Biochemistry Nucleotide Chemistry and Metabolism 3

Question 1

Methotrexate inhibits nucleotide synthesis by targeting which enzyme?

Accepted Answer

Dihydrofolate reductase (DHFR)

Answer

Methotrexate competitively inhibits DHFR, preventing reduction of dihydrofolate to tetrahydrofolate; without THF cofactors, one-carbon transfer reactions required for both purine ring synthesis and dTMP formation are blocked.

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

ADA deficiency allows deoxyadenosine to accumulate and be phosphorylated to dATP, which is selectively toxic to lymphocytes (inhibits ribonucleotide reductase and induces apoptosis), resulting in SCID.

Question 3

Allopurinol treats hyperuricemia and gout by inhibiting which enzyme?

Accepted Answer

Xanthine oxidase

Answer

Allopurinol (and its oxidized form oxipurinol) inhibits xanthine oxidase, the enzyme that converts hypoxanthine to xanthine and xanthine to uric acid, thereby lowering serum urate levels.

Question 4

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

Accepted Answer

Protein kinase A (PKA)

Answer

cAMP binds to the regulatory subunits of protein kinase A, causing their dissociation and releasing active catalytic subunits that phosphorylate serine/threonine residues on downstream target proteins.

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

Ribonucleotide reductase class I uses a stable tyrosyl radical (generated by a diferric iron center in the R2 subunit) to initiate the radical-based mechanism that removes the 2'-OH of ribonucleotides.

Biochemistry Practice Test

Biochemistry Practice Test

Biochemistry Nucleotide Chemistry and Metabolism 3