Biosynthesis of Lipids in Plants (Step by Step Notes)
Lipids in plants are synthesized through a series of well-organized steps. These include the formation of fatty acids, their elongation and desaturation, the production of triacylglycerols, phospholipids, sphingolipids, eicosanoids, and sterols. Below is a clear explanation with full names of enzymes and equations.
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1. From Glucose to Pyruvate
- Starting Molecule: Glucose
- Pathway: Glycolysis
- Process: Glucose is broken down into pyruvate through several steps.
Equation:
Glucose → Glyceraldehyde-3-phosphate → Pyruvate
2. Pyruvate to Acetyl-Coenzyme A (Mitochondria)
- Pyruvate enters the mitochondria.
- Enzyme: Pyruvate dehydrogenase complex
- Equation:
3. Citrate Transport to Cytosol
- Acetyl-Coenzyme A cannot cross mitochondrial membrane directly.
- It combines with oxaloacetate to form citrate.
- Citrate is transported out into the cytosol.
Equation:
Acetyl-Coenzyme A + Oxaloacetate → Citrate
4. Formation of Acetyl-Coenzyme A in Cytosol
- Enzyme: Adenosine triphosphate citrate lyase
- Equation:
5. Formation of Malonyl-Coenzyme A
- Enzyme: Acetyl-Coenzyme A carboxylase
- Equation:
6. Synthesis of Palmitic Acid (16-carbon Saturated Fatty Acid)
- Enzyme: Fatty acid synthase
- Equation (simplified):
7. Formation of Unsaturated and Polyunsaturated Fatty Acids
- Enzyme for Monounsaturated fatty acids: Stearoyl-Coenzyme A desaturase
- Converts palmitic acid or stearic acid into monounsaturated fatty acids.
-
Enzymes for Polyunsaturated fatty acids: Fatty acid desaturases and Elongases
- Add more double bonds and extend the carbon chain.
8. Glycerol Backbone and Triacylglycerol Synthesis
- Glycerol-3-phosphate is formed from glucose.
Stepwise process:
Glycerol-3-phosphate acyltransferase → Lysophosphatidic acid
- Glycerol-3-phosphate + Fatty acyl-Coenzyme A → Lysophosphatidic acid
- Lysophosphatidic acid + Fatty acyl-Coenzyme A → Phosphatidic acid
- Phosphatidic acid → Diacylglycerol + Inorganic phosphate
- Diacylglycerol + Fatty acyl-Coenzyme A → Triacylglycerol
9. Phospholipid Biosynthesis
From diacylglycerol, different phospholipids are formed:
- Phosphatidylcholine
- Phosphatidylethanolamine
- Phosphatidylserine
- Phosphatidylinositol
These are key components of plant membranes.
10. Sphingolipid Biosynthesis
- Starting molecule: Serine + Palmitoyl-Coenzyme A
- Process: Through a series of steps, ceramides and sphingosine are formed.
- Enzyme example: Sphingosine kinase → Sphingosine-1-phosphate
11. Eicosanoid Biosynthesis (From Arachidonic Acid)
- Arachidonic acid is produced from polyunsaturated fatty acids.
- Enzyme: Cyclooxygenase
- Produces prostaglandins like Prostaglandin H₂ and Prostaglandin E₂.
12. Sterol and Hormone Biosynthesis
- From acetyl-Coenzyme A → Hydroxymethylglutaryl-Coenzyme A → Mevalonate → Farnesyl pyrophosphate → Cholesterol.
- Cholesterol is then used to make:
- Cholesterol esters
- Steroid hormones (like brassinosteroids in plants)
Final Summary
- Glucose → Pyruvate → Acetyl-Coenzyme A (Mitochondria)
- Citrate transport → Acetyl-Coenzyme A (Cytosol)
- Acetyl-Coenzyme A → Malonyl-Coenzyme A
- Fatty Acid Synthase → Palmitic Acid
- Desaturases & Elongases → Monounsaturated and Polyunsaturated fatty acids
- Glycerol pathway → Triacylglycerols (storage)
- Phospholipids for membranes
- Sphingolipids → Signaling molecules
- Eicosanoids → Derived from arachidonic acid
- Cholesterol and Steroid hormones synthesis
Lipid Biosynthesis in Plants:
- Location: In plants, lipid biosynthesis primarily occurs in the chloroplasts, endoplasmic reticulum (ER), and to some extent in the cytosol.
- Acetyl-CoA Source: Acetyl-CoA for lipid biosynthesis in plants is derived from the products of photosynthesis in chloroplasts and from glycolysis in the cytosol.
- Main Lipid Products: Plants synthesize a diverse array of lipids, including triglycerides, phospholipids, and sterols, which are essential for membrane structure, energy storage, and signaling.
- Role of Enzymes: Key enzymes involved in lipid biosynthesis in plants include acetyl-CoA carboxylase (ACC), fatty acid synthase (FAS), glycerol-3-phosphate acyltransferase, and various desaturases and elongases.
- Regulation: Lipid biosynthesis in plants is regulated by various factors, including light intensity, temperature, hormonal signals, and developmental stages. For example, the expression and activity of lipid biosynthetic enzymes are modulated by environmental cues to meet the plant's metabolic needs.
Lipid Biosynthesis in Animals:
- Location: In animals, lipid biosynthesis primarily occurs in the cytosol and endoplasmic reticulum (ER) of cells, particularly hepatocytes (liver cells) and adipocytes (fat cells).
- Acetyl-CoA Source: Acetyl-CoA for lipid biosynthesis in animals is derived from the metabolism of carbohydrates, proteins, and fats through processes such as glycolysis, β-oxidation, and the citric acid cycle.
- Main Lipid Products: Animals synthesize primarily triglycerides, cholesterol, and phospholipids, which serve as structural components of cell membranes, energy storage molecules, and precursors for steroid hormones.
- Role of Enzymes: Key enzymes involved in lipid biosynthesis in animals include acetyl-CoA carboxylase (ACC), fatty acid synthase (FAS), stearoyl-CoA desaturase, and HMG-CoA reductase (for cholesterol synthesis).
- Regulation: Lipid biosynthesis in animals is tightly regulated by hormonal signals (e.g., insulin), nutrient availability, and metabolic demands. For instance, insulin promotes lipid synthesis in response to high blood glucose levels, while fasting stimulates lipolysis to release stored fats for energy production.
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