Question 1

Which of the following statements best describes the role of IMP (inosine 5'-monophosphate) in purine biosynthesis?

Accepted Answer

IMP serves as a branch point from which both AMP and GMP are synthesized via separate pathways.

Question 2

Which enzyme catalyzes the conversion of adenosine to inosine in purine catabolism, and what type of reaction is this?

Accepted Answer

Adenosine deaminase; deamination

Question 3

A deficiency in HGPRT (hypoxanthine-guanine phosphoribosyltransferase) can lead to elevated uric acid levels. Which TWO mechanisms contribute to this increase?

Accepted Answer

Increased conversion of PRPP to IMP and decreased salvage of hypoxanthine and guanine, both leading to more uric acid production.

Question 4

Which of the following best explains why purinosome formation is beneficial for de novo purine biosynthesis?

Accepted Answer

It facilitates channeling of intermediates, maintaining high local concentrations and increasing pathway efficiency.

Question 5

Which enzyme is responsible for converting nucleosides to nucleobases in purine catabolism, and what is the reaction type?

Accepted Answer

Purine nucleoside phosphorylase; phosphorolysis

Question 6

Elevated uric acid in the blood can lead to which of the following medical conditions?

Accepted Answer

Gout, kidney stones, and chronic kidney disease due to uric acid crystallization.

Question 7

Allopurinol is used to treat gout. Which statement best describes its mechanism of action?

Accepted Answer

Allopurinol is converted to alloxanthine, a suicide inhibitor of xanthine dehydrogenase, reducing uric acid production.

Question 8

Which two molecules provide all six atoms in the pyrimidine ring, and which atoms do they contribute?

Accepted Answer

Aspartate (C1, C4, C5, N1) and Carbamoyl phosphate (C2, N3)

Question 9

What is the main benefit of 'channeling' in carbamoyl phosphate synthetase during pyrimidine biosynthesis?

Accepted Answer

It allows unstable intermediates to be transferred efficiently between active sites without diffusing into the cytosol.

Question 10

Which step in the pyrimidine biosynthetic pathway is regulated by feedback inhibition from CTP?

Accepted Answer

Step 2 (carbamoyl aspartate formation) and Step 9 (conversion of UTP to CTP)

Question 11

Which enzyme is responsible for reducing ribonucleotides to deoxyribonucleotides, and which carbon in ribose is reduced?

Accepted Answer

rNDP reductase; C1 carbon

Question 12

Which class of rNDP reductase (RNR) is most common in humans, and what is its mechanism for generating a free radical?

Accepted Answer

Class I; uses a catalytic tyrosine radical

Question 13

In the regulation of RNR, what is the effect of dATP binding to the activity site?

Accepted Answer

dATP inhibits the reduction of all substrates (ADP, GDP, CDP, UDP) by RNR.

Question 14

Which molecule serves as both the carbon and electron donor in the conversion of dUMP to dTMP by thymidylate synthase?

Accepted Answer

$$5,10	ext{-methyl tetrahydrofolate}$$

Question 15

During the thymidylate synthesis cycle, which enzyme reduces dihydrofolate back to tetrahydrofolate?

Accepted Answer

Dihydrofolate reductase

Question 16

Which of the following best describes the structural difference between a nucleotide, nucleoside, and nucleobase?

Accepted Answer

Nucleotide = base + sugar + phosphate; Nucleoside = base + sugar; Nucleobase = base only

Question 17

Which of the following is the correct sequence for the conversion of UMP to UTP in pyrimidine biosynthesis?

Accepted Answer

UMP $$\rightarrow$$ UDP $$\rightarrow$$ UTP

Question 18

Which of the following statements about the electron delivery to RNR is correct?

Accepted Answer

Electrons are delivered from NADPH via either thioredoxin or glutaredoxin.

Question 19

Which enzyme catalyzes the conversion of hypoxanthine to xanthine in purine catabolism?

Accepted Answer

Xanthine dehydrogenase

Step-by-Step Guidance for Purine and Pyrimidine Metabolism Review

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