15 - Biosynthesis of Lipids

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Biosynthesis of Lipids

Overview: Lipid Classes

Category	Type	Alcohol Component
Storage lipids (neutral)	Triacylglycerols	Glycerol
Membrane lipids (polar)	Glycerophospholipids	Glycerol
Membrane lipids (polar)	Sphingolipids (phospholipids)	Sphingosine + Choline
Membrane lipids (polar)	Sphingolipids (glycolipids)	Sphingosine + Mono/oligosaccharide

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Biosynthesis of Triacylglycerols

Biosynthesis of Phosphatidic Acid

Step 1 — Activation of Fatty Acids to Fatty Acyl-CoA

Catalyzed by fatty acyl-CoA synthetase and inorganic pyrophosphatase, in two sub-steps:

1. The carboxylate of the fatty acid displaces $P_{\beta }$ and $P_{\gamma }$ of ATP → forms a fatty acyl-adenylate (mixed anhydride of carboxylic acid + phosphoric acid) + $P{P}_i$
2. The thiol group of CoA performs nucleophilic attack on the enzyme-bound mixed anhydride → displaces AMP → forms fatty acyl-CoA (thioester)

$\text{Fatty acid}+\text{ATP}+\text{CoA-SH}\stackrel{\text{acyl-CoA synthetase}}{\to }\text{Fatty acyl-CoA}+\text{AMP}+P{P}_i$

$P{P}_i+H_{2}O\stackrel{\text{inorganic pyrophosphatase}}{\to }2P_i$

The hydrolysis of $P{P}_i$ to $2P_i$ pulls the overall reaction strongly forward.

$\Delta G^{\prime \circ }\approx -19\text{kJ/mol}$ (two-step process combined)

Step 2 — Acylation of L-Glycerol 3-Phosphate

L-Glycerol 3-phosphate is formed by two routes:

Source	Enzyme	Substrate
Glycolysis	Glycerol 3-phosphate dehydrogenase (uses $\text{NADH}$)	Dihydroxyacetone phosphate (DHAP)
Free glycerol	Glycerol kinase (uses ATP)	Glycerol

Two sequential acyl-CoA transferase reactions add fatty acyl chains to positions sn-1 and sn-2 → Phosphatidic acid (diacylglycerol 3-phosphate)

Phosphatidic acid carries the correct stereochemistry at C-2 of glycerol.

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Phosphatidic Acid as Central Precursor

OC(COC(=O)CCCCCCC)COC(=O)CCCCCCC

Phosphatidic acid is the branch-point precursor for both triacylglycerols and glycerophospholipids.

Pathway	First step from phosphatidic acid	Product
Triacylglycerol synthesis	Phosphatidic acid phosphatase → 1,2-diacylglycerol; then acyl transferase adds 3rd acyl chain	Triacylglycerol
Glycerophospholipid synthesis	CDP-diacylglycerol pathway or head-group attachment	Glycerophospholipid

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Regulation of Triacylglycerol Synthesis by Insulin

Insulin stimulates conversion of dietary carbohydrates and proteins to fat.

In type 1 diabetes mellitus (absence of insulin):

• Fatty acid synthesis is diminished
• Acetyl-CoA from catabolism of carbohydrates and proteins is shunted to ketone body production instead

$\text{Acetyl-CoA}\stackrel{\text{(diabetes)}}{\to }\text{Ketone bodies (e.g., }\beta \text{-hydroxybutyrate, acetoacetate)}$

Uncontrolled diabetes → increased ketogenesis due to lack of insulin-driven lipogenesis.

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In the absence of insulin (as in diabetes mellitus), Acetyl-CoA is shunted away from fatty acid synthesis toward {1:ketone body production}.

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The Triacylglycerol Cycle

During starvation in mammals, triacylglycerol is continuously broken down and resynthesized in a triacylglycerol cycle.

Flow of the cycle:

1. Lipolysis in adipose tissue → releases free fatty acids into bloodstream
2. Fatty acids used for energy (e.g., in muscle) OR taken up by liver → re-synthesized into triacylglycerol
3. Liver triacylglycerol transported back to adipose tissue (via blood)
4. Extracellular lipoprotein lipase cleaves fatty acids → taken up by adipocytes → re-esterified into triacylglycerol

This cycle may maintain a rapidly mobilizable energy reserve in the bloodstream during fasting, more accessible than stored triacylglycerol in a "fight or flight" scenario.

Problem: Source of Glycerol 3-Phosphate During Starvation

During starvation, glycolysis is suppressed by glucagon and epinephrine → little DHAP available. Adipose tissue also lacks glycerol kinase → cannot phosphorylate released glycerol directly.

Solution: Glyceroneogenesis

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Glyceroneogenesis

Glyceroneogenesis is an abbreviated version of gluconeogenesis: from pyruvate → DHAP → glycerol 3-phosphate (used for triacylglycerol synthesis). It does not proceed all the way to glucose.

Pathway:

$\text{Pyruvate}\stackrel{\text{pyruvate carboxylase}}{\to }\text{Oxaloacetate}\stackrel{\text{PEPCK}}{\to }\text{Phosphoenolpyruvate}\stackrel{\text{multistep}}{\to }\text{DHAP}\stackrel{\text{glycerol 3-P dehydrogenase}}{\to }\text{Glycerol 3-phosphate}$

These enzymes are present in adipose tissue, where glucose is not synthesized — glyceroneogenesis provides glycerol 3-phosphate exclusively for local triacylglycerol synthesis.

Regulation of Glyceroneogenesis

Glucocorticoids have reciprocal effects on PEPCK expression:

Tissue	Glucocorticoid Effect on PEPCK	Consequence
Liver	Stimulate	↑ gluconeogenesis + glyceroneogenesis → glycerol → glucose
Adipose tissue	Suppress	↓ glyceroneogenesis → ↑ flux through triacylglycerol cycle

Glycerol released from adipose lipolysis → liver → converted primarily to glucose (via glycerol kinase + gluconeogenesis); some converted to glycerol 3-phosphate.

Thiazolidinediones (Type 2 Diabetes Treatment)

In type 2 diabetes, elevated free fatty acids in blood interfere with glucose utilization in muscle and promote insulin resistance.

Mechanism of thiazolidinediones:

1. Activate nuclear receptor PPAR (peroxisome proliferator-activated receptor $\gamma$)
2. PPAR induces expression of PEPCK
3. ↑ Glyceroneogenesis in adipose tissue
4. ↑ Re-esterification of fatty acids → ↓ free fatty acids in blood

Examples: Rosiglitazone, Pioglitazone

CC(=O)Nc1ccc(OCC2CS(=O)(=O)NC2=O)cc1

(Rosiglitazone — representative thiazolidinedione core)

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Thiazolidinediones treat type 2 diabetes by activating {1:PPARγ}, which upregulates {2:PEPCK}, increasing {3:glyceroneogenesis} in adipose tissue and reducing circulating {4:free fatty acids}.

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Biosynthesis of Membrane Phospholipids

Glycerophospholipid Structure

General structure: sn-1 saturated fatty acid, sn-2 unsaturated fatty acid, sn-3 phosphate + head group X.

Glycerophospholipid	Head group (X)	Net charge (pH 7)
Phosphatidic acid	—H	−1
Phosphatidylethanolamine	Ethanolamine $(-C{H}_2-C{H}_2-N{H}_3^{+})$	0
Phosphatidylcholine	Choline $(-C{H}_2-C{H}_2-N(C{H}_3{)}_3^{+})$	0
Phosphatidylserine	Serine $(-C{H}_2-CH(N{H}_3^{+})-CO{O}^{-})$	−1
Phosphatidylglycerol	Glycerol	−1
Phosphatidylinositol 4,5-bisphosphate	myo-Inositol 4,5-bisphosphate	−4
Cardiolipin	Phosphatidylglycerol	−2

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Sphingolipid Structure

Sphingolipid	Head group (X)
Ceramide	—H
Sphingomyelin	Phosphocholine
Glucosylcerebroside	Glucose
Lactosylceramide (globoside)	Di-, tri-, or tetrasaccharide
Ganglioside GM2	Complex oligosaccharide

Blood group antigens (A, B, O) are determined by oligosaccharide head groups on glycosphingolipids (ceramide backbone).

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Strategies for Phosphodiester Bond Formation

In both strategies, CDP (cytidine diphosphate) supplies the phosphate group of the phosphodiester bond. The high-energy phosphoanhydride bond drives the reaction.

Strategy	What is activated with CDP	Used for
Strategy 1	Diacylglycerol (CDP-diacylglycerol)	Phosphatidylglycerol, cardiolipin, phosphatidylinositol
Strategy 2	Head group (CDP-head group)	Phosphatidylcholine (in mammals)

Strategy 1 — CDP-Diacylglycerol Pathway (Eukaryotes)

Used to synthesize phosphatidylglycerol, cardiolipin, and phosphatidylinositol:

• CDP-diacylglycerol + inositol → Phosphatidylinositol + CMP
• CDP-diacylglycerol + glycerol 3-phosphate → phosphatidylglycerol → Cardiolipin (via cardiolipin synthase)

The hydroxyl groups of cardiolipin's glycerol bridge can also be esterified with $-P{O}_4^{3-}$.

Strategy 2 — CDP-Head Group Pathway (Mammals)

Example: Phosphatidylcholine synthesis from choline

$\text{Choline}\stackrel{\text{choline kinase, ATP}}{\to }\text{Phosphocholine}\stackrel{\text{CTP-choline cytidylyl transferase, CTP}}{\to }\text{CDP-choline}+P{P}_i$

$\text{CDP-choline}+\text{1,2-Diacylglycerol}\stackrel{\text{CDP-choline–diacylglycerol phosphocholine transferase}}{\to }\text{Phosphatidylcholine}+\text{CMP}$

Phosphatidylserine and Phosphatidylethanolamine Interconversion

Phosphatidylserine and phosphatidylethanolamine are interconverted by a reversible head-group exchange reaction. In mammals, phosphatidylserine is derived from phosphatidylethanolamine by reversal of this reaction.

$\text{Phosphatidylethanolamine}+\text{Serine}\rightleftharpoons \text{Phosphatidylserine}+\text{Ethanolamine}$

$\text{Phosphatidylserine}\stackrel{\text{phosphatidylserine decarboxylase}}{\to }\text{Phosphatidylethanolamine}+C{O}_2$

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Phosphatidylcholine synthesis in mammals uses Strategy {1:2} (CDP-{2:head group} activation), while cardiolipin and phosphatidylinositol use Strategy {3:1} (CDP-{4:diacylglycerol} activation).

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Synthesis of Ether Lipids and Plasmalogens

Ether lipids have an ether linkage (C–O–C) at sn-1 instead of an ester linkage.

Pathway Overview:

1. Fatty acyl-CoA + DHAP → 1-acyldihydroxyacetone 3-phosphate (acyltransferase)
2. Long-chain alcohol displaces the acyl chain → 1-alkyldihydroxyacetone 3-phosphate (1-alkylDHAP synthase) → ether bond formed
3. Reduction with NADPH → 1-alkylglycerol 3-phosphate
4. Addition of acyl group at sn-2 → 1-alkyl-2-acylglycerol 3-phosphate (ether analog of phosphatidic acid)
5. Head-group attachment (same mechanisms as ester-linked analogs)
6. For plasmalogens: a mixed-function oxidase introduces a characteristic vinyl ether double bond at sn-1 in a final step

The newly formed ether linkage and the vinyl ether double bond (plasmalogen-defining feature) are introduced by distinct enzymatic steps.

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Biosynthesis of Sphingolipids

Pathway:

1. Condensation: Palmitoyl-CoA + Serine $\stackrel{-\text{CoA-SH},-C{O}_2}{\to }$ 3-ketosphinganine → reduced by NADPH → sphinganine
2. N-acylation: Sphinganine + Fatty acyl-CoA → N-acylsphinganine (ceramide)
3. Desaturation: Mixed-function oxidase introduces double bond → ceramide with sphingosine backbone (sphingosine = trans-4-sphingenine)
4. Head-group addition:
   • • UDP-Glucose → glucosylcerebroside (neutral glycolipid)
   • • Phosphatidylcholine → sphingomyelin + diacylglycerol

CCCCCCCCCCCCCC/C=C/[C@@H](O)[C@@H](N)CO

(Sphingosine)

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Sphingolipid synthesis begins with condensation of {1:palmitoyl-CoA} and {2:serine}, producing 3-ketosphinganine, which is reduced to {3:sphinganine}. A mixed-function oxidase then introduces a {4:double bond} to yield ceramide with a sphingosine backbone.

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Lipid Composition of Membranes

Rat Hepatocyte Membrane Lipid Composition

Lipid	Plasma membrane	Inner mitochondrial	Notes
Cholesterol	High	Barely detectable	Prominent only in plasma membrane
Cardiolipin	Absent	Major component	Unique to inner mitochondrial membrane
Sphingolipids	Present	Low	Variable across membranes
Phosphatidylcholine	Present	Present	Widespread
Phosphatidylethanolamine	Present	Present	Widespread
Phosphatidylserine, PI, PG	Minor	Minor	Critical functions despite low abundance
Glycolipids	Virtually absent	Virtually absent	Absent from animal cells generally

Each membrane type has a unique lipid composition reflecting its functional specialization.

Phosphatidylinositol and its phosphorylated derivatives (e.g., $PI{P}_2$) are critical second messengers in signal transduction triggered by hormones, despite being minor membrane components.

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Cardiolipin is a major lipid component of the {1:inner mitochondrial membrane} and is virtually absent from the {2:plasma membrane}.

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{1:Cholesterol} is prominent in the plasma membrane but barely detectable in {2:mitochondrial membranes}.

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TLDR

Biosynthesis of Lipids — Summary

• Fatty acid activation: fatty acid + CoA-SH + ATP → fatty acyl-CoA + AMP + $P{P}_i$; $P{P}_i$ hydrolysis drives reaction forward; $\Delta G^{\prime \circ }\approx -19\text{kJ/mol}$
• L-Glycerol 3-phosphate is formed from DHAP (glycolysis, via glycerol 3-phosphate dehydrogenase) or from glycerol (glycerol kinase)
• Phosphatidic acid = diacylglycerol 3-phosphate; central branch-point precursor for triacylglycerols AND glycerophospholipids
• Triacylglycerol: phosphatidic acid → (phosphatase) → 1,2-diacylglycerol → (acyltransferase) → triacylglycerol
• Insulin stimulates lipogenesis; its absence (diabetes) shunts acetyl-CoA to ketone body production
• Triacylglycerol cycle: continuous lipolysis + re-esterification even during starvation; may maintain rapidly mobilizable energy reserve
• Glyceroneogenesis: pyruvate → OAA → PEP → DHAP → glycerol 3-phosphate (abbreviated gluconeogenesis); supplies glycerol 3-phosphate in adipose during starvation when glycerol kinase is absent and DHAP is scarce
• Glucocorticoids stimulate PEPCK in liver (↑ gluconeogenesis + glyceroneogenesis) but suppress PEPCK in adipose (↑ triacylglycerol cycle flux)
• Thiazolidinediones (rosiglitazone, pioglitazone) activate PPARγ → ↑ PEPCK → ↑ glyceroneogenesis in adipose → ↓ free fatty acids → treat type 2 diabetes insulin resistance
• Two strategies for phosphodiester bond in glycerophospholipids: Strategy 1 = CDP-diacylglycerol (cardiolipin, PI); Strategy 2 = CDP-head group (phosphatidylcholine in mammals)
• Phosphatidylserine ↔ phosphatidylethanolamine: reversible head-group exchange; PS → PE via decarboxylase ($-C{O}_2$)
• Ether lipids/plasmalogens: ether bond at sn-1 formed from long-chain alcohol + DHAP; plasmalogens have additional vinyl ether double bond introduced by mixed-function oxidase
• Sphingolipid synthesis: palmitoyl-CoA + serine → sphinganine → ceramide (N-acylation) → sphingomyelin (+ phosphatidylcholine) or glucosylcerebroside (+ UDP-Glucose)
• Membrane lipid distribution is organelle-specific: cholesterol high in plasma membrane, cardiolipin exclusive to inner mitochondrial membrane, $PI{P}_2$ critical for signaling despite low abundance, glycolipids virtually absent from animal cells