Cholesterol

• What type of compound is cholesterol classified as?

  Cholesterol is an animal sterol.
• What functional group is present in cholesterol?

  Cholesterol has an –OH group at C3.
• In what forms does cholesterol occur?

  Cholesterol occurs both as free form and in ester form.
• What is the endogenous source of cholesterol?

  Cholesterol is formed in the body from acetyl-CoA.
• How much cholesterol is formed endogenously in the body per day?

  About 700 mg/day.
• What are the exogenous sources of cholesterol?

  Cholesterol occurs in food of animal origin such as egg yolk, meat, liver, and brain.
• How much cholesterol does diet supply per day?

  Diet supplies about 400 mg/day.
• What role does cholesterol play in body cells?

  Cholesterol is a component of every body cell, including cell membranes.
• What is cholesterol a precursor for?

  Cholesterol is a precursor of Vitamin D3, steroid hormones, and bile acids.
• What is a major pathological role of cholesterol?

  Cholesterol is a factor in the genesis of atherosclerosis of vital arteries.
• What are the main functions of cholesterol?

  • Component of cell membranes
  • Precursor of Vitamin D3
  • Precursor of steroid hormones
  • Precursor of bile acids
  • Major constituent of gallstones
  • Factor in atherosclerosis
• Where is cholesterol primarily synthesized in the body?

  The liver is the major site of cholesterol synthesis.
• What is the precursor for cholesterol synthesis?

  Acetyl-CoA is the precursor for cholesterol synthesis.
• What are the stages of cholesterol synthesis?

  1. Formation of acetoacetyl CoA
  2. Synthesis of HMG CoA
  3. Formation of mevalonate (rate-limiting step)
  4. Production of isoprenoid units
  5. Synthesis of squalene
  6. Conversion of squalene to cholesterol
• What is the rate-limiting enzyme in cholesterol biosynthesis?

  HMG CoA reductase is the rate-limiting enzyme.
• What is the role of HMG CoA reductase in cholesterol synthesis?

  It catalyzes the reduction of HMG CoA to mevalonate.
• How does the synthesis of squalene occur?

  IPP and DPP condense to produce geranyl pyrophosphate, which then forms farnesyl pyrophosphate and finally squalene.
• What happens to squalene during cholesterol synthesis?

  Squalene undergoes hydroxylation and cyclization to convert to lanosterol.
• How many enzymatic reactions are involved in the formation of cholesterol from lanosterol?

  About 19 enzymatic reactions are involved.
• What are the regulatory mechanisms of HMG CoA reductase?

  • Feedback control
  • Hormonal control
  • Inhibition by glucagon and glucocorticoids
  • Activation by insulin and thyroxine
  • Inhibition by drugs like compactin and lovastatin
• What is the normal range for total plasma cholesterol?

  Total plasma cholesterol is 140-220 mg/dl.
• What can cause hypercholesterolemia?


  Diet rich in carbohydrates, cholesterol, and saturated fatty acids can cause hypercholesterolemia.
• How does hypothyroidism affect cholesterol levels?

  Hypothyroidism can lead to increased cholesterol levels as thyroxine stimulates conversion of cholesterol to bile acids.
• What is hypocholesterolemia?

  Hypocholesterolemia is defined as decreased plasma cholesterol concentration below 140 mg/dl.
• What are some causes of hypocholesterolemia?

  Prolonged fasting, liver diseases, and hyperthyroidism can cause hypocholesterolemia.
• What happens to the steroid nucleus of cholesterol in humans?

  The steroid nucleus of cholesterol cannot be metabolized in humans.
• What are the products of cholesterol degradation?

  • Bile acids
  • Steroid hormones
  • Vitamin D
  • Coprostanol
  • Cholestanol
• What are bile acids and their characteristics?

  Bile acids possess 24 carbon atoms, 2 or 3 hydroxyl groups, and a side chain ending in a carboxyl group.
• What is the role of bile acids in the intestine?

  Bile acids serve as emulsifying agents in the intestine and participate in the digestion and absorption of lipids.
• What is the rate-limiting reaction in bile acid synthesis?

  The step catalyzed by 7-hydroxylase is the rate-limiting reaction.
• What are primary bile acids and their examples?

  Cholic acid and chenodeoxycholic acid are primary bile acids.
• What happens to primary bile acids in the intestine?

  Primary bile acids undergo deconjugation and dehydroxylation to form secondary bile acids.
• What is enterohepatic circulation?


  • Conjugated bile salts are secreted into the small intestine.
  • They serve as emulsifying agents for digestion.
  • A large portion is reabsorbed and returned to the liver.
  • Bile salts are recycled and reused several times a day.
  • About 0.5 g/day is lost in feces and replaced by synthesis in the liver.
• What is the role of cholesterol in steroid hormone synthesis?

  Cholesterol is the precursor for the synthesis of all five classes of steroid hormones.
• What are the five classes of steroid hormones synthesized from cholesterol?

  • Glucocorticoids (e.g., cortisol)
  • Mineralocorticoids (e.g., aldosterone)
  • Progestins (e.g., progesterone)
  • Androgens (e.g., testosterone)
  • Estrogens (e.g., estradiol)
• How is vitamin D synthesized from cholesterol?

  1. Dehydrocholesterol is converted to cholecalciferol (vitamin D3) by ultraviolet rays.
• What is the role of ultraviolet rays in vitamin D synthesis?

  Ultraviolet rays convert 7-dehydrocholesterol to cholecalciferol (vitamin D3).