Phospholipid (PL)s and their Classification, Structure and Synthesis

Structure of a phospholipid (PL)

• The phospholipid structure, like triacylglycerol (energy storage form), uses a glycerol backbone with two fatty acids and one phosphate that is linked to a head group.

• Amphipathic

• R1 = hydrocarbon chain 1 – almost always saturated fatty acid, sometimes MUFAs

• R2 = hydrocarbon chain 2 – almost always unsaturated fatty acid

• Head group = H, choline, serine, ethanolamine, inositol

Glycerol backbone on PL

• 3 carbons

• Primary OH grp of glycerol: C-1 position: usually esterified with saturated or monounsaturated FA for PL

• C-2 position of the glycerol backbone of PL: middle C of the glycerol molecule, esterified with PUFA naturally in most PL, make kink, facilitate signaling and membrane fluidity and flexibility that was tight by s-1 position saturated FAs in the PL bilayer

• C-3: primary OH grp, typically links to phosphate group in PL, In triacylglycerol this is third FAs

Phospholipids

• Several pathways exist for generating PLs

• PLs play several roles, including being an integral component of cell membranes and plasma lipoproteins, storage of fatty acids, and signalling molecules

Phospholipid Synthesis

• Phosphatidic acid (phosphatidate) - precursor for phospholipid synthesis, takes place in the endoplasmic reticulum (ER).

  • ER cytoplasmic side of leaflet

• Phosphatidate is combined with an alcohol to generate the phospholipid:

  • Choline = phosphatidylcholine

  • Inositol = phosphatidylinositol

  • Serine = phosphatidylserine

  • Ethanolamine = phosphatidylethanolamine

Synthesis of Triacylglycerols (TAG) and Phospholipids (PL)

• Most glycerol 3-phosphate comes from dihydroxyacetone phosphate (DHAP) from glycolysis / gluconeogenesis.

• The glycerol → glycerol 3-phosphate reaction is not present in muscle or adipose tissue

• Some glycerol 3-phosphate is made from glycerol via glycerol kinase – a minor pathway in liver and kidney

• Phosphatidic acid (Phosphatidate) is the precursor to TAGs (or TGs) and phospholipids (PL).

  • → DAG → phosphatidylethanolamine via CDP-ethanolamine

  • → DAG → TAG via CDP-choline

    • Dietary choline

PC Synthesis (PEMT Pathway)

1. Glycerol 3-phosphate → Phosphatidic acid (uses 2 fatty acyl-CoA and releases 2 coA)

2. Phosphatidic acid → DAG

3. DAG → phosphatidylethanolamine (uses CDP-ethanolamine and released CDP)

4. Phosphatidylethanolamine → PC (uses (3) S-adenosylmethionine and releases (3) S-adenosylhomocysteine

• De novo pathway

PC Synthesis (CDP-Choline Pathway)

1. Glycerol 3-phosphate → Phosphatidic acid (uses 2 fatty acyl-CoA and releases 2 coA)

2. Phosphatidic acid → DAG

3. DAG → PC (uses CDP-choline and releases CMP)

Synthesis of Phosphatidic Acid Occurs Before Triacylglycerol – Step 1

• Phosphatidic acid is the precursor to TAG and phospholipids synthesis.

  • Activation of fatty acids

  • Fatty acids attached by acyl transferases

  • releases CoA

• Costs 4 ATP

Biosynthesis of Phospholipids – Step 2

• Begin with phosphatidic acid or diacylglycerol

• Attach head group to C-3 OH group

  • C-3 has OH; head group has OH

  • new phospho-head group created when phosphoric acid condenses with these two alcohols

  • eliminates two H2O

  • PL synthesis occurs exclusively in the cytosolic leaflet of the ER membrane

Synthesis from an Activated Alcohol - Step 3

Phosphatidylethanolamine (PE) in mammals can be synthesized from the alcohol ethanolamine through the formation of CDP-ethanolamine (activated form of alcohol).

• Cytidine triphosphate – CTP → donate cytidine monophosphate

Cytidine triphosphate – CTP

Donate cytidine monophosphate

Synthesis from an Activated Alcohol - Step 3

• Phosphatidylethanolamine (PE) in mammals can be synthesized from the alcohol ethanolamine through the formation of CDP-ethanolamine.

• ATP phosphorylates ethanolamine to form the precursor, phosphorylethanolamine.

• This precursor then reacts with CTP to form the activated alcohol, CDP-ethanolamine.

• The phosphorylethanolamine unit of CDP-ethanolamine is subsequently transferred to a diacylglycerol to form PE.

PC Synthesis: Two Ways

• PEMT pathway = Kennedy pathway

• ~30% of PC synthesis in the liver - de novo synthesis – hepatocytes have higher activity

• CDP-Choline pathway: ~ 70% of PC synthesis in the liver

Sources for choline to synthesize PC:

• Free choline from diet

• Derived from cellular membrane – continuously re-incorporate into PC

• Insufficient choline affects PC synthesis via PREMT pathway

Phosphatidylcholine – CDP-Choline Pathway

• Most common/abundant phospholipid in mammalian membranes

• Approximately 50% of the membrane mass.

• Dietary choline is activated in a series of reactions.

• CTP-phosphocholine cytidylyltransferase (CCT) catalyzes the formation of CDP-choline, the rate-limiting step in PC synthesis.

• It is an amphitropic enzyme—a class of enzymes whose regulator ligand is the membrane itself.

• It is normally associated with the membrane.

• CCT detect low PC

• That activates CCT

• Increase PC synthesis

The enzyme phosphatidic acid phosphatase (PAP) plays a key role in lipid synthesis regulation:

• It is also called Lipin 1 in mammals.

• It works together with diacylglycerol kinase.

• It controls the extent to which triacylglycerols are synthesized relative to phospholipids.

• It regulates the type of phospholipid synthesized.

Making Phosphatidylcholine from Phosphatidylethanolamine PEMT Pathway

• The liver possesses the enzyme, phosphatidylethanolamine methyltransferase (PEMT), which synthesizes PC from PE when dietary choline is insufficient.

• The amino group of this PE is methylated three times to form PC.

• S-Adenosylmethionine (SAM) is the methyl donor.

• Thus, PC can be produced by two distinct pathways in mammals, ensuring that this phospholipid can be synthesized even if the components for one pathway are in limited supply.

Phosphatidylcholine (PC)

• Constitutes 30 to 60% of the phospholipid mass of eukaryotic cell membranes.

• Depriving cells of PC by inhibiting synthesis or the availability of precursors results in growth arrest and apoptosis.

• PC is also a major component of lipoproteins, bile and lung surfactant.

• PC is a reservoir for second messengers such as fatty acids, diacylglycerol, phosphatidic acid and lyso-PC.

• PC is an important precursor for the synthesis of phosphatidylserine.

Phosphatidylserine

• Crucial.

• In neural tissues cofactor for signaling

• Apoptotic cell clearance

• Phosphatidylserine (PS) makes up 10% of the phospholipids in mammals

Base-Exchange Reactions Can Generate Phospholipids

• This phospholipid is synthesized by phosphatidyl serine synthase

• base-exchange reaction of serine with PC or PE.

• In the reaction, serine replaces choline or ethanolamine.

• Phosphatidylcholine + serine → choline + phosphatidylserine

• Phosphatidylethanolamine + serine → ethanolamine + phosphatidylserine

Liver lipid deposition – histology of liver – when gut bypassed:

• CDCA with TPN : normalize serum bile acid level, liver TG and liver lipid deposition and color of the liver.

• Bile synthesis also use PC, utilize liver lipids.

Phospholipid Catabolism

• The catabolism of PLs involve the hydrolysis of fatty acids or head groups by a group of enzymes known as phospholipases (PLase)

  • A1, A2, C, and D

• PLase’s are essential for providing several critical molecules involved in the regulation of various cell activities

Phospholipase A1 and A2

• Phospholipase A1 or A2 liberate a single fatty acid from PL and generate a lysophospholipid (e.g. phosphatidylcholine to lysophosphatidylcholine)

• Many toxic snake venoms contain highly active PLaseA1 and PLaseA2 enzymes that cause cell membrane damage, induce severe blood clotting/anti-clotting, inflammatio

Mammalian phospholipase A2 & Eicosanoids

• Mammals use phospholipase A2 to release arachidonic acid (20:4n-6) from the sn-2 position of membrane phospholipids

• The released arachidonic acid is rapidly enzymatically modified to produce eicosanoids

• Potent lipid signaling molecules involved in regulating many physiological events, including blood clotting, reproduction, inflammation, pain, fever, blood pressure

• Omega-3 fatty acids inhibit the production of omega-6 fatty acid derived eicosanoids (omega-6 fatty acids are generally inflammatory while omega-3 fatty acids are anti (less) inflammatory)

Phospholipase C

• Liberates a phospho-head group (ie. phosphocholine) and diacylglycerol from PL

• Diacylglycerol liberated by PLase C functions as a potent intracellular signaling molecules that regulate cellular [Ca2+]

Phospholipase D

• Hydrolyzes conversion of a phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, or phosphatidylinositol into phosphatidic acid.

• Phosphatidic acid produced through this route can be used toward the synthesis of triacylglycerol.