrome)

Also called Sticky blood syndrome, in which the body produces antibodies against its own phospholipids and protein/phospholipid complexes (the major one being an anticardiolipin antibody)

This is because people with it have an increased tendency to form clots in blood vessels (also known as thrombosis). Any blood vessel can be affected including the veins and the arteries

Antiphospholipid syndrome is tested for in the laboratory using both liquid phase coagulation assays (lupus anticoagulant) and solid phase ELISA assays (anti-cardiolipin antibodies).

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Glycosphingolipids

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Glycosphingolipids - chemistry

Components: carbohydrate + lipid

Derivatives of ceramide:

ALWAYS

Alcohol-Sphingosine

N-linked long chain fatty acid ( together = ceramide)

O-Glycosidically linked mono (glucose) or oligosaccharide

OFTEN

Other sugars include galactose and amino or N-acetyl amino sugars

Terminal acidic sugar – NANA

Sulfate group on 1st galactose

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Glycolipids are sphingosine-based lipids containing sugars

There are four types

Cerebrosides

Globosides

Gangliosides

Sulfatides

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USED FOR SURFACTANT (SIMILAR)

Cerebrosides

Ceramide linked to a single sugar

Two types in humans: Glucocerebroside:

common glycosphingolipid in nonneuronal tissue

breakdown product of globosides and gangliosides

Galactocerebroside: common in brain and peripheral nervous tissue with high concentration in myelin sheath

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Globosides

Ceramide linked to two or more sugars, usually glucose, galactose, or N-acetylgalactosamine

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Gangliosides

Most complex of glycosphingolipids

Derivatives of ceramide -linked to several sugars and one or more molecules of sialic acid (NANA)

Gangliosides are concentrated in the outer surface of cells, where they present points of recognition for extracellular molecules

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Sulfatides

Ceramide linked to sulfated galactose

A major lipid of myelin together with galactocerebroside & also found in kidney

Synthesized from oligodendrocytes of CNS

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Glycosphingolipid functions

They are mainly found in the plasma membrane and in particular in the noncytosolic half of the bilayer

Their sugar groups are therefore exposed to the exterior of the cell.

Play a role in the regulation of cellular interactions, growth & development

Antigenic & are a source of blood group antigens, various embryonic antigens specific for particular stages of fetal development & some tumor antigens

Serve as cell surface receptors for cholera & tetanus toxins as well as for certain viruses & microbes

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Phospholipids and glycolipids are asymmetrically distributed in the lipid bilayer

Phosphatidylinositol

Phosphatidylethanolamine

Phosphatidylcholine

Phosphatidylserine

Sphingomyelin

Sugars of glycolipids

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Glycosphingolipids in relation to other organisms in the human body

Helpful

Retention of beneficial bacteria – the normal gut ‘flora’

Harmful

Mechanism to allow entry of toxins into cells – leading to disease

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Some bacterial endotoxins require gangliosides for binding to enterocytes and allowing active toxin subunit to enter cell

Examples include cholera, pertussis, Shiga toxins and heat-labile E. coli enterotoxin influenza virus

Gangliosides help to bind bacteria, which are normal component organisms of the gut ‘flora’

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Biosynthesis of glycosphingolipids

It takes place in ER and Golgi

It begins with the synthesis of ceramide (N-acyl sphingosine)

Thereafter, the sugars are added one-at-a time in their activated forms (UDP-sugars and CMP-NANA)

For sulfatide, galactose from UDP-Gal is first added to ceramide, then sulfate from PAPS (3’-phosphoadenosine-5’-phosphosulphate) is added

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It requires synthesis of ceramide from palmitoyl CoA, serine and acyl CoA, NADPH, PLP, Mn2+, FAD

It takes place in smooth endoplasmic reticulum of most cells

Fatty acids in sphingomyelin

Skin – C30

Myelin – lignoceric and nervonic acids

Brain gray matter – stearic acid

Biosynthesis of ceramide and sphingomyelin

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Summary of pathways

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Degradation of glycosphingolipids

Glycosphingolipids are internalized by endocytosis

Endocytotic vesicles fuse with lysosomes to form secondary lysosome

The lysosomal enzymes hydrolytically and irreversibly cleave specific bonds in the glycosphingolipid

The lysosome contains all the enzymes necessary to degrade glycosphingolipids

The degradation is sequential – “last on – first off”

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Individual enzymes

α- and β-galactosidases

β-glucosidase

Neuraminidase

Hexosaminidases

Aryl sulfatase A

Sphingomyelinase

Deficiency of which lead to glycosphingolipid storage diseases

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Sphingolipidoses

If a mutation causes a deficiency of any enzyme, then its substrate will accumulate

For every affected enzyme, very to less severe forms of deficiency have been documented

Very severe forms lead to neurological regression and/or other symptoms and death in infanthood

All compounds are endogenous and, therefore, conditions can’t be treated by dietary restriction

Enzyme or gene replacement therapy holds out hope

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Abnormal accumulations of membrane lipids: some inherited human diseases

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Sphingolipidosis Diagnosis: check: enzymes/metabolites in cultured fibroblasts or leukocytes DNA analysis Histologic examination of affected tissues: shell-like inclusion bodies in Tay-Sach’s,

wrinkled tissue paper appearance of cytosol in Gaucher’s Prenatal diagnosis using amniocytes or chorionic villi Treatment: Recombinant human enzyme replacement therapy -bone marrow transplantation

in Gaucher’s

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Sphingomyelinase- lysosomal enzyme

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Niemann-Pick disease

Autosomal recessive lysosomal storage disease, due to absence of sphingomyelinase- thus inhibiting sphingomyelin degradation. More common in Ashkenazi Jewish population

Characterized by formation of lipid-laden phagocytes (foam cells) engorged with sphingomyelin

Two types: Type A: severe infantile form- sphingomyelin deposition in liver, spleen & CNS. Death in early childhood due to rapid & progressive neurodegeneration

Type B: lungs, liver, spleen & bone marrow affected. Neural tissue little or no damage. Life expectancy to early adulthood

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