Lipid Metabolism
Storage, Digestion, Transport & Mobilisation of Triacylglycerols
1. Synthesis and Storage of Triacylglycerols (TAG)
Triacylglycerols (TAGs, also designated as neutral fats or triglycerides) are highly concentrated energy reserves consisting of three fatty acyl chains esterified to a single glycerol backbone. While almost all eukaryotic cells possess some capacity to synthesize TAGs, the liver and intestinal mucosal cells represent the most active metabolic sites in animals.
1.1 Physical Storage: Cytoplasmic Lipid Droplets
Triacylglycerols are completely hydrophobic and physically insoluble in aqueous cellular compartments. Consequently, they are stored intracellularly as spherical lipid droplets (also designated as fat globules, oil bodies, lipid particles, or adiposomes).
Figure: Ultrastructure of a lipid droplet. A hydrophobic core of triacylglycerols and cholesterol esters is enclosed by a single monolayer of polar phospholipids, studded with surface proteins — primarily perilipins in animal adipose tissue and oleosins in plant seeds — that regulate access to the core.
1.2 Biosynthetic Pathways
There are two primary metabolic pathways for the synthesis of triacylglycerols.
1.2.1 The sn-Glycerol-3-Phosphate (Kennedy) Pathway
This is the universal de novo pathway that predominates in the liver, kidney, and adipose tissue. It proceeds via four sequential enzymatic reactions.
Figure: The Kennedy pathway. Two sequential acylations of glycerol-3-phosphate form phosphatidic acid; dephosphorylation yields diacylglycerol, which is acylated a third time to produce the fully hydrophobic neutral fat.
- First Acylation: A fatty acyl group from a fatty acyl-CoA is transferred to the C-1 position of sn-glycerol-3-phosphate to form lysophosphatidic acid (1-acyl-glycerol-3-phosphate).Glycerol-3-P + Fatty acyl-CoA GPAT→ Lysophosphatidic Acid
- Second Acylation: A second fatty acyl group is transferred to the C-2 position of lysophosphatidic acid to produce phosphatidic acid (1,2-diacylglycerol-3-phosphate).Lysophosphatidic Acid + Fatty acyl-CoA AGPAT→ Phosphatidic Acid
- Dephosphorylation: The phosphate group at the C-3 position of phosphatidic acid is hydrolytically removed to yield diacylglycerol (DAG).Phosphatidic Acid + H2O PAP (lipin)→ DAG + Pi
- Third Acylation: A third fatty acyl chain is transferred to the remaining C-3 hydroxyl group of diacylglycerol to form the fully hydrophobic neutral fat, triacylglycerol (TAG).DAG + Fatty acyl-CoA DGAT→ Triacylglycerol (TAG)
1.2.2 The Monoacylglycerol Pathway
This specialized alternative pathway operates exclusively within intestinal mucosal cells (enterocytes) during the absorption of dietary fats.
2. Lipid Digestion, Transport, and Mobilisation
From the Gut to the Bloodstream to the Adipocyte
2. Lipid Digestion, Transport, and Mobilisation
The complete insolubility of lipids in water presents a major physiological barrier for digestion, transport through the aqueous bloodstream and lymph, and intracellular mobilization.
2.1 Digestion of Dietary Lipids
The digestion of dietary triacylglycerols is a multi-step process designed to convert insoluble fat globules into water-soluble micelles that can be absorbed by the intestinal mucosa.
Figure: Dietary lipid digestion. Lingual and gastric lipases perform limited initial hydrolysis; bile salts emulsify fat globules into micelles in the duodenum; pancreatic lipase cleaves the C-1 and C-3 ester bonds, releasing two free fatty acids and one 2-monoacylglycerol per triacylglycerol, which are absorbed and re-esterified for chylomicron assembly.
- Lingual and Gastric Lipases: initiate lipid digestion in the mouth and stomach. These acidic lipases perform limited hydrolysis of short- and medium-chain triacylglycerols.
- Bile Salt Emulsification: in the duodenum, large fat globules are emulsified by bile salts (such as cholic acid and chenodeoxycholic acid) — amphipathic molecules synthesized from cholesterol in the liver and stored in the gallbladder. They act as biological detergents, coat lipid droplets, and form microscopic micelles, drastically increasing the surface area accessible to water-soluble digestive enzymes.
- Pancreatic Lipase: secreted by the pancreas into the small intestine, this enzyme acts at the lipid-water interface. It specifically cleaves the ester bonds at the C-1 and C-3 positions of triacylglycerols, yielding two free fatty acids and one 2-monoacylglycerol.
- Intestinal Lipases: hydrolyze any remaining ester linkages at the C-2 position. Fewer than 10% of dietary triacylglycerols remain completely unhydrolyzed.
2.2 Re-esterification and Chylomicron Assembly
Once absorbed across the microvilli of the intestinal mucosa, free fatty acids and 2-monoacylglycerols are transported to the endoplasmic reticulum and Golgi complex of the enterocyte, where they are re-esterified back into triacylglycerols. To be transported through the aqueous lymphatic and circulatory systems, these triacylglycerols are packaged with dietary cholesterol and specific proteins to form chylomicrons.
Figure: Chylomicron structure. A hydrophobic core of triacylglycerols and cholesterol esters is surrounded by a polar monolayer of phospholipids, free cholesterol, and apolipoproteins.
2.3 Lipoprotein Classification and Transport Pathways
Lipoproteins are classified into five distinct classes based on their density, determined by their relative lipid-to-protein ratio. The pathways of lipid transport are divided into the exogenous pathway (dietary lipids) and the endogenous pathway (liver-derived lipids).
Figure: Exogenous and endogenous lipid transport pathways. The exogenous pathway carries dietary lipids from the intestine to the liver via chylomicrons and their remnants. The endogenous pathway carries liver-synthesized lipids outward via VLDL, which is progressively remodeled by capillary lipases into IDL and then cholesterol-rich LDL.
2.3.1 Lipoprotein Classes and Densities
| Class | Density (g/mL) | Role |
|---|---|---|
| Chylomicrons | < 0.95 | Synthesized in the intestine; carry exogenous (dietary) triacylglycerols |
| VLDL | 0.95–1.006 | Synthesized in the liver; transport endogenous triacylglycerols and cholesterol to peripheral tissues |
| IDL | 1.006–1.019 | Formed as VLDL is depleted of triacylglycerols by peripheral tissue lipases |
| LDL | 1.019–1.063 | Formed from IDL; primary transport vehicle for cholesterol in human blood plasma |
| HDL | 1.063–1.21 | Synthesized in the liver and intestine; mediates reverse cholesterol transport back to the liver |
2.3.2 The Lipase Cleavage Network
- Lipoprotein Lipase (LPL): a capillary wall-associated enzyme activated by apolipoproteins. LPL hydrolyzes the triacylglycerol component of circulating chylomicrons and VLDL into free fatty acids and glycerol, which are rapidly absorbed by muscle cells (oxidized for energy) and adipose cells (re-esterified and stored).
- Chylomicron Remnants: as chylomicrons lose their TAG core via LPL activity, they shrink, leaving behind a remnant particle rich in cholesterol and proteins. These remnants are cleared from circulation within minutes by liver hepatocytes via receptor-mediated endocytosis.
- Hepatic Lipase (HL / HTGL): located on the hepatic capillary endothelium, this enzyme hydrolyzes residual lipids in IDL, converting these particles into cholesterol-dense LDL.
- LDL Receptors: circulating LDL is cleared from the bloodstream by binding to specific LDL receptors. Approximately 70% of LDL clearance occurs in the liver, with the remaining 30% cleared by peripheral extrahepatic tissues.
2.4 Mobilisation of Adipose Triacylglycerols (Lipolysis)
During fasting, exercise, or stress, stored triacylglycerols in adipose tissue must be mobilized and broken down into free fatty acids and glycerol to be transported to energy-consuming tissues. This process is called lipolysis.
Figure: The hormonal lipolysis cascade. Glucagon and epinephrine raise intracellular cAMP and activate PKA, which triggers a three-enzyme cascade — hormone-sensitive lipase, diacylglycerol lipase, and monoacylglycerol lipase — that sequentially strips all three fatty acids from triacylglycerol.
- Hormonal Activation: lipolysis is triggered by the lipolytic hormones glucagon, epinephrine, and norepinephrine. These hormones bind to specific cell-surface receptors, driving an increase in intracellular cyclic AMP (cAMP), which activates protein kinase A (PKA).
- Hormone-Sensitive Lipase (TAG Lipase): PKA phosphorylates and activates hormone-sensitive lipase (HSL), which also translocates to the lipid droplet surface. HSL acts as the rate-limiting enzyme, cleaving the first fatty acid from triacylglycerol to yield diacylglycerol (DAG).
- Diacylglycerol Lipase: cleaves the second fatty acid from DAG to yield monoacylglycerol (MAG).
- Monoacylglycerol Lipase: cleaves the final fatty acid from MAG, releasing one molecule of free glycerol and three molecules of free fatty acids.
3. Biosynthesis of Fatty Acids
Building Long-Chain Fatty Acids from Two-Carbon Units
3. Biosynthesis of Fatty Acids
Fatty acid de novo synthesis is an anabolic pathway that constructs long-chain saturated fatty acids from two-carbon acetyl-CoA units. It occurs primarily in the cytosol of animal and yeast cells and within the chloroplast stroma of plants.
3.1 The Citrate Shuttle System
While fatty acid synthesis occurs exclusively in the cytosol, the starting material acetyl-CoA is generated within the mitochondrial matrix via pyruvate oxidation. Because the inner mitochondrial membrane (IMM) is completely impermeable to acetyl-CoA, cells transport these carbon units into the cytosol using the citrate shuttle system.
Figure: The citrate shuttle. Mitochondrial acetyl-CoA condenses with oxaloacetate to form citrate, which crosses the inner membrane and is cleaved in the cytosol to regenerate acetyl-CoA (for fatty acid synthesis) and oxaloacetate. Oxaloacetate is reduced to malate, then decarboxylated by malic enzyme to pyruvate — generating NADPH that directly powers synthesis — and pyruvate returns to the matrix to be re-carboxylated, closing the loop.
- Condensation: inside the mitochondrial matrix, acetyl-CoA condenses with oxaloacetate to form citrate, catalyzed by citrate synthase.
- Transport: citrate is transported across the inner membrane into the cytosol via a specific citrate transporter.
- Cleavage: in the cytosol, citrate is cleaved by citrate lyase in an ATP-dependent reaction to regenerate acetyl-CoA and oxaloacetate.Citrate + ATP + CoA-SH Citrate Lyase→ Acetyl-CoA + Oxaloacetate + ADP + Pi
- Oxaloacetate Return: since oxaloacetate cannot cross back into the mitochondria directly, it is first reduced to malate by cytosolic malate dehydrogenase (MDH), consuming one cytosolic NADH.
- Malic Enzyme Decarboxylation: malate is oxidatively decarboxylated to pyruvate by cytosolic malic enzyme, generating one molecule of cytosolic NADPH which directly powers fatty acid synthesis. Pyruvate is then transported back into the mitochondrial matrix.
- Carboxylation: inside the matrix, pyruvate is carboxylated to oxaloacetate by the ATP-dependent, biotin-containing enzyme pyruvate carboxylase, completing the shuttle loop.
3.2 Acetyl-CoA Carboxylase (The Committed Step)
The first committed and rate-limiting step of fatty acid synthesis is the carboxylation of acetyl-CoA to form the three-carbon intermediate malonyl-CoA, catalyzed by acetyl-CoA carboxylase (ACC).
- Prosthetic Group: ACC is a biotin-dependent enzyme. The biotin ring is covalently linked to a lysine residue, acting as an activated carrier of carbon dioxide.
- Allosteric Regulation: ACC is activated allosterically by citrate (signaling high energy and carbon availability) and inhibited allosterically by palmitoyl-CoA, the final product of the pathway, acting as a feedback inhibitor.
- Covalent Regulation — Inactivation: glucagon and epinephrine trigger phosphorylation of ACC, inactivating the enzyme to halt fatty acid synthesis.
- Covalent Regulation — Activation: insulin promotes dephosphorylation of ACC, activating the enzyme to stimulate fatty acid synthesis.
3.3 Fatty Acid Synthase (FAS) Machinery
The systematic elongation of the fatty acid chain is carried out by the multi-enzyme complex fatty acid synthase (FAS). During this process, all intermediates remain covalently attached to the sulfhydryl (–SH) group of a phosphopantetheine prosthetic group.
Figure: The phosphopantetheine prosthetic group. A long, flexible arm links the FAS protein (via serine) to a terminal sulfhydryl group, to which every growing acyl intermediate is covalently tethered during elongation.
3.3.1 Evolutionary Variants of FAS
| Variant | Organisms | Architecture |
|---|---|---|
| FAS I | Vertebrates | A massive, multifunctional homodimer; each monomer contains seven catalytic domains on one continuous polypeptide, including an integrated ACP domain |
| FAS I | Fungi | A heteromultimeric (α6β6) complex; the seven catalytic activities are divided across two different polypeptide chains |
| FAS II | Plants & Bacteria | Seven individual, separate, monofunctional polypeptides that associate as a loose multi-protein complex; ACP is a small, independent polypeptide |
3.4 The Elongation Cycle of Palmitate Synthesis
Before the elongation cycle begins, the FAS complex is primed: an acetyl group from acetyl-CoA is loaded onto the sulfhydryl group of FAS by acetyl transacylase, and a malonyl group from malonyl-CoA is loaded onto the sulfhydryl group of ACP by malonyl transacylase.
Figure: One round of the FAS elongation cycle. Condensation releases CO2 to drive the reaction forward; two NADPH-dependent reductions bracket a dehydration step, yielding a saturated four-carbon acyl-ACP two carbons longer than the starting unit.
- Condensation: the acetyl group (or growing acyl chain) is condensed with the malonyl group by β-ketoacyl-ACP synthase, releasing the malonyl carboxyl group as gaseous CO2 to yield the four-carbon acetoacetyl-ACP intermediate. This decarboxylation drives the endergonic condensation forward.
- First Reduction: the ketone group at C-3 is reduced to a hydroxyl group by β-ketoacyl-ACP reductase, consuming one molecule of NADPH to yield D-3-hydroxybutyryl-ACP.
- Dehydration: water is removed from the C-2 and C-3 positions by β-hydroxyacyl-ACP dehydratase to introduce a double bond, forming the unsaturated intermediate crotonyl-ACP.
- Second Reduction: the double bond is reduced by enoyl-ACP reductase, consuming a second molecule of NADPH to yield the saturated four-carbon intermediate butyryl-ACP.
3.4.1 Termination and Stoichiometry
This four-step cycle repeats a total of seven times, each round adding a two-carbon unit from malonyl-CoA to the carboxyl end of the growing chain. When the chain reaches 16 carbons (palmitoyl-ACP), palmitoyl thioesterase hydrolytically cleaves the thioester bond to release free palmitate.
3.5 Fatty Acid Modification
3.5.1 Elongation Systems
Palmitate can be further elongated by microsomal (endoplasmic reticulum) and mitochondrial enzyme systems.
- ER Elongation: adds two-carbon units using malonyl-CoA as the donor and NADPH as the reducing agent. A common product is the 18-carbon saturated fatty acid stearic acid.
- Mitochondrial Elongation: adds two-carbon units by reversing the steps of β-oxidation, utilizing acetyl-CoA as the carbon donor.
3.5.2 Unsaturation and Desaturase Complexes
Double bonds are introduced into saturated fatty acids by membrane-bound desaturases (mixed-function oxidases) located in the endoplasmic reticulum — an electron transport system of three membrane proteins.
Figure: The desaturase electron transport chain. Electrons pass from NADH through cytochrome b5 reductase and cytochrome b5 to the non-heme iron desaturase, which uses them to introduce a double bond into the fatty acyl-CoA substrate while reducing O2 to two molecules of water.
4. Fatty Acid Oxidation (β-Oxidation)
Catabolism of Fatty Acids in the Mitochondrial Matrix
4. Fatty Acid Oxidation (β-Oxidation)
β-oxidation is the primary catabolic pathway for fatty acids. It occurs within the mitochondrial matrix of animal cells, where fatty acids are sequentially oxidized from their carboxyl end to yield acetyl-CoA, NADH, and FADH2.
4.1 Activation and the Carnitine Shuttle
4.1.1 Activation in the Cytosol
Before entering the mitochondria, fatty acids in the cytosol must be activated to high-energy fatty acyl-CoA intermediates by outer-membrane acyl-CoA synthetase (acyl-CoA ligase / thiokinase), consuming ATP and releasing inorganic pyrophosphate.
4.1.2 The Carnitine Shuttle System
Short- and medium-chain fatty acids (fewer than 12 carbons) can diffuse directly into the mitochondrial matrix. Long-chain fatty acyl-CoAs, however, cannot cross the impermeable inner mitochondrial membrane and must be transported via the carnitine shuttle.
Figure: The carnitine shuttle. CPT I converts cytosolic fatty acyl-CoA to acyl-carnitine, translocase exchanges it across the inner membrane, and CPT II regenerates fatty acyl-CoA in the matrix.
- Transesterification: at the outer mitochondrial membrane, carnitine acyltransferase I (CPT I) transfers the fatty acyl group from coenzyme A to carnitine, forming fatty acyl-carnitine.
- Translocation: a specific carrier protein, carnitine-acylcarnitine translocase, moves fatty acyl-carnitine across the inner membrane into the matrix in exchange for free carnitine.
- Regeneration: on the matrix side, carnitine acyltransferase II (CPT II) transfers the fatty acyl group back to mitochondrial coenzyme A, regenerating fatty acyl-CoA and free carnitine.
4.2 The Saturated Even-Chain β-Oxidation Cycle
Once inside the matrix, the saturated, even-chain fatty acyl-CoA is degraded through a cycle of four sequential reactions.
Figure: One round of β-oxidation. An FAD-linked oxidation, a hydration, an NAD+-linked oxidation, and a thiolytic cleavage together remove one acetyl-CoA and shorten the acyl chain by two carbons per turn.
- First Oxidation: acyl-CoA is oxidized by FAD-linked acyl-CoA dehydrogenases to introduce a trans double bond between the α and β carbons (C-2 and C-3), yielding trans-Δ²-enoyl-CoA and FADH2.
- Hydration: water is added across the double bond of trans-Δ²-enoyl-CoA by enoyl-CoA hydratase to form L-3-hydroxyacyl-CoA.
- Second Oxidation: L-3-hydroxyacyl-CoA is oxidised to 3-ketoacyl-CoA by L-3-hydroxyacyl-CoA dehydrogenase, converting the C-3 hydroxyl into a ketone while reducing NAD+ to NADH.
- Thiolysis: the C–C bond between the α and β carbons of 3-ketoacyl-CoA is cleaved by thiolase, consuming a molecule of coenzyme A to release acetyl-CoA and a fatty acyl-CoA shortened by two carbons.
4.3 Stoichiometry and ATP Ledger for Palmitate Oxidation
Complete oxidation of the 16-carbon saturated palmitate requires seven rounds of β-oxidation, yielding 8 molecules of acetyl-CoA, 7 molecules of NADH, and 7 molecules of FADH2.
| Metabolic process | Yield / cost | ATP equivalent |
|---|---|---|
| Activation cost | ATP → AMP + 2 Pi | −2.0 ATP |
| 7 rounds of β-oxidation | 7 FADH2 × 1.5 ATP | +10.5 ATP |
| 7 NADH × 2.5 ATP | +17.5 ATP | |
| 8 Acetyl-CoA (TCA cycle) | 8 GTP/ATP × 1.0 ATP | +8.0 ATP |
| 24 NADH × 2.5 ATP | +60.0 ATP | |
| 8 FADH2 × 1.5 ATP | +12.0 ATP | |
| Net ATP Yield | 106.0 ATP | |
4.4 Specialised Fatty Acid Oxidation Pathways
4.4.1 Oxidation of Saturated Odd-Chain Fatty Acids
Odd-chain fatty acids undergo normal β-oxidation until the final round of thiolysis, which yields one acetyl-CoA and one three-carbon propionyl-CoA. Propionyl-CoA is converted to the citric acid cycle intermediate succinyl-CoA through a three-step pathway.
Figure: Odd-chain propionyl-CoA salvage. A biotin-dependent carboxylation, an epimerisation, and a B12-dependent mutase reaction convert the three-carbon leftover into succinyl-CoA, which feeds directly into the citric acid cycle.
- Carboxylation: propionyl-CoA is carboxylated to D-methylmalonyl-CoA by the biotin-dependent enzyme propionyl-CoA carboxylase, consuming one ATP.
- Epimerisation: D-methylmalonyl-CoA is racemized to L-methylmalonyl-CoA by methylmalonyl-CoA epimerase.
- Isomerisation: L-methylmalonyl-CoA is rearranged to succinyl-CoA by methylmalonyl-CoA mutase, an enzyme that requires coenzyme B12 (adenosylcobalamin). Succinyl-CoA then enters the citric acid cycle.
4.4.2 Oxidation of Unsaturated Fatty Acids
Unsaturated fatty acids contain double bonds in the cis configuration, which cannot serve as substrates for the standard enzymes of β-oxidation. Their degradation requires two auxiliary enzymes.
- Monounsaturated Fatty Acids: β-oxidation proceeds normally until the double bond reaches the C-3 position, forming cis-Δ³-enoyl-CoA. Enoyl-CoA isomerase converts this into trans-Δ²-enoyl-CoA, which re-enters the standard pathway. Because this step bypasses the first FAD-linked oxidation, it yields 1.5 fewer ATP per double bond than a corresponding saturated fatty acid.
- Polyunsaturated Fatty Acids: multiple double bonds can form conjugated intermediates such as 2,4-dienoyl-CoA. This barrier is resolved by the cooperative action of enoyl-CoA isomerase and the NADPH-dependent 2,4-dienoyl-CoA reductase, converting conjugated double bonds into a single trans double bond metabolized by standard β-oxidation.
Linoleoyl-CoA (18C, cis-Δ⁹, cis-Δ¹²) walkthrough: three standard rounds of β-oxidation release 3 acetyl-CoA, producing cis-Δ³, cis-Δ⁶-dienoyl-CoA → enoyl-CoA isomerase converts this to trans-Δ², cis-Δ⁶-dienoyl-CoA → one further round releases a 4th acetyl-CoA, producing cis-Δ⁴-enoyl-CoA → acyl-CoA dehydrogenase introduces a trans-Δ² bond, producing the conjugated trans-Δ², cis-Δ⁴-dienoyl-CoA → 2,4-dienoyl-CoA reductase (consuming NADPH) yields trans-Δ³-enoyl-CoA → 3,2-enoyl-CoA isomerase (in mammals) converts this to trans-Δ²-enoyl-CoA, which re-enters standard β-oxidation.
4.4.3 Peroxisomal β-Oxidation
In eukaryotic cells, β-oxidation also occurs within peroxisomes, functioning as a chain-shortening system particularly for very-long-chain fatty acids (>C20).
4.4.4 Alpha (α)-Oxidation of Branched-Chain Fatty Acids
Branched-chain fatty acids with a methyl group at C-3 (such as phytanic acid, a chlorophyll degradation product) cannot undergo standard β-oxidation because the methyl substituent blocks formation of the 3-ketoacyl-CoA intermediate. These are degraded via α-oxidation in peroxisomes.
- Phytanic acid is activated to phytanoyl-CoA by phytanoyl-CoA synthetase.
- Phytanoyl-CoA is hydroxylated at the α-carbon (C-2) by phytanoyl-CoA hydroxylase to form 2-hydroxyphytanoyl-CoA.
- 2-hydroxyphytanoyl-CoA lyase cleaves the bond between C-1 and C-2, releasing C-1 as formyl-CoA and yielding the 19-carbon branched aldehyde pristanal.
- Pristanal is oxidised to pristanic acid by aldehyde dehydrogenase. Because pristanic acid's methyl branches sit at even-numbered carbon positions, it can undergo normal β-oxidation.
4.4.5 Omega (ω)-Oxidation
A minor pathway occurring in the endoplasmic reticulum of liver and kidney cells, oxidizing the terminal methyl (ω) carbon of fatty acids to a carboxyl group and converting the fatty acid into a dicarboxylic acid. It is catalyzed by cytochrome P450 monooxygenases, alcohol dehydrogenase, and aldehyde dehydrogenase, utilizing molecular oxygen and NADPH.
4.5 Synthesis versus Degradation Summary
| Feature | Fatty Acid Synthesis | Fatty Acid Degradation (β-Oxidation) |
|---|---|---|
| Major tissue site | Primarily liver, adipose tissue, lactating mammary glands | Muscle, liver, adipose tissue |
| Subcellular location | Cytosol (chloroplast stroma in plants) | Mitochondrial matrix (peroxisomes for VLCFA) |
| Redox cofactor | Uses NADPH as reducing agent | Uses NAD+ and FAD as electron acceptors |
| Two-carbon carrier / product | Malonyl-CoA (two-carbon donor) | Acetyl-CoA (two-carbon product) |
| Acyl carrier | Bound to Acyl Carrier Protein (ACP) | Bound to Coenzyme A (CoA-SH) |
| Hormonal state | Stimulated by high insulin/glucagon ratio | Stimulated by low insulin/glucagon ratio |
5. Ketone Bodies and Ketogenesis
The Liver's Alternate Fuel Export System
5. Ketone Bodies and Ketogenesis
When the rate of fatty acid oxidation in the liver exceeds the capacity of the citric acid cycle to process acetyl-CoA (such as during starvation or untreated diabetes), excess acetyl-CoA is converted into water-soluble organic acids called ketone bodies: acetoacetate, β-hydroxybutyrate, and acetone.
5.1 The Pathway of Ketogenesis
Ketogenesis occurs exclusively within the mitochondrial matrix of liver cells.
Figure: Ketogenesis. Two acetyl-CoA units condense to acetoacetyl-CoA, which combines with a third acetyl-CoA to form HMG-CoA. HMG-CoA lyase releases acetoacetate, which either decarboxylates spontaneously to volatile acetone or is reduced to β-hydroxybutyrate for export to peripheral tissues.
- Condensation: two molecules of acetyl-CoA are condensed by acetoacetyl-CoA thiolase to form acetoacetyl-CoA, releasing one free coenzyme A.
- HMG-CoA Formation: acetoacetyl-CoA condenses with a third molecule of acetyl-CoA to form β-hydroxy-β-methylglutaryl-CoA (HMG-CoA), catalyzed by HMG-CoA synthase.
- Cleavage: HMG-CoA is cleaved by HMG-CoA lyase to release one molecule of acetyl-CoA and yield the first ketone body, acetoacetate.
- Reduction / Decarboxylation: acetoacetate can either be reduced to β-hydroxybutyrate by mitochondrial β-hydroxybutyrate dehydrogenase in a reversible, NADH-consuming reaction, or undergo slow, spontaneous non-enzymatic decarboxylation to form volatile acetone.
5.2 Ketolysis (Utilization in Extrahepatic Tissues)
Ketone bodies are exported from the liver and transported via the bloodstream to extrahepatic tissues (heart, skeletal muscle, renal cortex, and brain during starvation), where they are converted back into acetyl-CoA to generate energy.
- β-hydroxybutyrate is re-oxidised to acetoacetate by β-hydroxybutyrate dehydrogenase, generating one molecule of NADH.
- Acetoacetate is activated to acetoacetyl-CoA by β-ketoacyl-CoA transferase (thiophorase), which transfers a coenzyme A group from succinyl-CoA.Acetoacetate + Succinyl-CoA Thiophorase→ Acetoacetyl-CoA + Succinate
- Acetoacetyl-CoA is cleaved by thiolase using free coenzyme A to yield two molecules of acetyl-CoA, which enter the citric acid cycle.
6. Cholesterol, Steroid Hormones & Bile Acids
From Acetyl-CoA to the Steroid Nucleus
6. Biosynthesis of Cholesterol, Steroid Hormones, and Bile Acids
Cholesterol is a vital structural constituent of eukaryotic plasma membranes and serves as the precursor for the synthesis of all steroid hormones, bile acids, and vitamin D. Virtually all 27 carbon atoms of cholesterol are derived from the acetyl group of acetyl-CoA.
6.1 The Four Stages of Cholesterol Biosynthesis
Cholesterol synthesis occurs primarily in the liver (accounting for ~50% of total synthesis) and proceeds in four major stages.
Figure: The four stages of cholesterol biosynthesis. HMG-CoA reductase — the statin target — commits carbon flow to mevalonate; sequential phosphorylation and decarboxylation build the active C5 isoprenoid pool, which polymerises to squalene and is finally cyclised into the steroid nucleus.
Stage 1: Synthesis of Mevalonate from Acetate
- Two molecules of acetyl-CoA condense to form acetoacetyl-CoA, which condenses with a third acetyl-CoA to form HMG-CoA.
- HMG-CoA is reduced to mevalonate by the integral membrane protein HMG-CoA reductase, consuming two molecules of NADPH. This is the committed, rate-limiting step of cholesterol biosynthesis.
Stage 2: Conversion of Mevalonate to Active Isoprene Pools
Mevalonate undergoes three sequential phosphorylations, consuming three molecules of ATP, followed by a decarboxylation, to yield the active five-carbon isoprenoid intermediate isopentenyl pyrophosphate (IPP).
Stage 3: Condensation of Isoprenes to Squalene
Isopentenyl pyrophosphate isomerizes to dimethylallyl pyrophosphate. These five-carbon units undergo head-to-tail condensations to form geranyl pyrophosphate (C10) and farnesyl pyrophosphate (C15). Two molecules of farnesyl pyrophosphate then condense head-to-head in an NADPH-dependent reaction to yield the 30-carbon hydrocarbon squalene.
Stage 4: Cyclisation of Squalene to the Steroid Nucleus
Squalene is oxidized and cyclized to form the four-ring steroid precursor lanosterol (C30). Lanosterol then undergoes approximately 20 subsequent enzymatic reactions, including the removal of three methyl groups, to yield the final 27-carbon product, cholesterol.
6.2 Biosynthetic Routes to Steroid Hormones
Cholesterol serves as the precursor for all five major classes of steroid hormones, synthesized via a series of side-chain cleavage and hydroxylation reactions catalyzed by mixed-function oxygenases.
Figure: Steroid hormone biosynthesis flow. Cholesterol is converted to pregnenolone and then progesterone, the shared precursor for glucocorticoids, mineralocorticoids, and androgens; testosterone is further converted (below the diagram) to estradiol via aromatase.
- Progestins (Progesterone, C21): the initial step in steroidogenesis is removal of a six-carbon segment from the side chain of cholesterol (cleavage between C-20 and C-22) by desmolase to form pregnenolone (C21), subsequently oxidised to progesterone (C21). Progesterone regulates pregnancy and serves as the precursor for all other steroid hormones.
- Glucocorticoids (Cortisol, C21): synthesized in the adrenal cortex, these hormones promote gluconeogenesis and suppress inflammatory reactions.
- Mineralocorticoids (Aldosterone, C21): synthesized in the adrenal cortex, aldosterone regulates ionic balance by promoting reabsorption of sodium, chloride, and bicarbonate ions in the kidney.
- Androgens (Testosterone, C19): synthesized in the testes, androgens promote male sexual development and maintain male secondary sex characteristics.
- Estrogens (Estradiol, C18): synthesized in the ovaries, estrogens support female secondary sex characteristics.
6.3 Bile Acid Biosynthesis and Excretion
Bile acids are polar steroid derivatives synthesized in the liver from cholesterol.
- Cholesterol is oxidized and hydroxylated to form the primary bile acids, cholic acid and chenodeoxycholic acid.
- In the liver, these bile acids are conjugated via amide bonds to the amino acids glycine or taurine to yield glycocholic acid and taurocholic acid (bile salts).
- Because bile salts contain both a highly polar, negatively charged region (from the conjugated amino acid) and a hydrophobic steroid nucleus, they act as highly effective amphipathic detergents — emulsifying dietary lipids in the intestine to facilitate digestion and absorption.
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