The process of new synthesis of fatty acids from the acetyl-CoA is called lipogenesis.

Location:- It mainly occurs in liver and mammary glands; but are also formed in minute amounts in apidose tissue, kidney and brain cells.

Steps of lipogenesis

1. transport of acetyl-CoA from mitochondria to cytosol

2.  carboxylation of acetyl-CoA to malonyl- CoA.

The carboxylation of acetyl-CoA is the rate limiting step of fatty acid synthesis

3. Reactions of fatty acid synthase complex

Fatty acid synthase complex is a multi-enzyme complex which have different catalytic reaction site on it. The active one is the dimeric and is arranged in head to tail form. This synthase complex have 7 enzymatic site on it and one ACP (acylcamer protein) site.

Pathway for De novo synthesis

Regulation of lipogenesis

These are divided in three types

• Enzymatic regulation:- acetyl-CoA carboxylase is a rate limiting enzyme for this pathway. This enzyme is activated by citrate and is inhibited by palmitoyl-CoA.
• Hormonal regulation:-

1. Insulin activates the acetyl-CoA carboxylase.

  2. Glucagon inhibits the acetyl-CoA carboxylase enzyme

• Nutritional regulation:-

  1. if the diet is rich in carbohydrate and contains low fat than it stimulates the rate limiting enzyme

  2. In case of starvation, diabetes mellitus and rich fat diet; the rate limiting enzyme is inhibited and thus inhibits the fatty acid synthesis.

Multiple choice questions (MCQs)

1. What is lipogenesis?
   A. synthesis of lipid B. catabolism of phospholipid
   C. synthesis of fatty acids D. catabolism of fatty acids
2. which fatty acid contains 16 carbon skeleton?
   A. palmitic acid B. glutamic acid
   C. Lauric acid D. Myristic acid
3. Where does lipogenesis takes place?
   A. cytosol B. mitochondria
   C. ribosomes D. nucleus
4. In which organ, lipogenesis occur mostly?
   A. liver B. lungs
   C. mammary glands D. both A and C
5. Which of the following is the 2nd step of lipogenesis?
   A. transport of acetyl-CoA from mitochondria to cytosol
   B. carboxylation of acetyl-CoA to malonyl-CoA
   C. Reactions of synthesis using fatty acid synthase complex
   D. none of the above
6. Match the following –
   a. acetyl-CoA carboxylase 1. stimulates acetyl-CoA carboxylase
   b. thioesterase 2. inhibits acetyl-CoA carboxylase
   c. insulin 3. malonyl-CoA
   d. glucagon 4. palmitic acid
7. In which situation, fatty acid synthesis is stimulated?
   A. carbohydrate rich diet D. citrate
   C. insulin D. all of the above
8. Which of the following statement is NOT true?
   A. lipogenesis is same as lypolysis
   B. fatty acids are synthesized from acetyl-CoA
   C. Insulin activates the acetyl-CoA carboxylase.
   D. Glucagon inhibits the acetyl-CoA carboxylase enzyme
9. Which type of diet inhibits lipogenesis?
   A. carbohydrate rich B. fat rich diet
   C. protein rich diet D. all of the above
10. Which enzyme is known as the rate limiting enzyme?
    A. thioesterase B. enolreductase
    C. ketoacyl synthase D. acetyl-CoA carboxylase
11. How many enzymatic sites does fatty acid synthase complex have?
    A. 8 B. 6
    C. 7 D. 5
12. Which form of energy is required for the conversion of acetyl-CoA to malonyl-CoA?
    A. NADH B. GTP
    C. AMP D. None of the above
13. In which tissue, lipogenesis occur in minute amounts?
    A. adipose tissue B. areolar tissue
    C. epithelium D. elastic tissue
14. In which type of cell lipogenesis occur in minute amounts?
    A. nerve cell B. kidney cell
    C. brain cell D. both B and C
15. Synthesis of fatty acids begins from which compound?
    A. acyl-CoA b. acetyl-CoA
    C. phospholipid D. lipoproteins

ANSWERS:-

1. synthesis of fatty acids

2. palmitic acid

3. cytosol

4. both A and c

5. carboxylation of acetyl-CoA to malonyl-CoA

6. a – 3 b – 4 c – 1 d – 2

7. all of the above

8. lipogenesis is same as lypolysis

9. fat rich diet

10. acetyl-CoA carboxylase

11. 7

12. none of the above

13. adipose tissue

14. both B and C

15. acetyl-CoA

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REFERENCE:- Pankaja Naik- Biochemistry; 4^th edition; page no:- 224-230 .