Question

1- Which of the following statements about cholesterol regulation is false?

A) Insulin stimulates the production of cholesterol.

B) High intracellular cholesterol levels stimulate proteolysis of HMG-CoA reductase.

C) High intracellular cholesterol levels inhibit the uptake of extracellular cholesterol.

D) Glucagon stimulates the production of cholesterol.

2- Which statement is incorrect about the synthesis of fatty acids by fatty acid synthase in mammals?

A) It uses FADH2 as a reductant of carbon-carbon double bonds.

B) It uses NADPH as a reductant of carbonyl groups.

C) The end product of synthesis is palmitic acid.

D) All the carbons in the fatty acid are derived from acetyl-CoA.

3- NADPH is used for:

A) oxidizing vitamin C.

B) β-oxidation of fatty acids.

C) oxidizing vitamin E.

D) reductive biosynthesis of fatty acids.

E) oxidizing glutathione.

Answer 1

1. The enzyme HMG CoA reductase is the enzyme responsible for catalyzing the last step of cholesterol synthesis and is hormonally regulated. It's phosphorylated form is inactive form of enzyme and dephosphorylated form is active form. Glucagon stimulates phosphorylation of the enzyme inactivating it while insulin stimulates it's dephosphorylation and hence activating it. So insulin stimulates cholesterol synthesis while glucagon inhibits cholesterol synthesis. Hence statement D. is false.

2. Fatty acid synthesis uses Acetyl CoA and NADPH to produce the 16 carbon palmitic acid using enzyme fatty acid synthase. Hence the end product is palmitic acid and all the carbons of the fatty acids are derived from acetyl CoA and NADPH is a reducing agent that is used as a reductant of carbonyl groups. Hence all statements are correct except A. Hence A. is incorrect.

3. NADPH, being the reduced form of cofactor NADP+, is itself a reducing agent and hence is involved in several biosynthetic pathways including fatty acid synthesis as a reducing agent. NADPH cannot be used for oxidizing and hence in the given question the answer would be D. ie NADPH is used for reductive biosynthesis of fatty acids.