BIOCHEMISTRY in Questions and Answers - Alimov A.M. - 2016

SECTION 7. LIPID METABOLISM

Question 1. What are Lipids?

Answers: This is a heterogeneous group of ester-based compounds that are insoluble in Water but highly soluble in organic Solvents.

Question 2. What is the Biological Role of Lipids in the Human and Animal body?

Answer. Lipids serve as an energy source, a structural component of Cell membranes, solvents for certain Vitamins, protection against cold and mechanical trauma, and perform the function of biological regulators and mediators involved in vital physiological processes in animal and human Cells, among others. In addition, lipids are a source of metabolic water: the biochemical oxidation of 100 g of fat yields 107.1 g of water, compared to 55.5 g for starch and 41.3 g for protein.

Question 3. How are lipids classified according to their composition?

Answer: Simple and complex. Upon Hydrolysis, Simple Lipids break down into glycerol and Fatty acids.

Question 4. Name representatives of simple lipids.

Answer: Simple lipids include neutral fats (triglycerides), Waxes, and sterides.

Question 5. Name representatives of Complex Lipids.

Answer: Complex lipids include Phospholipids, Sphingolipids, and Glycolipids.

Question 6. What Fatty acids are found in neutral fats?

Answer: Saturated and Unsaturated fatty acids.

Question 7. What is the difference between simple and mixed triglycerides?

Answer: Simple triglycerides contain identical fatty acid residues at all three carbon positions of glycerol. Mixed triglycerides contain different fatty acids.

Question 8. What are neutral fats (triacylglycerols)?

Answer: Triacylglycerols are esters of glycerol and Higher Fatty Acids.

Question 9. Which fatty acids are most commonly found in human and animal triacylglycerols?

Answer: Palmitic, oleic, and stearic acids.

Question 10. What are Essential Fatty Acids?

Answer: Essential fatty acids are polyunsaturated fatty acids that cannot be synthesized by the body and must be obtained from the diet.

Question 11. What are the main polyunsaturated fatty acids?

Answer: Linoleic, linolenic, and arachidonic acids.

Question 12. Which compounds belong to Glycerophospholipids?

Answer: Phosphatidic acid, phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, phosphatidylinositol, plasmalogens, cardiolipin

Question 13. What components are contained in the phosphatidylcholine molecule?

Answer: Glycerol, two fatty acid residues, a phosphoric acid residue, Choline

Question 14. What components are identified in a cerebroside molecule?

Answer: The complex amino alcohol sphingosine, a fatty acid residue, galactose or glucose

Question 15. What components are contained in the sphingomyelin molecule?

Answer: The complex amino alcohol sphingosine, a fatty acid residue, a phosphoric acid residue, choline

Question 16. What are gangliosides?

Answer: Gangliosides are glycosphingolipids containing sphingosine, a fatty acid residue, an oligosaccharide, and one or more sialic acid molecules

Question 17. What is the biological role of glycerophospholipids and sphingolipids?

Answer: These compounds are Structural components of membranes; phosphatidylinositol-4,5-bisphosphate in membranes is cleaved into diacylglycerol and Inositol trisphosphate, which act as secondary messengers in hormonal signal Transduction into The Cell

Question 18. To what Class of lipids does Cholesterol belong?

Answer: Cholesterol is a steroid. Steroids are substances containing a cyclopentanoperhydrophenanthrene ring

Question 19. Which Enzymes are involved in the hydrolysis of lipids in the intestine?

Answer. Pancreatic triacylglycerol lipase and phospholipase A2

Question 20. Which acids are called Bile acids?

Answer: Cholic, deoxycholic, chenodeoxycholic, lithocholic

Question 21. From which substance are bile acids formed?

Answer: From cholesterol

Question 22. What is The Role of bile acids?

Answer: 1. Together with bicarbonates and phospholipids, they cause the emulsification of lipids, i.e., they increase the dispersion of fat droplets

2. By forming "choleic acids," they ensure the absorption of triacylglycerol (TAG) Digestion products.

Question 23. What are chylomicrons?

Answer: Chylomicrons are small fat particles formed after Absorption in the intestinal wall; they are Lipoproteins surrounded by phospholipids and a protein coat, containing fat molecules and bile acids inside.

Question 24. Where are lipid transport particles formed?

Answer: They are formed in the enterocytes of the intestine. They contain 88-90% TAG, 5-6% cholesterol, 4-7% glycerophospholipids, and 1-2% protein (apolipoprotein B-48)

Question 25. Which lipoproteins are called plasma lipoproteins?

Answer: 1. High-density lipoproteins (HDL), or α-lipoproteins

2. Low-density lipoproteins (LDL), or β-lipoproteins

3. Very low-density lipoproteins (VLDL), pre-β-lipoproteins

Question 26. Which lipoproteins contain the highest amount of endogenous triacylglycerols (TAG) and where are they synthesized?

Answer. VLDL; they are synthesized in The Liver and deliver TAG to Muscles and other Tissues

Question 27. Which lipoproteins contain the highest amount of cholesterol?

Answer: LDL; they deliver cholesterol from the liver to tissues

Question 28. Which enzymes are involved in intracellular lipolysis?

Answer: Triacylglycerol lipase, diacylglycerol lipase, monoacylglycerol lipase

Question 29. Which intracellular lipase is hormone-sensitive?

Answer: Triacylglycerol lipase

Question 30. Where does fatty acid β-oxidation take place?

Answer. In the mitochondrial matrix

Question 31. How is a fatty acid activated?

Answer: In the Cytosol, a fatty acid binds to HS-CoA to form acyl-CoA: R-COOH + HS-CoA + ATF = R-CO~CoA + AMF +H₃PO₄.

Question 32. Which compound is involved in The transport of fatty acids from the Cytoplasm into the mitochondrial matrix?

Answer: Carnitine

Question 33. Which metabolite is formed during fatty acid β-oxidation?

Answer: Acetyl-CoA combines with carnitine under the action of carnitine acyltransferase to form acyl-carnitine, which enters the Mitochondria with the participation of translocase.

Question 34. What happens after acylcarnitine crosses the mitochondrial membrane?

Answer: The reverse Cleavage reaction of acylcarnitine takes place involving mitochondrial HS-CoA and carnitine acyltransferase. Then carnitine returns to the cytoplasm, while acyl-CoA undergoes oxidation in the mitochondria.

Question 35. What is the specific feature of unsaturated fatty acid β-oxidation?

Answer: During the β-Oxidation of Unsaturated fatty acids, ∆3 -cis-enoyl-CoA is formed, which must be converted into ∆2 -trans-enoyl-CoA

Question 36. Which fatty acid is predominantly synthesized in The Human Body?

Answer. Palmitic acid

Question 37. Which reactions produce NADPH, which is required for fatty acid synthesis?

Answer: 1. In dehydrogenase Reactions of the Pentose Phosphate Pathway

2. During the decarboxylation of malate by NADP⁺-dependent malate dehydrogenase

3. In the isocitrate dehydrogenase reaction in the cytoplasm

Question 38. Where do the Elongation and desaturation of FAs take place?

Answer: In the membranes of The Endoplasmic reticulum

Question 39. Which reaction is key in cholesterol synthesis?

Answer: The formation of mevalonic acid catalyzed by HMG-CoA reductase

Question 40. How is The activity of HMG-CoA reductase regulated?

Answer: 1. Cholesterol activates repressor Proteins of the HMG-CoA reductase Gene, thereby suppressing enzyme synthesis

2. Cholesterol acts as an allosteric inhibitor of HMG-CoA reductase

3. The activity of HMG-CoA reductase is regulated by covalent modification. The phosphorylated form of the enzyme is inactive, while the dephosphorylated form is active. Consequently, Glucagon and adrenaline, acting via the adenylate cyclase mechanism, inhibit cholesterol synthesis, whereas Insulin accelerates it

Question 41. What are the Functions of cholesterol?

Answer: Cholesterol serves as:

1. A component of cell membranes

2. A precursor of vitamin D₃

3. A precursor of bile acids

4. A precursor of Steroid Hormones (Sex Hormones and Adrenocortical Hormones)

Question 42. The Pathogenesis of which disease is associated with hypercholesterolemia?

Answer: Atherosclerosis

Question 43. Which substances are classified as Ketone Bodies?

Answer: 1. Acetoacetic acid

2. β-Hydroxybutyric acid

3. Acetone

Question 44. Where does the synthesis of ketone bodies take place?

Answer: In the liver

Question 45. Can the liver utilize ketone bodies?

Answer: No, because the liver lacks the corresponding enzymes

Question 46. What is the role of ketone bodies?

Answer: They serve as an energy source

Question 47. In which tissues are ketone bodies utilized?

Answer: In Skeletal Muscle, myocardium, Kidneys, and Lungs

Question 48. What is Ketosis?

Answer: It is a significant accumulation of ketone bodies in the Blood (ketonemia) and their excretion in milk and urine (ketonuria) resulting from increased production of ketone bodies in the liver and reduced utilization in peripheral tissues

Question 49. What does the accumulation of ketone bodies in the body lead to?

Answer. Metabolic acidosis

Question 50. Under what conditions does ketosis develop?

Answer: In Diabetes Mellitus, prolonged fasting, severe toxicosis of Pregnancy, and a decrease in the Energy Sources required for the synthesis of fat and cholesterol from acetic acid.

Question 51. What is fatty liver infiltration?

Answer: Significant deposition of TAG in the hepatocyte cytosol

Question 52. What are lipotropic factors?

Answer: Lipotropic factors are substances that prevent TAG deposition in hepatocytes and promote the synthesis of glycerophospholipids. These include choline, Methionine, complete proteins, vitamins B₆, B₁₂, F, and Folic acid

Question 53. What is obesity?

Answer: It is an increase in stored body fat associated with an imbalance between the rates of TAG deposition and mobilization. It occurs due to overeating and slowed fat degradation

Question 54. Which hormones enhance lipolysis (the mobilization of fat from adipose depots)?

Answer: Adrenaline, thyroxine, Male Sex Hormones, and somatotropin

Question 55. Which hormones enhance Lipogenesis?

Answer: Insulin and Female Sex Hormones

Question 56. What are fatty acids synthesized from?

Answer: FAs are synthesized from Acetyl-CoA in the cytosol, yielding malonyl-CoA.

Question 57. Describe the general pathway for palmitic acid synthesis.

Answer: Acetyl-CoA + 7 malonyl-CoA + 14 NADPH₂ = palmitic acid [CH₃(CH₂)₁₄-COOH] + 7 CO₂ + 8 HS-CoA + 14 NADP + 7 H₂O.