Plant Biochemistry / Lipid Metabolism / Lipid Metabolism

πŸ«’ Lipid Metabolism in Plants Fatty Acids to Oils

Biosynthesis, storage, and degradation of lipids – from membrane lipids to seed oils
πŸ§ͺ fatty acid synthase Β· ACCase 🌻 triacylglycerol (TAG) Β· oil bodies πŸ«’ Ξ²-oxidation Β· glyoxylate cycle Β· membranes

πŸ” 1. Why Lipid Metabolism Matters

Lipids are essential components of plant cells, serving as:

🌍 Ethiopian perspective: Lipid metabolism is crucial for:

  • Niger seed (Guizotia abyssinica): Major oilseed crop, rich in linoleic acid.
  • Sesame (Sesamum indicum): High-quality oil with antioxidants (sesamin, sesamolin).
  • Castor bean (Ricinus communis): Source of ricinoleic acid for industrial uses.
  • Sunflower and safflower: Grown for edible oils.
  • Oil quality traits: Fatty acid composition affects nutritional value and shelf life.
πŸ”„
[Diagram: Overview of lipid metabolism – synthesis, storage, degradation]
Figure 1: Lipid metabolism – fatty acid synthesis in plastids, TAG assembly in ER, and degradation in peroxisomes.

βš™οΈ 2. Fatty Acid Biosynthesis

2.1 Location and Substrates

Fatty acid synthesis occurs primarily in plastids (chloroplasts in leaves, leucoplasts in seeds).

Substrates:

  • Acetyl-CoA: Derived from pyruvate (via PDH) or from acetate.
  • Malonyl-CoA: Formed by acetyl-CoA carboxylase (ACCase).
  • NADPH: From pentose phosphate pathway or photosynthesis.
  • ATP: Required for ACCase.

2.2 Acetyl-CoA Carboxylase (ACCase)

Reaction: Acetyl-CoA + COβ‚‚ + ATP β†’ Malonyl-CoA + ADP + Pi

ACCase is the rate-limiting enzyme in fatty acid synthesis. Two forms exist:

  • Heteromeric ACCase: In plastids of most plants (except Poaceae). Multisubunit, sensitive to herbicides (aryloxyphenoxypropionates, cyclohexanediones).
  • Homomeric ACCase: In cytosol and in plastids of Poaceae (grasses). Single multifunctional protein. Grass ACCase is the target of graminicide herbicides.

2.3 Fatty Acid Synthase (FAS)

Fatty acids are synthesized by the fatty acid synthase (FAS) complex, a type II dissociated system in plants (similar to bacteria).

Each cycle adds two carbons (from malonyl-CoA) and involves four steps:

  1. Condensation: Malonyl-ACP + Acetyl-ACP β†’ Acetoacetyl-ACP (KAS III for first cycle, KAS I for subsequent).
  2. Reduction: Ξ²-ketoacyl-ACP β†’ Ξ²-hydroxyacyl-ACP (Ξ²-ketoacyl-ACP reductase).
  3. Dehydration: Ξ²-hydroxyacyl-ACP β†’ trans-2-enoyl-ACP (hydroxyacyl-ACP dehydratase).
  4. Reduction: trans-2-enoyl-ACP β†’ acyl-ACP (enoyl-ACP reductase).

Products: Mainly 16:0-ACP (palmitoyl-ACP) and 18:0-ACP (stearoyl-ACP).

βš™οΈ
[Diagram: Fatty acid synthesis cycle – condensation, reduction, dehydration, reduction]
Figure 2: Fatty acid synthase cycle – each turn adds two carbons to the growing acyl chain.

2.4 Desaturation and Elongation

Desaturases

  • Stearoyl-ACP desaturase (SAD): Introduces first double bond at Ξ”9 position (18:0 β†’ 18:1). Soluble enzyme in plastids.
  • Membrane-bound desaturases (FAD2, FAD3): In ER, introduce additional double bonds (18:1 β†’ 18:2 β†’ 18:3).
  • FAD2 (oleate desaturase): 18:1 β†’ 18:2 (linoleic acid).
  • FAD3 (linoleate desaturase): 18:2 β†’ 18:3 (Ξ±-linolenic acid).

Elongases

  • Very-long-chain fatty acids (VLCFAs, >18C) synthesized by elongase complexes (FAE1, KCS) in ER.
  • Important for seed oils (erucic acid in rapeseed), waxes, sphingolipids.

🌻 3. Triacylglycerol (Oil) Biosynthesis

3.1 The Kennedy Pathway

TAGs are assembled in the endoplasmic reticulum (ER) via the Kennedy pathway:

  1. Glycerol-3-phosphate acyltransferase (GPAT): acyl-CoA + G3P β†’ lysophosphatidic acid (LPA).
  2. Lysophosphatidic acid acyltransferase (LPAAT): LPA + acyl-CoA β†’ phosphatidic acid (PA).
  3. Phosphatidate phosphatase (PAP): PA β†’ diacylglycerol (DAG).
  4. Diacylglycerol acyltransferase (DGAT): DAG + acyl-CoA β†’ triacylglycerol (TAG).

DGAT is the only committed step in TAG synthesis and is rate-limiting. An alternative pathway uses phospholipid:diacylglycerol acyltransferase (PDAT) transferring acyl from phosphatidylcholine to DAG.

3.2 Oil Body Formation

TAGs accumulate in oil bodies (oleosomes) – spherical organelles 0.5-2 ΞΌm in diameter.

  • Structure: TAG core surrounded by a phospholipid monolayer embedded with oleosins (structural proteins).
  • Oleosins: Prevent coalescence of oil bodies, provide binding sites for lipases during germination.
  • Caleosins and steroleosins: Other oil body proteins with signaling roles.
πŸ”¬
[Diagram: Oil body structure – TAG core, phospholipid monolayer, oleosins]
Figure 3: Oil body – TAGs stored in seeds, surrounded by phospholipid monolayer and oleosin proteins.

πŸ§ͺ 4. Membrane Lipids

4.1 Glycerolipids

Lipid ClassStructureLocationFunction
Phosphatidylcholine (PC)Glycerol + 2 fatty acids + phosphate + cholineER, plasma membrane, organelle membranesMajor membrane phospholipid; acyl donor for TAG synthesis
Phosphatidylethanolamine (PE)Glycerol + 2 FA + phosphate + ethanolamineAll membranesMembrane structure
Phosphatidylglycerol (PG)Glycerol + 2 FA + phosphate + glycerolChloroplast membranesEssential for photosynthetic function
Phosphatidylinositol (PI)Glycerol + 2 FA + phosphate + inositolAll membranesSignaling precursor (phosphoinositides)
Monogalactosyldiacylglycerol (MGDG)Glycerol + 2 FA + galactoseChloroplast thylakoidsMajor chloroplast lipid (50% of thylakoid lipids)
Digalactosyldiacylglycerol (DGDG)Glycerol + 2 FA + 2 galactoseChloroplast thylakoidsStabilizes photosystems
Sulfolipid (SQDG)Glycerol + 2 FA + sulfoquinovoseChloroplast thylakoidsInvolved in phosphate starvation response

4.2 Sphingolipids

Sphingolipids are based on a sphingoid base (e.g., sphingosine). Important in:

  • Membrane rafts: Glycosylinositolphosphoceramides (GIPC) abundant in plasma membrane.
  • Signaling: Ceramides, sphingosine-1-phosphate act in stress signaling.

4.3 Cutin, Suberin, and Waxes

🌿 Cutin

Polyester of C16 and C18 hydroxy and epoxy fatty acids. Forms cuticle with waxes. Prevents water loss, pathogen entry.

🌳 Suberin

Polyester of fatty acids (including Ο‰-hydroxy acids) and phenolics. Found in cork cells, endodermis, root periderm. Waterproofs tissues.

πŸ•―οΈ Waxes

Very-long-chain aliphatics (alkanes, alcohols, aldehydes, esters). Deposited on cuticle surface. Provide waterproofing, reflection.

πŸ”₯ 5. Lipid Degradation

5.1 Lipolysis (TAG breakdown)

During germination of oilseeds, TAGs are mobilized by lipases:

  • Oil body lipases (e.g., SDP1): TAG β†’ DAG + fatty acid.
  • Fatty acids are activated to acyl-CoA and enter Ξ²-oxidation.

5.2 Ξ²-Oxidation

Fatty acids are broken down in peroxisomes (glyoxysomes) via Ξ²-oxidation:

  1. Acyl-CoA oxidase (ACX): Acyl-CoA β†’ trans-2-enoyl-CoA + Hβ‚‚Oβ‚‚.
  2. Enoyl-CoA hydratase: trans-2-enoyl-CoA β†’ 3-hydroxyacyl-CoA.
  3. 3-hydroxyacyl-CoA dehydrogenase: 3-hydroxyacyl-CoA β†’ 3-ketoacyl-CoA + NADH.
  4. Thiolase (KAT): 3-ketoacyl-CoA + CoA β†’ acetyl-CoA + (n-2) acyl-CoA.

Each cycle produces acetyl-CoA. For oilseeds, acetyl-CoA enters the glyoxylate cycle to produce succinate for gluconeogenesis.

5.3 Glyoxylate Cycle

The glyoxylate cycle operates in glyoxysomes during germination, converting acetyl-CoA to succinate (which enters gluconeogenesis). Key enzymes:

  • Isocitrate lyase (ICL): Isocitrate β†’ succinate + glyoxylate.
  • Malate synthase (MS): Glyoxylate + acetyl-CoA β†’ malate + CoA.

See the Glyoxylate Cycle resource for detailed information.

πŸ”₯
[Diagram: Ξ²-oxidation and glyoxylate cycle in germinating oilseeds]
Figure 4: Ξ²-oxidation produces acetyl-CoA; glyoxylate cycle converts it to succinate for gluconeogenesis.

πŸ“‘ 6. Lipid Signaling

🟑 Phosphatidic acid (PA)

Produced by phospholipase D (PLD) or diacylglycerol kinase (DGK). Regulates stress responses, vesicle trafficking.

🟒 Phosphoinositides

PI(4,5)Pβ‚‚, PI3P, PI4P – regulate cytoskeleton, membrane trafficking, signaling.

🟠 Jasmonates

Oxylipins derived from linolenic acid via lipoxygenase pathway. Defense hormones.

πŸ”΅ Sphingolipids

Ceramides, sphingosine-1-phosphate regulate programmed cell death, stress responses.

🟣 N-acylethanolamines (NAEs)

Signaling lipids involved in seed germination, stress responses.

πŸ‡ͺπŸ‡Ή 7. Lipid Metabolism in Ethiopian Oilseed Crops

🌱 Niger Seed (Guizotia abyssinica)

  • Oil content: 35-40%.
  • Fatty acid composition: Linoleic acid (18:2, ~75%), oleic (18:1, ~10%), palmitic (16:0, ~8%), stearic (18:0, ~5%).
  • High linoleic acid: Essential fatty acid (omega-6).
  • Uses: Edible oil, traditional medicine.

🌿 Sesame (Sesamum indicum)

  • Oil content: 50-55%.
  • Fatty acid composition: Oleic (18:1, ~40%), linoleic (18:2, ~45%), palmitic (16:0, ~10%), stearic (18:0, ~5%).
  • Antioxidants: Sesamin, sesamolin (lignans) – high oxidative stability.
  • Uses: High-quality edible oil, confectionery.

πŸ«’ Castor Bean (Ricinus communis)

  • Oil content: 45-50%.
  • Unique fatty acid: Ricinoleic acid (12-hydroxy-9-octadecenoic acid, ~90%).
  • Industrial uses: Lubricants, paints, plastics, cosmetics.
  • Toxic protein: Ricin (not lipid, but present in seed).

🌻 Sunflower (Helianthus annuus)

  • Oil content: 40-50%.
  • Fatty acid composition: High oleic or high linoleic varieties.
  • Uses: Edible oil.

🌼 Safflower (Carthamus tinctorius)

  • Oil content: 30-40%.
  • High linoleic or high oleic types.
  • Uses: Edible oil, birdseed.

πŸ₯œ Peanut (Arachis hypogaea)

  • Oil content: 45-50%.
  • Fatty acid composition: Oleic (18:1, ~50%), linoleic (18:2, ~30%), palmitic (16:0, ~10%).
  • Uses: Edible oil, roasted nuts.
🌱
[Graph: Fatty acid composition of niger seed oil – high linoleic acid]
Figure 5: Niger seed oil – rich in linoleic acid (omega-6), an essential fatty acid.

🧬 8. Genetic Engineering of Oil Composition

Oilseed crops can be engineered to produce modified oils with improved nutritional or industrial properties:

  • High oleic oils: Suppression of FAD2 (oleate desaturase) increases oleic acid (18:1), improving oxidative stability.
  • High stearic oils: Suppression of stearoyl-ACP desaturase (SAD).
  • Long-chain polyunsaturated fatty acids (LC-PUFA): Engineering EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) in oilseeds.
  • Industrial oils: Production of unusual fatty acids (ricinoleic, vernolic, etc.) in transgenic crops.

πŸ“ 9. Methods to Study Lipid Metabolism

πŸ§ͺ Lipid Extraction

  • Folch method: Chloroform:methanol (2:1).
  • Bligh-Dyer method: For small samples.

πŸ”¬ Lipid Separation

  • TLC: Lipid classes.
  • GC-FID: Fatty acid methyl esters (FAME).
  • HPLC-MS: Complex lipids (phospholipids, galactolipids).

⚑ Enzyme Assays

  • ACCase activity: Incorporation of ¹⁴C-bicarbonate.
  • DGAT activity: Using radiolabeled acyl-CoA.

🧬 Gene Expression

  • qRT-PCR of lipid biosynthesis genes (FAD2, FAD3, DGAT1, etc.).

πŸ“š 10. Open Access Resources & Further Reading

πŸ“Œ 11. Key References

TopicCitation
Fatty acid synthesisOhlrogge & Jaworski (1997) Annu Rev Plant Physiol Plant Mol Biol; Harwood (1996) Biochim Biophys Acta
ACCase structure and herbicidesSasaki et al. (1995) J Biol Chem; DΓ©lye (2005) Weed Sci
TAG biosynthesis (Kennedy pathway)Kennedy (1961) Fed Proc; Coleman & Lee (2004) Prog Lipid Res
Oil bodies and oleosinsHuang (1992) Annu Rev Plant Physiol Plant Mol Biol; Tzen et al. (1993) J Cell Biol
Membrane lipidsJoyard et al. (1998) Prog Lipid Res; DΓΆrmann & Benning (2002) Trends Plant Sci
Ξ²-oxidation and glyoxylate cycleGraham & Eastmond (2002) J Exp Bot; Beevers (1980) The Biochemistry of Plants
🌱 Plant Biochemistry – Dilla University πŸ“… March 2026 Β· Lipid Metabolism