Biochemistry Vitamins and Cofactors 2

Question 1

Vitamin C (ascorbic acid) acts as an antioxidant primarily by:

Accepted Answer

Donating electrons to neutralize free radicals

Answer

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?

Accepted Answer

Clotting factors II, VII, IX, and X

Answer

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:

Accepted Answer

1,25-dihydroxyvitamin D (calcitriol)

Answer

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?

Accepted Answer

Vitamin E (alpha-tocopherol)

Answer

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:

Accepted Answer

Thymidylate (dTMP) for DNA synthesis

Answer

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.

Biochemistry Practice Test

Biochemistry Practice Test

Biochemistry Vitamins and Cofactors 2