Biochemistry Vitamins and Cofactors 2 — Questions and Answers
Question 1: Vitamin C (ascorbic acid) acts as an antioxidant primarily by:
- Activating superoxide dismutase
- Donating electrons to neutralize free radicals (Correct answer)
- Chelating iron to prevent Fenton reactions
- Inhibiting lipid peroxidation by binding membranes
Correct answer: Donating electrons to neutralize free radicals
Ascorbic acid is a potent reducing agent that donates electrons to neutralize reactive oxygen species and can regenerate other antioxidants such as vitamin E.
Question 2: Vitamin K is required for the gamma-carboxylation of which proteins critical for blood clotting?
- Fibrinogen and thrombin
- Clotting factors II, VII, IX, and X (Correct answer)
- Platelet glycoproteins
- Von Willebrand factor
Correct answer: Clotting factors II, VII, IX, and X
Vitamin K-dependent gamma-glutamyl carboxylase modifies prothrombin (factor II) and factors VII, IX, and X, adding extra carboxyl groups that allow Ca²⁺ binding essential for clot formation.
Question 3: Vitamin D3 (cholecalciferol) is hydroxylated first in the liver to 25-hydroxyvitamin D, then in the kidney to its active form:
- 1,24-dihydroxyvitamin D
- 1,25-dihydroxyvitamin D (calcitriol) (Correct answer)
- 24,25-dihydroxyvitamin D
- 7-dehydrocholesterol
Correct answer: 1,25-dihydroxyvitamin D (calcitriol)
1,25-dihydroxyvitamin D₃ (calcitriol) is the biologically active hormone that binds to nuclear vitamin D receptors to increase intestinal calcium absorption and regulate bone mineralization.
Question 4: Which fat-soluble vitamin acts as the most important lipid-soluble antioxidant in membranes?
- Vitamin A (retinol)
- Vitamin D (calcitriol)
- Vitamin E (alpha-tocopherol) (Correct answer)
- Vitamin K (phylloquinone)
Correct answer: Vitamin E (alpha-tocopherol)
Vitamin E (alpha-tocopherol) is the primary chain-breaking antioxidant in lipid membranes, donating a hydrogen atom to lipid peroxyl radicals to terminate lipid peroxidation chain reactions.
Question 5: Folate (Vitamin B9) deficiency causes megaloblastic anemia primarily because folate is required for synthesis of:
- Heme precursors
- Iron-binding proteins
- Thymidylate (dTMP) for DNA synthesis (Correct answer)
- Globin protein chains
Correct answer: Thymidylate (dTMP) for DNA synthesis
Folate as 5,10-methylene-THF is required for thymidylate synthase to convert dUMP to dTMP; without sufficient folate, DNA synthesis is impaired, causing large, immature red blood cells.
Question 6: Coenzyme A (CoA) is derived from which vitamin?
- Niacin (B3)
- Pantothenic acid (B5) (Correct answer)
- Riboflavin (B2)
- Thiamine (B1)
Correct answer: Pantothenic acid (B5)
Coenzyme A contains pantothenic acid (vitamin B5) as a core component, along with adenosine 3',5'-bisphosphate and a phosphopantetheine moiety that provides the reactive thiol group.
Biochemistry Vitamins and Cofactors 2
Vitamin C (ascorbic acid) acts as an antioxidant primarily by: