Lipids Flashcards
Classes of Lipids
-
Glycerolipids
- Triacylglycerols
- Phospholipids
-
Sphingolipids
- Sphingophospholipids
- Sphingoglycolipids
- Isoprenoids
- Eicosanoids

Phospholipids
- Major lipid component of cell membranes
- Composed of an alcohol attached via a phosphodiester bond to either:
-
Diacylglycerol (DAG) ⇒ glycerolphospholipid
- DAG is an intermediate of TAG synthesis
-
Ceramide ⇒ sphingophospholipid
- An amino alcohol spingosine with a fatty acid esterified to the amino group
-
Diacylglycerol (DAG) ⇒ glycerolphospholipid
Triacylglycerol Synthesis
- Glucose or glycerol used to synthesize glycerol-3-phosphate
- Fatty acid tails added via esterification to glycerol-3-phosphate to produce triacylglycerol.
- Phosphatidic acid (aka DAG-P) can be dephosphorylated to DAG then acylated to form TAG.

TAG Functions
- An intermediate for glycerolphospholipid synthesis
- Energy storage medium
- Structural component of lipoproteins
Glycerophospholipids
The major class of phospholipids.
Phosphatidic acid is the simplest glycerophospholipid.
All others are derived from it by esterification of an alcohol.

Cardiolipid
Two phosphatidic acids esterified to an additional glycerol molecule.
- Class of glycerophospholipids
- Found only in the inner mitochondrial membrane
- Required for functioning of the mitochondrial ETC

Plasmalogens
Glycerophospholipid where fatty acid at C-1 of glycerol is attached via an ether linkage.
- Most common plasmalogens are:
- Phosphatidalethanolamine
- Phosphatidalcholine
- -al suffix instead of -yl
- Almost 30% of glycerophospholipids in the brain are plasmologens
- Reduced levels of plasmologens associated with Alzheimer’s disease

Platelet-activating Factor
(PAF)
Ether glycerophospholipid with the fatty acid chain on C-2 replaced with an acetyl group.
- PAF is a mediator of many physiological processes including:
- Platelet aggregation and degranulation
- Inflammation
- Anaphylaxis
Head Groups
Various alcohols which can be esterified to phosphatidic acid:

Glycerophospholipid Charge
- Net charge on a glycerophospholipid depends upon the head group.
- Charge affects the nature of the membrane surface.

Glycerophospholipid
Acyl Tails
- Acyl tails can vary from lipid to lipid and cell type to cell type
- Acyl tails effect the ability to pack lipids and thus membrane fluidity:
- Inc length = dec fluidity
- Double bonds = inc fluidity
- Unsaturated FA usually at C-2
- Saturated FA usually at C-1
- Naturally occuring lipids have saturated or cis-DB
- Changing membrane fluidity effects:
- Transport
- Signal transduction
- Etc.

Sphingophospholipids
- Use the long chain amino alcohol sphingosine as a backbone to which FA are attached
- Ceramide is a sphingosine with an extra FA attached
- Major sphingophospholipid in humans is sphingomyelin
- Has a choline head group
- Sometimes ethanolamine found instead
- Sphingomyelin is a major component of the myelin sheath

Phosphatidic Acid Synthesis
- Glucose ⇒ DHAP ⇒ Glycerol-3-Phosphate
or
- Glycerol ⇒ Glycerol-3-Phosphate
- Two fatty acyl tails attached to glycerol-3-phosphate using fatty acyl CoA substrates
- Water esterified as phosphate headgroup to form phosphatidic acid

Phospholipid Synthesis
from
Phosphatidic Acid
- Phosphatidic acid is dephosphorylated to diacylglycerol (DAG)
- CDP is used to activate choline or ethanolamine.
- DAG is condensed with CDP-activated alcohols to form phospholipids.
- DAG + CDP-choline ⇒ Phosphatidylcholine (PC aka lecithin)
- DAG + CDP-ethanolamine ⇒ Phosphatidylethanolamine (PE)
OR
- Phosphatidic acid is dephosphorylated to diacylglycerol (DAG)
- DAG is activated by CDP forming CDP-DAG
- CDP-DAG is condensed with inositol to form Phosphatidylinositol (PI)
- CDP-DAG is condensed with glycerol to form Phosphatidylglycerol (PG)

Phospholipid Interconversion
PC can be made through methylation of PE using SAM.
PS can be made from PE through base exchange of ethanolamine for serine.

Glycerolphospholipid
Grand Scheme

Phospholipases
- Phospholipases hydrolyze phosphodiester bonds.
- Different phospholipases cleave specific phosphodiester bonds.
- Functions to:
- Degrade glycerolphospholipids
- Generate secondary messengers
- Release acyl chains which serve as intermediates in the synthesis of biologically-active molecules
- PL-A1 : cleaves at C-1 producing a lysolipid
-
PL-A2: cleaves at C-2 producing arachidonic acid
- serves as a precursor for eicosanoids
- Cortisol’s anti-inflammatory action via PL-A2 inhibition
-
PL-C: cleaves at ester bond on C-3
- can be activated by specific GPCR’s
- acts on PIP2 to produce DAG and IP3

Pulmonary Surfactant
- Type II pneumocytes synthesize and secrete dipalmitoyl PC (DPPC) which is the main component of surfactant.
- Surfactant prevents alveoli collapse during expiration and reduces pressure needed to inflate alveoli during inspiration.
- DPPC synthesis markedly increases around 32 weeks.
- Lack of surfactant main cause of infant respiratory distress syndrome (IRDS)
- An amniotic fluid lecithin-sphingomyelin (L/S) ratio > 2 indicative of sufficient surfactant synthesis
Synthesis of Plasmalogens
- Ether lipids synthesized from:
- DHAP
- Fatty acids
- Fatty alcohols
- Fatty acid esterified at C-1
- FA exchanged with a long chain fatty alcohol in peroxisomes
- Patients with Zellweger disease are unable to synthesize plasmalogens

Sphingomyelin Synthesis
-
Palmitoyl-CoA condensed with serine to produce sphinganine by Serine Palmitoyl Transferase.
- Reaction requires PLP and NADPH
- Second acyl tail is added from a fatty acyl-CoA to produce ceramide.
- Choline is transfered from PC to ceramide producing sphingomyelin.

Sphingomyelin Degradation
-
Sphingomyelinase (SMase) removes phosphorylcholine from sphingomyelin producing a ceramide.
- Several isoforms of SMase classified according to pH optima.
- Defects in lysosomal acid SMase leads to type A and B Niemann-Pick Disease
- Ceramide degraded into sphingosine and free fatty acid.
Niemann-Pick Disease
Caused by a deficiency of lysosomal acid SMase.
Autosomal recessive.
Results in build-up of sphingomyelin in liver, spleen, and CNS.
- Type A has < 1% enzyme activity.
- Extensive developmental disability
- Neurodegeneration
- Death in early childhood
- Type B has ~ 5-10% enzyme activity.
Glycosphingolipids
- Derived from ceramide
- Do not have a phosphoester moiety
- Polar portion provided by a monosaccharide or oligosaccharide attached via an O-glycosidic linkage
- Depending on number and type of sugar attached, glycosphingolipids subclassified as:
- neutral glycosphingolipids
- acidic glycosphingolipids

Acidic Glycosphingolipids
- Negatively charged at physiological pH due to the presence of:
- N-acetylneuraminic acid (NANA, a sialic acid) in gangliosides
- Sulfates in sulfatides
- Gangliosides
- Primarily found in ganglion cells of CNS
- Named based on number of NANA moieties attached
- Mono - GM series
- Di - GD series
- Tri - GT series
- Quarto - GQ series
- Carbohydrate moiety protrudes from cell surface
- Cell recognition
- Cell-cell communication
- “receptors” for bacterial toxins
- Cholera toxin interacts with GM1












