Biosynthesis of lipids in plants
Lipid biosynthesis is the process through which plants produce fatty acids, storage oils, membrane lipids and protective surface materials.
Plants use lipids to:
- Build cell and organelle membranes.
- Store energy in seeds and fruits.
- Reduce water loss through protective surface layers.
- Support cellular signalling and responses to environmental changes.
The central pathway involves building fatty acid chains, followed by attaching those chains to suitable molecular backbones.
Where lipid biosynthesis occurs
| Location | Main function |
|---|---|
| Plastid stroma | Most new fatty acid synthesis |
| Plastid envelope | Fatty acid export and parts of membrane lipid synthesis |
| Endoplasmic reticulum | Storage-oil assembly, fatty acid elongation and many membrane lipid reactions |
| Cytosol | Carbon supply, glycerol-backbone production and connecting reactions |
| Golgi apparatus | Modification of certain complex membrane lipids |
| Oil bodies | Storage of triacylglycerol |
| Cell surface and cell wall | Deposition of waxes and protective lipid polymers |
A plastid is a family of plant organelles. Chloroplasts are photosynthetic plastids, while non-green tissues also contain plastids that synthesize fatty acids.
Essential Molecules Before The Pathway Begins
| Full name | Function |
|---|---|
| Acetyl coenzyme A | Supplies two-carbon acetyl groups |
| Malonyl coenzyme A | Supplies activated carbon for chain extension |
| Acyl carrier protein | Holds the growing fatty acid during plastid synthesis |
| Adenosine triphosphate | Supplies energy |
| Reduced nicotinamide adenine dinucleotide phosphate | Supplies electrons for many reduction reactions |
| Reduced nicotinamide adenine dinucleotide | Supplies electrons for particular reactions |
| Glycerol-3-phosphate | Provides the backbone for glycerolipid assembly |
An acyl group is the fatty-acid-derived group that attaches to another molecule. Reduction means gaining electrons. Desaturation introduces a carbon–carbon double bond.
Step 1: Formation Of Acetyl Coenzyme A
Location: Plastid stroma.
Enzyme: Pyruvate dehydrogenase complex.
Pyruvate contains three carbon atoms. The enzyme complex removes one carbon atom as carbon dioxide and transfers the remaining two-carbon acetyl group to coenzyme A.
The reaction also converts oxidized nicotinamide adenine dinucleotide into reduced nicotinamide adenine dinucleotide.
Main carbon conversion:
Pyruvate → acetyl coenzyme A + carbon dioxide.
Within the complex:
- Pyruvate dehydrogenase begins carbon dioxide removal using thiamine diphosphate.
- Dihydrolipoyl acetyltransferase transfers the acetyl group to coenzyme A.
- Dihydrolipoyl dehydrogenase regenerates the oxidized carrier system.
Why this matters: Acetyl coenzyme A provides the starting material for fatty acid synthesis.
Step 2: Activation Of The Carboxyl-Group Donor
Location: Plastid stroma.
Enzyme activity: Biotin carboxylase activity of acetyl coenzyme A carboxylase.
The enzyme uses bicarbonate and energy from adenosine triphosphate to attach a carboxyl group to biotin.
Biotin is attached to a carrier protein and temporarily holds this additional carbon.
Products: Carboxylated biotin, adenosine diphosphate and inorganic phosphate.
Why this matters: The reaction prepares a transferable carboxyl group for the next step.
Step 3: Formation of Malonyl Coenzyme A
Location: Plastid stroma.
Enzyme activity: Carboxyltransferase activity of acetyl coenzyme A carboxylase.
The enzyme transfers the carboxyl group from biotin to acetyl coenzyme A.
The two-carbon acetyl group becomes a three-carbon malonyl group.
Product: Malonyl coenzyme A.
Important distinction: Malonyl contains three carbon atoms, but only two remain in the growing fatty acid. The extra carboxyl carbon leaves during condensation.
Step 4: Transfer of Malonyl to Acyl carrier Protein
Location: Plastid stroma.
Enzyme: Malonyl coenzyme A:acyl carrier protein transacylase.
The enzyme transfers the malonyl group from coenzyme A to the flexible arm of acyl carrier protein.
Reaction:
Malonyl coenzyme A + acyl carrier protein → malonyl acyl carrier protein + coenzyme A.
Why this matters: The extension unit now sits on the carrier used by the plastid fatty acid synthesis machinery.
Step 5: First Condensation
Location: Plastid stroma.
Enzyme: 3-ketoacyl acyl carrier protein synthase three.
The enzyme combines the two-carbon acetyl starter with malonyl acyl carrier protein.
The malonyl group loses carbon dioxide. Its remaining two carbons join the acetyl group, producing a new carbon–carbon bond.
Product: Acetoacetyl acyl carrier protein.
The product contains:
- Four carbon atoms.
- A keto group at carbon three.
- A linkage to acyl carrier protein.
Carbon calculation: Two starter carbons + two retained extension carbons = four carbons.
Step 6: First Reduction
Location: Plastid stroma.
Enzyme: 3-ketoacyl acyl carrier protein reductase.
Reduced nicotinamide adenine dinucleotide phosphate supplies electrons.
The enzyme changes the keto group at carbon three into a hydroxyl group.
Conversion:
Acetoacetyl acyl carrier protein → 3-hydroxybutyryl acyl carrier protein.
Why this matters: The reaction begins converting the oxygen-containing intermediate into a more reduced hydrocarbon chain.
The chain still contains four carbon atoms.
Step 7: Dehydration
Location: Plastid stroma.
Enzyme: 3-hydroxyacyl acyl carrier protein dehydratase.
The enzyme removes the hydroxyl group from carbon three and hydrogen from carbon two as water.
A double bond forms between carbon two and carbon three.
Conversion:
3-hydroxybutyryl acyl carrier protein → trans-2-butenoyl acyl carrier protein + water.
Why this matters: The temporary double bond prepares the intermediate for the second reduction.
Step 8: Second Reduction
Location: Plastid stroma.
Enzyme: Enoyl acyl carrier protein reductase.
The enzyme reduces the double bond to a single bond. Plant plastid enzymes commonly use reduced nicotinamide adenine dinucleotide for this reaction.
Conversion:
trans-2-butenoyl acyl carrier protein → butyryl acyl carrier protein.
Product: A saturated four-carbon acyl chain attached to acyl carrier protein.
One complete extension cycle has now finished.
Step 9: Repetition of the extension cycle
Location: Plastid stroma.
Main condensing enzyme: 3-ketoacyl acyl carrier protein synthase one.
The four-carbon chain enters another cycle with a fresh malonyl acyl carrier protein molecule.
Each cycle contains:
- Condensation.
- First reduction.
- Dehydration.
- Second reduction.
Each completed cycle adds two carbon atoms.
| Completed extension cycles | Carbon atoms in the chain |
|---|---|
| Starting acetyl group | 2 |
| One | 4 |
| Two | 6 |
| Three | 8 |
| Four | 10 |
| Five | 12 |
| Six | 14 |
| Seven | 16 |
The sixteen-carbon saturated product is palmitoyl acyl carrier protein.
Starting with acetyl coenzyme A, making this chain requires one acetyl starter, seven malonyl extension units, seven adenosine triphosphate molecules for malonyl formation, and fourteen reduction reactions.
Step 10: Extension to eighteen carbon atoms
Location: Plastid stroma.
Condensing enzyme: 3-ketoacyl acyl carrier protein synthase two.
Palmitoyl acyl carrier protein undergoes one additional condensation with malonyl acyl carrier protein.
The usual reduction, dehydration and second reduction follow.
Conversion:
Palmitoyl acyl carrier protein → stearoyl acyl carrier protein.
Product: An eighteen-carbon saturated chain.
Some sixteen-carbon chains leave the pathway earlier or enter plastid membrane lipids instead.
Step 11: Introduction of the first persistent double bond
Location: Plastid stroma.
Enzyme: Stearoyl acyl carrier protein desaturase.
The enzyme introduces a cis double bond between carbon nine and carbon ten, counted from the carboxyl end.
Oxygen and electrons supplied through reduced ferredoxin support this reaction.
Conversion:
Stearoyl acyl carrier protein → oleoyl acyl carrier protein.
Product: An eighteen-carbon chain with one double bond.
Unlike the temporary double bond in Step 7, this double bond remains in the fatty acid product.
Step 12: Release of the Fatty Acid
Location: Plastid stroma.
Enzyme: Acyl acyl carrier protein thioesterase.
Water breaks the thioester bond between the fatty acid and acyl carrier protein.
Products: A free fatty acid and reusable acyl carrier protein.
Different thioesterases favour different chains, helping determine the fatty acids that leave the plastid.
Why this matters: Release makes selected fatty acids available for export and further lipid assembly.
Step 13: Export And Activation
Location: Plastid envelope and associated cellular acyl pools.
Enzyme: Long-chain acyl coenzyme A synthetase.
Export-associated processes move fatty acids out of the plastid.
The synthetase joins a fatty acid to coenzyme A, using adenosine triphosphate.
Reaction:
Fatty acid + coenzyme A + adenosine triphosphate → fatty acyl coenzyme A + adenosine monophosphate + pyrophosphate.
Pyrophosphate hydrolysis helps drive the process forward.
Why this matters: Fatty acyl coenzyme A supplies many reactions at the endoplasmic reticulum.
Step 14: Formation Of Additional Double Bonds
Location: Endoplasmic reticulum membranes. Related reactions also occur in plastid membranes.
Enzymes: Omega-six and omega-three fatty acid desaturases.
At the endoplasmic reticulum, desaturation often acts on fatty acid chains attached to phosphatidylcholine.
The sequence is:
- An oleic acid chain contains one double bond.
- An omega-six desaturase introduces another double bond, producing a linoleic acid chain.
- An omega-three desaturase introduces a third double bond, producing an alpha-linolenic acid chain.
Why this matters: The number of double bonds influences membrane properties and the composition of plant oils.
Step 15: Elongation beyond eighteen carbon atoms
Location: Endoplasmic reticulum.
This pathway uses fatty acyl coenzyme A rather than acyl carrier protein.
Its four reactions are:
| Reaction | Enzyme | Change |
|---|---|---|
| Condensation | 3-ketoacyl coenzyme A synthase | Adds two retained carbon atoms from malonyl coenzyme A |
| First reduction | 3-ketoacyl coenzyme A reductase | Converts the keto group into a hydroxyl group |
| Dehydration | 3-hydroxyacyl coenzyme A dehydratase | Removes water |
| Second reduction | Enoyl coenzyme A reductase | Reduces the temporary double bond |
Reduced nicotinamide adenine dinucleotide phosphate supplies reducing power.
Products: Very long-chain fatty acids used in waxes, sphingolipids and other specialized lipids.
Step 16: Formation of the glycerol backbone
Location: Cytosol and plastids.
Enzyme: Glycerol-3-phosphate dehydrogenase.
The enzyme reduces dihydroxyacetone phosphate, an intermediate of carbohydrate metabolism.
Conversion:
Dihydroxyacetone phosphate → glycerol-3-phosphate.
An alternative route uses glycerol kinase to phosphorylate glycerol with adenosine triphosphate.
Why this matters: Glycerol-3-phosphate provides the three-carbon backbone onto which fatty acids attach.
Step 17: Attachment of the first fatty acid
Location: Endoplasmic reticulum for the storage-oil route.
Enzyme: Glycerol-3-phosphate acyltransferase.
The enzyme transfers an acyl group from fatty acyl coenzyme A to the first position of glycerol-3-phosphate.
Product: Lysophosphatidic acid.
This molecule contains:
- One fatty acid.
- A glycerol backbone.
- Phosphate at the third position.
Coenzyme A leaves the reaction.
Step 18: Attachment of the second fatty acid
Location: Endoplasmic reticulum.
Enzyme: Lysophosphatidic acid acyltransferase.
The enzyme transfers another acyl group to the second position of glycerol.
Conversion:
Lysophosphatidic acid + fatty acyl coenzyme A → phosphatidic acid + coenzyme A.
Product: Phosphatidic acid, containing two fatty acids and a phosphate group.
Why this matters: Phosphatidic acid is a branch point between several membrane-lipid pathways and storage-oil synthesis.
Step 19: Removal of phosphate
Location: Endoplasmic reticulum-associated pathway.
Enzyme: Phosphatidic acid phosphatase.
The enzyme uses water to remove phosphate from the third position of glycerol.
Reaction:
Phosphatidic acid + water → diacylglycerol + inorganic phosphate.
The two fatty acid ester bonds remain intact.
Why this matters: The third position becomes available for another fatty acid or an appropriate membrane-lipid head group.
Step 20: Formation of triacylglycerol
Location: Endoplasmic reticulum.
Enzyme: Diacylglycerol acyltransferase.
The enzyme transfers a third acyl group from fatty acyl coenzyme A onto diacylglycerol.
Reaction:
Diacylglycerol + fatty acyl coenzyme A → triacylglycerol + coenzyme A.
Triacylglycerol contains three fatty acids attached to one glycerol backbone through ester bonds.
An alternative enzyme, phospholipid:diacylglycerol acyltransferase, transfers the final acyl group from a phospholipid instead of using fatty acyl coenzyme A as the immediate donor.
Step 21: Formation of an oil body
Location: Endoplasmic reticulum and cytosol.
Newly synthesized triacylglycerol accumulates between the two leaflets of the endoplasmic reticulum membrane.
The sequence is:
- Triacylglycerol molecules collect into a small lipid lens.
- The lens grows as more storage lipid enters.
- A lipid droplet emerges toward the cytosol.
- A phospholipid monolayer surrounds the neutral-lipid core.
- Proteins such as oleosins stabilize many seed oil bodies.
Why this matters: Oil bodies store lipid reserves that can later support seed germination and early grow
Membrane lipid synthesis
Phosphatidylcholine
Location: Cytosol and endoplasmic reticulum.
- Choline kinase converts choline into phosphocholine using adenosine triphosphate.
- Phosphocholine cytidylyltransferase combines phosphocholine with cytidine triphosphate, producing cytidine diphosphate choline.
- Cholinephosphotransferase transfers the phosphocholine group to diacylglycerol.
- The products are phosphatidylcholine and cytidine monophosphate.
Phosphatidylcholine is a major membrane lipid and an important site of fatty acid modification.
Phosphatidylinositol
Location: Mainly endoplasmic reticulum-associated membranes.
- Phosphatidate cytidylyltransferase activates phosphatidic acid using cytidine triphosphate.
- The product is cytidine diphosphate diacylglycerol.
- Phosphatidylinositol synthase combines this activated lipid with myo-inositol.
- Phosphatidylinositol forms, and cytidine monophosphate leaves.
Phosphatidylglycerol
Location: Plastid membranes for the photosynthetic pathway described here.
- Phosphatidylglycerophosphate synthase combines cytidine diphosphate diacylglycerol with glycerol-3-phosphate.
- Phosphatidylglycerophosphate forms.
- Phosphatidylglycerophosphate phosphatase removes the terminal phosphate.
- Phosphatidylglycerol forms.
This lipid contributes to the organization and function of photosynthetic membranes.
Galactolipid synthesis
Location: Plastid envelope.
First galactose addition
Monogalactosyldiacylglycerol synthase transfers galactose from uridine diphosphate galactose to diacylglycerol.
Product: Monogalactosyldiacylglycerol.
Second galactose addition
Digalactosyldiacylglycerol synthase transfers another galactose onto the existing sugar head group.
Product: Digalactosyldiacylglycerol.
These lipids form a large part of photosynthetic membranes and contain no phosphate.
Sulfolipid synthesis
Location: Plastid-associated pathway.
- Uridine diphosphate sulfoquinovose synthase uses uridine diphosphate glucose and sulfite to produce an activated sulfur-containing sugar.
- Sulfoquinovosyldiacylglycerol synthase transfers this sugar to diacylglycerol.
- Sulfoquinovosyldiacylglycerol forms.
This negatively charged lipid supports photosynthetic membranes without requiring a phosphate-containing head group.
Sphingolipid synthesis
Location: The backbone forms at the endoplasmic reticulum. Some later modifications occur in the Golgi apparatus.
- Serine palmitoyltransferase combines serine with palmitoyl coenzyme A, forming 3-ketosphinganine.
- 3-ketosphinganine reductase reduces the keto group to form sphinganine.
- Hydroxylation and desaturation can modify the long-chain base.
- Ceramide synthase attaches a fatty acyl group to its amino group, forming an amide bond.
- Addition of glucose produces glucosylceramide.
- Other head-group and sugar additions produce additional complex sphingolipids.
Key distinction: Sphingolipids use a long-chain amino-alcohol backbone rather than glycerol.
Wax, cutin and suberin synthesis
Location: Precursor formation mainly involves the endoplasmic reticulum. Deposition or polymer assembly occurs at the cell surface or in cell walls.
Waxes
Very long-chain fatty acyl precursors enter several branches:
- Fatty acyl coenzyme A reductases produce primary alcohols.
- Wax ester synthases join suitable alcohols to fatty acyl groups.
- An alkane-forming enzyme system produces alkanes.
- Additional reactions can produce secondary alcohols and ketones.
Cutin and suberin
- Enzymes hydroxylate and otherwise modify selected fatty acid chains.
- Further oxidation can produce dicarboxylic acids.
- Acyltransferases form suitable glycerol-containing building blocks.
- Transport systems deliver the building blocks to their deposition sites.
- Polymer-forming reactions assemble protective polyester structures.
Cutin contributes to the aerial cuticle. Suberin forms barriers in tissues such as root endodermis and cork. Their detailed composition and assembly vary among tissues and species.
Sterol synthesis
Location: Mainly the cytosol and endoplasmic reticulum-associated pathway.
Sterols have fused carbon rings and follow a different construction route.
- Acetyl coenzyme A acetyltransferase combines two acetyl coenzyme A molecules to form acetoacetyl coenzyme A.
- 3-hydroxy-3-methylglutaryl coenzyme A synthase adds another acetyl-derived unit.
- 3-hydroxy-3-methylglutaryl coenzyme A reductase produces mevalonate.
- Mevalonate kinase produces mevalonate-5-phosphate.
- Phosphomevalonate kinase produces mevalonate-5-diphosphate.
- Mevalonate diphosphate decarboxylase produces isopentenyl diphosphate.
- Isopentenyl diphosphate isomerase interconverts this molecule with dimethylallyl diphosphate.
- Prenyltransferases assemble fifteen-carbon farnesyl diphosphate.
- Squalene synthase joins two farnesyl diphosphate molecules to form squalene.
- Squalene epoxidase produces 2,3-oxidosqualene.
- Cycloartenol synthase produces the cyclic precursor cycloartenol.
- Further methylation, demethylation, reduction and double-bond changes produce plant sterols such as campesterol and sitosterol.
The terminal sterol reactions form a branched pathway rather than one identical sequence in every plant.
Regulation and examination points
Lipid synthesis responds to carbon supply, available energy, enzyme activity, tissue development and environmental conditions.
Remember these distinctions:
- Most new fatty acid chains form in plastids.
- Each extension cycle adds two carbon atoms.
- The cycle contains condensation, reduction, dehydration and reduction.
- Desaturases introduce persistent double bonds.
- The endoplasmic reticulum assembles much of the plant’s storage oil.
- Triacylglycerol contains three fatty acids and one glycerol backbone.
- Membrane lipids, sphingolipids and sterols have different structural features.
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