Phospholipids & Sphingolipids

Phospholipids and sphingolipids are the amphipathic backbone of every biological membrane, the precursors of signalling molecules, and — most importantly for NEET PG — the substrates whose accumulation causes the classic sphingolipidoses (lysosomal storage diseases). This note builds the structural logic first, then converts it into the enzyme-defect tables that the exam loves.

Classification of complex lipids

Membrane lipids divide into two great families based on their backbone alcohol:

• Glycerophospholipids (phosphoglycerides): backbone is glycerol-3-phosphate.
• Sphingolipids: backbone is sphingosine (an 18-carbon amino alcohol), not glycerol.

A simple way to remember the difference: a glycerophospholipid is built on glycerol and always carries a phosphate; a sphingolipid is built on a ceramide core (sphingosine + fatty acid), and that ceramide may then be capped with phosphocholine (→ sphingomyelin) or with sugars (→ glycosphingolipids).

Feature	Glycerophospholipids	Sphingolipids
Backbone alcohol	Glycerol-3-phosphate	Sphingosine
Number of fatty acids	2 (ester bonds at C1, C2)	1 (amide bond to sphingosine)
Defining linkage	Ester	Amide (in ceramide)
Always phosphate-containing?	Yes	Only sphingomyelin (others are glycolipids)
Examples	Phosphatidylcholine, cardiolipin	Sphingomyelin, gangliosides, cerebrosides

High-yield: Sphingomyelin is the only sphingolipid that contains phosphorus — it is simultaneously a phospholipid and a sphingolipid, the bridge between the two families. All other sphingolipids are glycolipids.

Glycerophospholipids — structure and key members

The parent compound is phosphatidic acid (diacylglycerol-3-phosphate), the metabolic precursor of all glycerophospholipids and of triacylglycerol. Esterifying the phosphate with a head-group alcohol generates the named lipids.

Phospholipid	Head group	Notable role / exam fact
Phosphatidylcholine (lecithin)	Choline	Most abundant membrane phospholipid; dipalmitoyl-lecithin = lung surfactant
Phosphatidylethanolamine (cephalin)	Ethanolamine	Inner leaflet; substrate for PC synthesis
Phosphatidylserine	Serine	Inner leaflet; flips to outer leaflet → apoptosis signal & platelet procoagulant surface
Phosphatidylinositol (PI)	Inositol	Source of IP₃/DAG second messengers; GPI anchor
Cardiolipin (diphosphatidylglycerol)	Glycerol bridging 2 phosphatidic acids	Inner mitochondrial membrane; antigen in VDRL; antibody target in antiphospholipid syndrome
Plasmalogens	Ether-linked alkenyl chain at C1	Abundant in heart and brain (myelin); deficient in Zellweger / peroxisomal disorders

High-yield: Dipalmitoylphosphatidylcholine (lecithin) is the principal surfactant. A lecithin:sphingomyelin (L:S) ratio ≥ 2:1 in amniotic fluid indicates fetal lung maturity. Surfactant deficiency → neonatal respiratory distress syndrome (hyaline membrane disease).

High-yield: Cardiolipin is unique to the inner mitochondrial membrane and is the antigen used in the VDRL/RPR non-treponemal test for syphilis. Anti-cardiolipin antibodies are a criterion for antiphospholipid antibody syndrome (paradoxically prolonging the in-vitro aPTT — lupus anticoagulant — while causing clinical thrombosis).

Phospholipases — directional cleavage

Phospholipase nomenclature maps directly onto bond position and is a recurring single-best-answer item:

• PLA₂ → cleaves the C2 ester, releasing arachidonic acid (eicosanoid precursor); inhibited by steroids via lipocortin/annexin; venom and pancreatic forms exist.
• PLA₁ → cleaves C1 ester.
• PLC → cleaves glycerophosphate bond, generating DAG + IP₃ (key in PIP₂ signalling).
• PLD → cleaves the head-group, releasing the alcohol (e.g. choline) and leaving phosphatidic acid.

A mnemonic: C for PLC gives Cytosolic messengers; D for PLD Detaches the head-group.

Sphingolipids — building from ceramide outward

Synthesis flows: serine + palmitoyl-CoA → sphinganine → (acylation) → dihydroceramide → ceramide. Ceramide is the central hub. From ceramide:

Ceramide + phosphocholine → Sphingomyelin (a phospholipid). Ceramide + 1 sugar (glucose/galactose) → Cerebroside (a monoglycosylceramide). Ceramide + oligosaccharide + sialic acid (NANA) → Ganglioside. Cerebroside + sulfate → Sulfatide.

So the hierarchy of glycosphingolipid complexity is:

Ceramide → Cerebroside (1 sugar) → Globoside (neutral oligosaccharide) → Ganglioside (contains NANA / sialic acid) → progressively more complex.

High-yield: Gangliosides are defined by their sialic acid (N-acetylneuraminic acid, NANA) content and are concentrated in neuronal grey matter / synaptic membranes. GM2 ganglioside accumulation is the hallmark of Tay–Sachs disease.

Ceramide itself is also pro-apoptotic, and sphingosine-1-phosphate (its downstream product) is a vascular/immune signalling lipid — the target of fingolimod in multiple sclerosis (S1P receptor modulator). That pharmacology link is increasingly examined.

Sphingolipidoses (lysosomal storage diseases) — the core exam material

These arise from deficiency of a lysosomal acid hydrolase that degrades sphingolipids, causing the upstream substrate to accumulate. They are mostly autosomal recessive, with two important X-linked recessive exceptions: Fabry and Hunter (Hunter is a mucopolysaccharidosis, not a sphingolipidosis, but is the classic X-linked trap paired with Fabry).

Master table

Disease	Deficient enzyme	Accumulated substrate	Inheritance	Cherry-red spot	Hepatosplenomegaly	Hallmark clue
Tay–Sachs	Hexosaminidase A	GM2 ganglioside	AR	Yes	No	Hyperacusis (exaggerated startle), Ashkenazi Jews, no organomegaly
Sandhoff	Hexosaminidase A & B	GM2 ganglioside + globoside	AR	Yes	Yes	Tay–Sachs + organomegaly
Niemann–Pick (A/B)	Sphingomyelinase	Sphingomyelin	AR	Yes (~50%)	Yes	Foamy/“sea-blue” histiocytes, Ashkenazi
Gaucher	Glucocerebrosidase (β-glucosidase)	Glucocerebroside	AR	No	Yes (massive)	Crumpled-tissue-paper Gaucher cells, bone crises, MOST COMMON
Krabbe	Galactocerebrosidase (β-galactosidase)	Galactocerebroside + psychosine	AR	No	No	Globoid cells, optic atrophy, peripheral neuropathy
Metachromatic leukodystrophy	Arylsulfatase A	Cerebroside sulfate (sulfatide)	AR	No	No	Demyelination, metachromatic granules, ataxia/dementia
Fabry	α-Galactosidase A	Ceramide trihexoside (globotriaosylceramide)	X-linked recessive	No	No	Angiokeratomas, acroparaesthesias, renal/cardiac disease, corneal whorls

High-yield: Gaucher disease is the most common lysosomal storage disease overall. Gaucher cells are lipid-laden macrophages with a “crumpled tissue paper / wrinkled silk” cytoplasm; clinically — hepatosplenomegaly, pancytopenia, Erlenmeyer-flask deformity of the distal femur, and bone pain crises.

High-yield: Tay–Sachs has NO hepatosplenomegaly; Niemann–Pick and Sandhoff DO. This single discriminator is the most frequently tested point in the GM2/sphingomyelin group. All three can show a cherry-red macular spot.

High-yield: Fabry and Hunter are the two X-linked recessive lysosomal storage diseases. Fabry = α-galactosidase A deficiency → angiokeratomas, burning pain in extremities (acroparaesthesias), corneal opacities, and progression to renal failure and cardiomyopathy.

Two diseases that DO NOT show a cherry-red spot but are heavily tested

• Gaucher and Fabry lack the cherry-red macular spot. Conversely the classic “cherry-red spot” diseases to memorise are Tay–Sachs, Niemann–Pick, Sandhoff, GM1 gangliosidosis (and, in a vascular non-storage context, central retinal artery occlusion).

Diagnosis and treatment principles

• Screening / confirmation: measure the specific leukocyte (WBC) lysosomal enzyme activity — the investigation of choice for most sphingolipidoses. Definitive confirmation is by molecular genetic testing.
• Tay–Sachs prenatal/carrier screening: hexosaminidase A assay; important in Ashkenazi Jewish populations.
• Enzyme replacement therapy (ERT): available and exam-relevant for Gaucher (imiglucerase/velaglucerase) and Fabry (agalsidase).
• Substrate reduction therapy: miglustat / eliglustat for Gaucher.
• Neuronopathic forms (Tay–Sachs, Krabbe, Niemann–Pick A) lack effective therapy and carry a poor prognosis — supportive care.

A stepwise diagnostic approach to a suspected storage disease:

1. Clinical phenotype (neurodegeneration ± organomegaly ± skeletal ± skin) → narrows the family.
2. Cherry-red spot present? → points to GM2/sphingomyelin group.
3. Hepatosplenomegaly present? → separates Niemann–Pick/Gaucher/Sandhoff from Tay–Sachs/Krabbe/MLD/Fabry.
4. Specific leukocyte enzyme assay → confirms the enzyme defect.
5. Genetic testing → mutation confirmation and counselling.

Mnemonics

• Tay–Sachs vs Sandhoff: “Tay = Hex A only; Sandhoff = Hex A + B (Both).” Sandhoff adds organomegaly.
• Gaucher = Glucocerebrosidase; Galactocerebrosidase = Krabbe. “Gaucher has Glucose; Krabbe has Galactose + Globoid cells.”
• Fabry = Female-sparing X-linked, α-galactosidase, Foot/Finger burning + skin angiokeratomas.
• “No man picks his nose with his sphinger” — Niemann–Pick = sphingomyelinase deficiency.
• Cherry-red spot diseases: “Tay-Niemann-Sandhoff” cluster (plus GM1).

Membrane biology and functional roles (frequently linked stems)

Beyond storage diseases, the exam asks why these lipids matter in the membrane and in signalling:

• Amphipathic architecture: every membrane phospholipid has a polar (phosphate + head group) end and two hydrophobic acyl tails, driving spontaneous bilayer formation. The degree of acyl-chain unsaturation and the cholesterol content govern membrane fluidity; saturated tails and cholesterol below the transition temperature reduce fluidity.
• Asymmetry: the bilayer is asymmetric. Phosphatidylcholine and sphingomyelin sit predominantly in the outer leaflet; phosphatidylserine and phosphatidylethanolamine in the inner leaflet. ATP-dependent flippases maintain this asymmetry. Loss of asymmetry, with phosphatidylserine exposure on the outer leaflet, is the universal eat-me apoptosis signal and the procoagulant platelet surface.
• Lipid rafts: cholesterol- and sphingomyelin-rich microdomains organise membrane signalling and receptor clustering.
• Second-messenger source: PIP₂ is cleaved by PLC into IP₃ (mobilises intracellular Ca²⁺ from ER) and DAG (activates protein kinase C). This is the single most examined signalling pathway derived from a phospholipid.
• GPI anchor: phosphatidylinositol anchors many proteins to the cell surface; defective GPI biosynthesis (acquired PIGA mutation) underlies paroxysmal nocturnal haemoglobinuria (PNH) — loss of GPI-anchored complement regulators CD55 and CD59.

High-yield: PNH results from a somatic PIGA mutation impairing GPI-anchor synthesis, causing loss of CD55 (DAF) and CD59 from blood cells → complement-mediated intravascular haemolysis. This ties phosphatidylinositol biochemistry to a haematology favourite; treat with eculizumab (anti-C5).

Synthesis and turnover essentials

• Phosphatidylcholine is made by two routes: the CDP-choline (Kennedy) pathway (major; choline → phosphocholine → CDP-choline → PC) and methylation of phosphatidylethanolamine by PEMT using SAM (mainly hepatic). Choline is a dietary essential and a methyl-group donor (via betaine).
• Cardiolipin is synthesised from phosphatidylglycerol and CDP-diacylglycerol exclusively at the inner mitochondrial membrane; defective cardiolipin remodelling (tafazzin gene) causes Barth syndrome (cardiomyopathy, neutropenia, 3-methylglutaconic aciduria) — an emerging X-linked stem.
• Sphingomyelin synthesis transfers phosphocholine from PC onto ceramide, simultaneously generating DAG — directly linking the two lipid families.

High-yield: Sphingolipid degradation requires lysosomal acid hydrolases plus the small activator protein saposin and GM2 activator protein; deficiency of the GM2 activator protein produces a Tay–Sachs-like (AB-variant) phenotype despite normal hexosaminidase levels — an advanced distractor.

Clinical features by organ pattern

• Pure CNS degeneration, no organomegaly: Tay–Sachs (developmental regression, hyperacusis, blindness), Krabbe (irritability, stiffness, peripheral neuropathy with optic atrophy), Metachromatic leukodystrophy (gait disturbance, dementia, demyelination).
• CNS + visceromegaly: Niemann–Pick A (severe neurodegeneration + hepatosplenomegaly), Sandhoff.
• Visceral / skeletal predominant, variable CNS: Gaucher type 1 (non-neuronopathic, adult, commonest) vs types 2/3 (neuronopathic).
• Multisystem in a male with skin + pain + renal disease: Fabry.

Key differentials and traps

Confusable pair	Discriminator
Tay–Sachs vs Niemann–Pick	Both cherry-red spot; organomegaly only in Niemann–Pick
Tay–Sachs vs Sandhoff	Hex A vs Hex A+B; organomegaly in Sandhoff
Gaucher vs Niemann–Pick (foamy cells)	Crumpled tissue-paper cell (Gaucher) vs foamy / sea-blue histiocyte (Niemann–Pick)
Krabbe vs MLD	Both demyelinate; globoid cells + galactocerebrosidase (Krabbe) vs metachromatic granules + arylsulfatase A (MLD)
Fabry vs Hunter	Both X-linked; Fabry = sphingolipidosis with angiokeratomas/pain; Hunter = MPS II with coarse facies, no corneal clouding

High-yield: Hunter has NO corneal clouding (X-linked), whereas Hurler (MPS I, AR) HAS corneal clouding — the eternal MPS distractor placed beside Fabry.

Complications

• Niemann–Pick / Gaucher: hypersplenism → cytopenias, pathological fractures, growth failure.
• Fabry: progressive renal failure, hypertrophic cardiomyopathy, early stroke.
• Tay–Sachs / Krabbe / NP-A: intractable seizures, blindness, death usually in early childhood.
• Surfactant (phospholipid) deficiency: neonatal RDS, the obstetric/paediatric crossover from the L:S ratio.

Recently asked / exam angle

• Enzyme–substrate matching is the single most common stem: e.g. “Glucocerebroside accumulates in deficiency of ___” (β-glucocerebrosidase, Gaucher).
• “Cherry-red spot but no hepatosplenomegaly” → Tay–Sachs; the same stem with organomegaly → Niemann–Pick / Sandhoff.
• Crumpled tissue-paper cell image → Gaucher; foamy histiocyte / sea-blue histiocyte → Niemann–Pick.
• X-linked lysosomal disease → Fabry (and Hunter as the MPS partner).
• Sphingomyelin is the only sphingolipid with phosphorus — pure-biochemistry one-liner.
• Cardiolipin location (inner mitochondrial membrane) + VDRL antigen — repeat favourite.
• Phospholipase A₂ releases arachidonic acid; steroids inhibit it — pharmacology-biochem link.
• L:S ratio ≥ 2:1 = fetal lung maturity — obstetrics crossover.
• Fingolimod = S1P receptor modulator — newer pharmacology angle tying sphingosine metabolism to MS therapy.

Rapid revision

1. Glycerophospholipids = glycerol backbone + 2 fatty acids (ester); sphingolipids = sphingosine backbone + 1 fatty acid (amide).
2. Sphingomyelin is the only phosphorus-containing sphingolipid and the only one that is a true phospholipid.
3. Phosphatidic acid is the common precursor of all glycerophospholipids and triacylglycerol.
4. Lecithin (dipalmitoyl-PC) is lung surfactant; L:S ≥ 2:1 = mature fetal lungs.
5. Cardiolipin lives in the inner mitochondrial membrane and is the VDRL antigen.
6. PLA₂ → arachidonic acid, inhibited by steroids (annexin/lipocortin).
7. Gangliosides contain sialic acid (NANA) and concentrate in grey matter; GM2 → Tay–Sachs.
8. Tay–Sachs: Hex A deficiency, cherry-red spot, no organomegaly, Ashkenazi, hyperacusis.
9. Niemann–Pick: sphingomyelinase deficiency, foamy/sea-blue histiocytes, cherry-red spot + organomegaly.
10. Gaucher: glucocerebrosidase deficiency, commonest LSD, crumpled tissue-paper cells, Erlenmeyer-flask femur, treat with ERT/substrate reduction.
11. Krabbe: galactocerebrosidase, globoid cells; MLD: arylsulfatase A, metachromatic granules.
12. Fabry (X-linked): α-galactosidase A, ceramide trihexoside, angiokeratomas, acroparaesthesias, renal/cardiac disease — diagnose by leukocyte enzyme assay.