Question

16. Briefly describe the action of Insulin on carbohydrate metabolism, fat metabolism, and on protein metabolism. I.e. do es it promote (+) or discourage (-) the following? a. Glycogenesis (production of glycogen) b. Glycogenolysis (breakdown of glycogen) c. Glycolysis: using glucose for fuel inside cells d. Gluconeogenesis: Production of glucose by liver (usually from aa) e. Uptake of glucose into fat and muscle cells? f. Lipid synthesis: Production of Triglycerides in fat cells g. Lipid breakdown/mobilization of fat from adipocytes(breakdown of TG to FFA, which are released into blood) h. Synthesis of protein i. Breakdown of muscle protein

Answer 1

Insulin has profound effect on the carbohydrate and fat metabolism . It also influences protein and mineral metabolism. It also arrange for the storage and oxidation of the fuels produced by metabolism.

1. Glycogenesis:

Insulin stimulate the synthesis of glycogen. It activates the enzyme hexokinase, which phosphorylates glucose, trapping it within the cell. Coincidently, insulin acts to inhibit the activity of glucose-6-phosphatase. Insulin also activates Phosphofructokinase and glycogen synthase and other enzymes that are directly involved in glycogen synthesis.

2. Glycogenolysis:

Insulin inhibits glycogenolysis. When levels of insulin in blood fall, glycogen synthesis in the liver diminishes. Then the enzymes responsible for breakdown of glycogen become active. The break down of glycogen or Glycogenolysis is supported by the absense of insulin and also by the presence of Glucagon hormone.

3. Glycolysis:

Insulin promotes Glycolysis through the dephosphorylation process. High glucose levels stimulate the production of Insulin. Insulin promote the entry of glucose into the cells and increase production of glycolysing enzymes.

4. Gluconeogenesis:

Gluconeogenesis is inhibited by insulin. Insulin inhibits the secretion of Glucagon, thereby inhibits Gluconeogenesis indirectly.

5. Uptake of glucose into fat and muscle cells:

Insulin promotes the entry of glucose to muscle and adipose tissues. Binding of insulin to receptors on GLUT4 glucose transporters cause rapid fusion of those vesicles with the plasma membrane and insertion of the glucose transporters, leading to efficiently take up glucose by the cells.

6. Lipid synthesis:

Insulin stimulates the synthesis of lipids. When the liver gets saturated with Glycogen, the additional glucose is diverted to pathways for the syntheisi of fatty acids.

7. Lipid break down:

Insulin inhibits lipolysis by inhibiting the intracellular lipase that hydrolyzes triglycerides to release fatty acids.

8. Protein synthesis:

Protein is formed in the absence of insulin but the formation is just accelerated by the insulin. Insulin stimulates the protein synthesis.

1. Break down of muscle protein:

Insulin inhibits the protein break down. The deficiency of insulin cause the break down of protein in the muscle and may cause muscle wasting.