Biochemistry Nucleotide Chemistry and Metabolism — Questions and Answers

Question 1: Which of the following nitrogenous bases is classified as a purine?

Cytosine
Adenine (Correct answer)
Uracil
Thymine

Correct answer: Adenine

Adenine and guanine are purines, characterized by a bicyclic structure of a pyrimidine ring fused to an imidazole ring, while cytosine, uracil, and thymine are monocyclic pyrimidines.

Question 2: What distinguishes a nucleotide from a nucleoside?

Nucleosides contain a phosphate group; nucleotides do not
Nucleotides contain one or more phosphate groups; nucleosides do not (Correct answer)
Nucleosides contain deoxyribose; nucleotides contain ribose
Nucleotides are found only in DNA; nucleosides are found only in RNA

Correct answer: Nucleotides contain one or more phosphate groups; nucleosides do not

A nucleotide consists of a nitrogenous base, a pentose sugar, and one or more phosphate groups, whereas a nucleoside contains only the base and the sugar without any phosphate.

Question 3: In the DNA double helix, adenine forms hydrogen bonds with which complementary base?

Guanine
Cytosine
Thymine (Correct answer)
Uracil

Correct answer: Thymine

Adenine pairs with thymine in DNA via two hydrogen bonds (A=T), following Chargaff's rules, while guanine pairs with cytosine via three hydrogen bonds.

Question 4: Which pyrimidine base is present in RNA but absent from DNA?

Thymine
Cytosine
Uracil (Correct answer)
Hypoxanthine

Correct answer: Uracil

Uracil replaces thymine in RNA; thymine contains a methyl group at C-5 that uracil lacks, and uracil pairs with adenine in RNA just as thymine does in DNA.

Question 5: De novo purine nucleotide synthesis begins with which metabolite as the core scaffold?

Hypoxanthine
Ribose-5-phosphate (Correct answer)
Xanthine
Inosine monophosphate (IMP)

Correct answer: Ribose-5-phosphate

De novo purine synthesis starts with ribose-5-phosphate (from the pentose phosphate pathway), which is converted to PRPP (5-phosphoribosyl-1-pyrophosphate), the activated ribose donor for ring assembly.

Question 6: Lesch-Nyhan syndrome results from a deficiency of which enzyme in purine metabolism?

Adenosine deaminase
Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) (Correct answer)
Purine nucleoside phosphorylase
Xanthine oxidase

Correct answer: Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)

HGPRT deficiency impairs the purine salvage pathway, leading to massive uric acid overproduction, causing gout, intellectual disability, and self-mutilating behavior characteristic of Lesch-Nyhan syndrome.

Question 7: What is the final end product of purine catabolism in humans?

Allantoin
Uric acid (Correct answer)
Xanthine
Hypoxanthine

Correct answer: Uric acid

Humans lack uricase and therefore cannot convert uric acid to allantoin as most other mammals do; uric acid is thus the terminal product of purine breakdown, and its accumulation causes gout.

Question 8: Which enzyme catalyzes the committed (first unique) step of pyrimidine biosynthesis?

PRPP synthetase
Carbamoyl phosphate synthetase II (CPS II)
Aspartate transcarbamoylase (ATCase) (Correct answer)
Dihydroorotase

Correct answer: Aspartate transcarbamoylase (ATCase)

ATCase (aspartate transcarbamoylase) catalyzes the condensation of carbamoyl phosphate with aspartate, forming carbamoyl aspartate, which is the committed step unique to pyrimidine biosynthesis.

Question 9: Which nucleotide intermediate serves as the branch point from which both AMP and GMP are synthesized in the purine de novo pathway?

XMP
PRPP
IMP (inosine monophosphate) (Correct answer)
Adenylosuccinate

Correct answer: IMP (inosine monophosphate)

IMP is the first complete purine nucleotide formed in de novo synthesis and serves as the branch-point intermediate from which AMP and GMP diverge via separate enzymatic routes.

Question 10: PRPP (5-phosphoribosyl-1-pyrophosphate) is best described as which of the following?

The final catabolic product of both purines and pyrimidines
An activated ribose donor used in both de novo synthesis and salvage pathways (Correct answer)
A substrate exclusive to pyrimidine de novo synthesis
An allosteric inhibitor of purine salvage enzymes

Correct answer: An activated ribose donor used in both de novo synthesis and salvage pathways

PRPP donates the activated ribose-5-phosphate moiety in both de novo nucleotide synthesis and salvage pathways for purines (HGPRT, APRT) and pyrimidines (OPRT).

Question 11: Orotic aciduria type I is caused by a deficiency of which bifunctional enzyme?

CPS II
ATCase
UMP synthase (UMPS) (Correct answer)
Dihydroorotate dehydrogenase

Correct answer: UMP synthase (UMPS)

UMP synthase possesses both orotate phosphoribosyltransferase and OMP decarboxylase activities; its deficiency blocks the last two steps of pyrimidine synthesis, causing orotic acid accumulation and megaloblastic anemia.

Question 12: 5-Fluorouracil (5-FU) exerts its anticancer activity primarily by inhibiting which enzyme?

Ribonucleotide reductase
Thymidylate synthase (Correct answer)
Dihydrofolate reductase
CTP synthetase

Correct answer: Thymidylate synthase

5-FU is converted to 5-FdUMP, which forms a covalent ternary complex with thymidylate synthase and N5,N10-methyleneTHF, irreversibly blocking dTMP synthesis and depleting the thymidine pool for DNA replication.

Question 13: PRPP amidotransferase, which catalyzes the first committed step of de novo purine synthesis, is allosterically inhibited by which molecules?

Carbamoyl phosphate and aspartate
AMP, ADP, ATP, GMP, GDP, and GTP (Correct answer)
Orotate and UMP
IMP and XMP only

Correct answer: AMP, ADP, ATP, GMP, GDP, and GTP

PRPP amidotransferase is subject to cumulative feedback inhibition by the end-product purine nucleotides (AMP/ADP/ATP and GMP/GDP/GTP), ensuring synthesis shuts down when purine pools are replete.

Question 14: In the pyrimidine salvage pathway, uridine is converted directly to UMP by which enzyme?

Uridine kinase (Correct answer)
UMP synthase
Uridine phosphorylase
CTP synthetase

Correct answer: Uridine kinase

Uridine kinase phosphorylates uridine (using ATP) to produce UMP in the salvage pathway, allowing cells to recycle preformed pyrimidine nucleosides without de novo synthesis.

Question 15: Methotrexate inhibits nucleotide synthesis by targeting which enzyme?

Thymidylate synthase
Dihydrofolate reductase (DHFR) (Correct answer)
Ribonucleotide reductase
PRPP synthetase

Correct answer: Dihydrofolate reductase (DHFR)

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

Gout, due to uric acid overproduction from adenosine catabolism
Severe combined immunodeficiency (SCID), due to toxic dATP accumulation in lymphocytes (Correct answer)
Orotic aciduria, due to blocked pyrimidine conversion
Hemolytic anemia, due to erythrocyte purine depletion

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

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 17: Allopurinol treats hyperuricemia and gout by inhibiting which enzyme?

HGPRT
Purine nucleoside phosphorylase
Xanthine oxidase (Correct answer)
Adenosine deaminase

Correct answer: Xanthine oxidase

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

Phospholipase C
Protein kinase A (PKA) (Correct answer)
Protein kinase C (PKC)
Calmodulin-dependent kinase

Correct answer: Protein kinase A (PKA)

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 19: Ribonucleotide reductase reduces ribonucleotides to deoxyribonucleotides using which catalytic mechanism?

FAD-mediated hydride transfer
NAD+-dependent oxidation
A stable tyrosyl radical stabilized by a dinuclear iron center (Correct answer)
Pyridoxal phosphate-mediated transamination

Correct answer: A stable tyrosyl radical stabilized by a dinuclear iron center

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.

Question 20: NAD+ is synthesized in cells using which vitamin as the key precursor of the nicotinamide ring?

Thiamine (B1)
Riboflavin (B2)
Niacin (B3) (Correct answer)
Pyridoxine (B6)

Correct answer: Niacin (B3)

Niacin (vitamin B3), supplied as nicotinic acid or nicotinamide, provides the nicotinamide moiety of NAD+ and NADP+; deficiency causes pellagra and impairs all NAD+-dependent redox reactions.

Question 21: S-adenosylmethionine (SAM) is the principal methyl donor in biochemical methylation reactions. How is SAM synthesized?

From GTP and methionine by methionine adenosyltransferase
From ATP and methionine by methionine adenosyltransferase (Correct answer)
From PRPP and methionine by a transferase
From AMP and homocysteine by methyltransferase

Correct answer: From ATP and methionine by methionine adenosyltransferase

Methionine adenosyltransferase transfers the adenosyl group of ATP to methionine, forming SAM and releasing inorganic pyrophosphate and phosphate; SAM then donates its methyl group in reactions from DNA methylation to creatine synthesis.

Biochemistry Nucleotide Chemistry and Metabolism

Which of the following nitrogenous bases is classified as a purine?

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