Biochemistry Nucleotide Chemistry and Metabolism 3

Question 1

Methotrexate inhibits nucleotide synthesis by targeting which enzyme?

Accepted Answer

Dihydrofolate reductase (DHFR)

Answer

Thymidylate synthase

Answer

Ribonucleotide reductase

Answer

PRPP synthetase

Question 2

Adenosine deaminase (ADA) deficiency causes which clinical condition, and why?

Accepted Answer

Severe combined immunodeficiency (SCID), due to toxic dATP accumulation in lymphocytes

Answer

Gout, due to uric acid overproduction from adenosine catabolism

Answer

Orotic aciduria, due to blocked pyrimidine conversion

Answer

Hemolytic anemia, due to erythrocyte purine depletion

Question 3

Allopurinol treats hyperuricemia and gout by inhibiting which enzyme?

Accepted Answer

Xanthine oxidase

Answer

HGPRT

Answer

Purine nucleoside phosphorylase

Answer

Adenosine deaminase

Question 4

Cyclic AMP (cAMP) functions as a second messenger primarily by activating which intracellular target?

Accepted Answer

Protein kinase A (PKA)

Answer

Phospholipase C

Answer

Protein kinase C (PKC)

Answer

Calmodulin-dependent kinase

Question 5

Ribonucleotide reductase reduces ribonucleotides to deoxyribonucleotides using which catalytic mechanism?

Accepted Answer

A stable tyrosyl radical stabilized by a dinuclear iron center

Answer

FAD-mediated hydride transfer

Answer

NAD+-dependent oxidation

Answer

Pyridoxal phosphate-mediated transamination

Question 6

NAD+ is synthesized in cells using which vitamin as the key precursor of the nicotinamide ring?

Accepted Answer

Niacin (B3)

Answer

Thiamine (B1)

Answer

Riboflavin (B2)

Answer

Pyridoxine (B6)

Question 7

S-adenosylmethionine (SAM) is the principal methyl donor in biochemical methylation reactions. How is SAM synthesized?

Accepted Answer

From ATP and methionine by methionine adenosyltransferase

Answer

From GTP and methionine by methionine adenosyltransferase

Answer

From PRPP and methionine by a transferase

Answer

From AMP and homocysteine by methyltransferase