1.  From the beginning or extract and save and reuse. Differentiate between the de novo synthesis of nucleotides and salvage-pathway synthesis. ✓ 5

2.  Activated ribose phosphate. Write a balanced equation for the synthesis of PRPP from glucose through the oxidative branch of the pentose phosphate pathway.

3.  Making a pyrimidine. Write a balanced equation for the synthesis of orotate from glutamine, CO₂, and aspartate. ✓ 5

4.  Identifying the donor. What is the activated reactant in the biosynthesis of each of the following compounds? ✓ 5

(a) Phosphoribosylamine

(b) Carbamoylaspartate

(c) Orotidylate (from orotate)

5.  An “s” instead of a “t”? Differentiate between a nucleoside and a nucleotide.

6.  Associate ‘em. Match the term on the right with the description on the left. ✓ 5

7.  The price of methylation. Write a balanced equation for the synthesis of TMP from dUMP that is coupled to the conversion of serine into glycine.

8.  Sulfa action. Bacterial growth is inhibited by sulfanilamide and related sulfa drugs, and there is a concomitant accumulation of 5-aminoimidazole-4-carboxamide ribonucleotide. This inhibition is reversed by the addition of p-aminobenzoate.



Propose a mechanism for the inhibitory effect of sulfanilamide.

9.  A generous donor. What major biosynthetic reactions utilize PRPP?

10.  Bringing equilibrium. What is the reciprocal substrate relation in the synthesis of ATP and GTP? ✓ 6

11.  Calculate the ATP footprint. How many molecules of ATP are required to synthesize one molecule of CTP from scratch?

12.  Find the label. Suppose that cells are grown on amino acids that have all been labeled at the α carbons with ¹³C. Identify the atoms in cytosine and guanine that will be labeled with ¹³C. ✓ 5

13.  On the trail of carbons. Tissue-culture cells were incubated with glutamine labeled with ¹⁵N in the amide group. Subsequently, IMP was isolated and found to contain some ¹⁵N. Which atoms in IMP were labeled? ✓ 5

14.  Blockages. What intermediate in purine synthesis will accumulate if a strain of bacteria is lacking each of the following biochemicals? ✓ 5

(a) Aspartate

(b) Tetrahydrofolate

(c) Glycine

(d) Glutamine

15.  Mechanism of action. What is the biochemical basis of allopurinol treatment for gout? ✓ 6

16.  Adjunct therapy. Allopurinol is sometimes given to patients with acute leukemia who are being treated with anticancer drugs. Why is allopurinol used? ✓ 6

17.  Folate deficiency. Suppose that someone was suffering from a folate deficiency. What cells would you think might be most affected? Symptoms may include diarrhea and anemia.

18.  Correcting deficiencies. Suppose that a person is found who is deficient in an enzyme required for IMP synthesis. How might this person be treated?

19.  Labeled nitrogen. Purine biosynthesis is allowed to take place in the presence of [¹⁵N]aspartate, and the newly synthesized GTP and ATP are isolated. What positions are labeled in the two nucleotides? ✓ 5

20.  Needed supplies. Why are cancer cells especially sensitive to inhibitors of TMP synthesis? ✓ 6

21.  They’re everywhere! Nucleotides play a variety of roles in the cell. Give an example of a nucleotide that acts in each of the following roles or processes.

(a) Second messenger

(b) Phosphoryl-group transfer

(c) Activation of carbohydrates

(d) Activation of acetyl groups

(e) Transfer of electrons

(f) Chemotherapy

(g) Allosteric effector

22.  Exercising muscle. Some interesting reactions take place in muscle tissue to facilitate the generation of ATP for contraction. ✓ 6



In muscle contraction, ATP is converted into ADP. Adenylate kinase converts two molecules of ADP into a molecule of ATP and AMP.

(a) Why is this reaction beneficial to contracting muscle?

(b) Why is the equilibrium for the adenylate kinase approximately equal to 1?

Muscle can metabolize AMP by using the purine nucleotide cycle. The initial step in this cycle, catalyzed by AMP deaminase, is the conversion of AMP into IMP.

(c) Why might the deamination of AMP facilitate ATP formation in muscle?

(d) How does the purine nucleotide cycle assist the aerobic generation of ATP?

23.  Different strokes. Human beings contain two different carbamoyl phosphate synthetase enzymes. One uses glutamine as a substrate, whereas the other uses ammonia. What are the functions of these two enzymes?

24.  Your pet duck. You suspect that your pet duck has gout. Why should you think twice before administering a dose of allopurinol-laced bread?

25.  Inhibiting purine biosynthesis. Amidotransferases are inhibited by the antibiotic and antitumor agent azaserine (O-diazoacetyl-l-serine), which is an analog of glutamine. ✓ 5



Which intermediates in purine biosynthesis would accumulate in cells treated with azaserine?

26.  HAT medium. Mutant cells unable to synthesize nucleotides by salvage pathways are very useful tools in molecular and cell biology. Suppose that cell A lacks thymidine kinase, the enzyme catalyzing the phosphorylation of thymidine to thymidylate, and that cell B lacks hypoxanthine-guanine phosphoribosyl transferase.

(a) Cell A and cell B do not proliferate in a HAT medium containing hypoxanthine, aminopterin or amethopterin (methotrexate), and thymine. However, cell C formed by the fusion of cells A and B grows in this medium. Why?

(b) Suppose that you wanted to introduce foreign genes into cell A. Devise a simple means of distinguishing between cells that have taken up foreign DNA and those that have not. ✓ 5

27.  Hyperuricemia. Many patients with glucose 6-phosphatase deficiency have high serum levels of urate. Hyperuricemia can be induced in normal people by the ingestion of alcohol or by strenuous exercise. Propose a common mechanism that accounts for these findings.

28.  Labeled carbon. Succinate uniformly labeled with ¹⁴C is added to cells actively engaged in pyrimidine biosynthesis. Propose a pathway by which carbon atoms from succinate can be incorporated into a pyrimidine. At what positions is the pyrimidine labeled? ✓ 5

29.  Something funny going on here. Cells were incubated with glucose labeled with ¹⁴C in carbon 2, shown in red in the following structure. Later, uracil was isolated and found to contain ¹⁴C in carbons 4 and 6. Account for this labeling pattern. ✓ 5