Lipid Metabolism

Abstract

Lipids are water-insoluble heterogeneous group of compounds, which are soluble in organic solvents. Besides being an important constituent of the diet, lipids have many other uses. A great diversity of lipids occurs in nature, and it is very important to get the right kind of lipid in a diet. Lipids are among the biomolecules required for structure and functions of the cell. Since they are mostly stored in seeds or fruits of some plants, these are the main sources for obtaining dietary lipids. Studying the diversity and the metabolism of lipids is important. A brief introduction to diversity in structure and functions of lipids is taken in this chapter besides learning about the classification and nomenclature of fatty acids. The chapter includes the pathways, the role of enzymes, and the organelles involved in the lipid metabolism. Studying the catabolism of lipids is significant in germinating fatty seeds, since they have primarily oils as the storage form. In fatty seeds the stored lipids are not the source of energy directly. Lipids are metabolized to produce soluble form of sugars in the storage organs, which are translocated to the meristematic regions in the growing seedlings. A study of pathways, enzymes, and organelles involved in lipid metabolism in germinating seeds is also included in the chapter. Though there is similarity in lipid metabolism of plants and animals, a great degree of disparity also exists. In plants lipid metabolism is more complex and involves many cell organelles. Fatty acid synthesis and fatty acid oxidation occur in different subcellular locations and involve different metabolic pathways and different sets of enzymes. There is successive removal of two-carbon units as acetyl-CoA during oxidation of fatty acids. Elongation of hydrocarbon chain also requires, at the time of fatty acid synthesis, addition of two carbons at a time. However, precursors for fatty acid synthesis are three-carbon compounds, malonyl-CoA, except one acetyl-CoA, which is required as a primer. The condensation reaction is coupled with simultaneous release of one carbon as CO₂ (Fig. 10.1).

Multiple-Choice Questions

• 1. The lipids which form the major component of thylakoids are:

  • (a) Phospholipids

  • (b) Sphingolipids

  • (c) Galactolipids

  • (d) Neutral lipids

• 2. In plants fatty acid biosynthesis occurs in:

  • (a) Plastids

  • (b) Peroxisomes

  • (c) Mitochondria

  • (d) Cytosol

• 3. First step in fatty acid biosynthesis is catalyzed by:

  • (a) Acetyl-CoA synthase

  • (b) Carboxyl transferase

  • (c) Malonyl-CoA:ACP transacylase

  • (d) Acetyl-CoA carboxylase

• 4. At the time of fatty acid synthesis, the cofactor to which acyl group is attached during reactions involving two reductions and dehydration is:

  • (a) Coenzyme A

  • (b) Fatty acid synthase

  • (c) ACP

  • (d) Ketoacyl-ACP synthase

• 5. Initial condensation of acetyl CoA with malonyl-ACP resulting in production of acetoacetyl-ACP is catalyzed by:

  • (a) KAS I

  • (b) KAS II

  • (c) KAS III

  • (d) 3-Ketoacyl-ACP reductase

• 6. In the endoplasmic reticulum, modification of hydrocarbon chain of fatty acids requires:

  • (a) Fatty acyl-ACP

  • (b) Fatty acyl-CoA

  • (c) Thioesterase

  • (d) FAS

• 7. In a prokaryotic way, synthesis of membrane lipids involves:

  • (a) Esterification of C1 hydroxyl group of glycerol phosphate with 16:0 FA and C2 hydroxyl group with 18:0 FA

  • (b) Esterification of C1 hydroxyl group of glycerol phosphate with 18:1 FA and C2 hydroxyl group with 16:0 FA

  • (c) Esterification of both C1 and C2 of glycerol phosphate with 18:1 FA

  • (d) Esterification of both C1 and C2 of glycerol phosphate with 16:0 FA

• 8. Conversion of stearoyl-ACP to oleoyl-ACP (tick the correct answer):

  • (a) Is catalyzed by a soluble desaturase present in the stroma of plastids

  • (b) Is catalyzed by desaturase present in the envelope of the plastid

  • (c) Is catalyzed by desaturase present in the ER membrane

  • (d) Is catalyzed by a soluble desaturase present in the cytosol

• 9. In plants β-oxidation of fatty acids occurs in the:

  • (a) Mitochondria

  • (b) Cytosol

  • (c) Oleosomes

  • (d) Glyoxysomes

• 10. Which of the following statements is correct?

  • (a) In plants acyl-CoA oxidase catalyzes transfer of electrons O₂ during first oxidation step in β-oxidation of fatty acids.

  • (b) Acyl-ACP is transported out of plastids to be used for further modifications in the hydrocarbon chain of fatty acids.

  • (c) Oleosin is the membrane lipid of oleosomes.

  • (d) Glyoxylate cycle occurs both in plants and animals.

Answers

1. c	2. a	3. d	4. c	5. c	6. b	7. b
8. a	9. d	10. a