BIO C HE MIA I I – ĆW ICZ E NI A

L I S T A 9

do wykładu dr. hab. inż. P. Dobryszyckiego

B i o s y n t e z a a m i n o k w a s ó w

1. Write the net equation for nitrogen fixation and describe the sources of the electrons and ATP.

2. Match the enzyme with the reaction it catalyzes.

(a) glutamine synthetase

(b) glutamate dehydrogenase

(c) glutamate synthase

Which of the reactions require the following?

(a) NH +

4

(b) ATP

(c) NADH

(d) NADPH

3. All organisms can incorporate NH +

4 into glutamate and glutamine using glutamate dehydrogenase

and glutamine synthetase. Why do prokaryotes have an additional enzyme, glutamate synthase, to perform this function?

4. Which of the following amino acids are essential dietary components for an adult human?

(a) alanine

(g) glutamine

(b) aspartate

(h) asparagine

(c) histidine

(i) glutamate

(d) tryptophan

(j) threonine

(e) leucine

(k) methionine

(f) phenylalanine

5. Which of the following amino acids are derived from pyruvate?

(a) phenylalanine

(e) valine

(b) alanine

(f) leucine

(c) tyrosine

(g) cysteine

(d) histidine

(h) glycine

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6. Which of the following amino acids are derived from α-ketoglutarate?

(a) glutamate

(e) glutamine

(b) proline

(f) arginine

(c) cysteine

(g) ornithine

(d) aspartate

(h) serine

7. Which of the following compounds provide the carbon skeletons of the six biosynthetic families of amino acids? Name the metabolic pathways from which each of the precursor compounds originates.

(a) pyruvate

(b) oxaloacetate

(c) α-ketoglutarate

(d) succinate

(e) 2-deoxyribose

(f) 3-phosphoglycerate

(g) ribose 5-phosphate

(h) glucose 6-phosphate

(i) phosphoenolpyruvate

(j) erythrose 4-phosphate

(k) α-ketobutyrate

8. Three coenzymes are involved in carrying activated one-carbon units. Match the activated group in the right column with the appropriate coenzyme in the left column.

(a) tetrahydrofolate

(1) —CH3

(b) S-adenosylmethionine

(2) —CH2—

(c) biotin

(3) —CHO

(4) —CHNH

(5) — C H =

(6) —CO2—

9. Which of the following answers completes the sentence correctly? The major source of one-carbon units for the formation of the tetrahydrofolate derivative N5,N10-methylenetetrahydrofolate is the conversion of

(a) methionine to homocysteine.

(b) deoxyuridine 5'-phosphate to deoxythymidine 5'-phosphate.

(c) 3-phosphoglycerate to serine.

(d) serine to glycine.

10. S-adenosylmethionine is involved directly in which of the following reactions?

(a) methyl transfer to phosphatidyl ethanolamine

(b) synthesis of glycine from serine

(c) DNA methylation

(d) conersion of homocysteine into methionine

(e) synthesis of ethylene in plants

11. How many high-energy bonds are expended during the synthesis of S-adenosylmethionine from ATP

and methionine?

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12. The conversion of homocysteine into methionine involves which of the following cofactors?

(a) N5-methyltetrahydrofolate

(b) N5,N10-methylenetetrahydrofolate

(c) methylcobalamin

(d) pyridoxal phosphate

13. In the following biosynthetic pathway A — > B — > C — > D — > E —> F —> G, which is likely to be the committed step? Which compound is likely to inhibit the committed step?

14. Since glutamine is an important source of nitrogen in biosynthetic reactions, the enzyme that synthesizes it is carefully regulated. Which of the following compounds act as inhibitors of glutamine synthetase in E. coli?

(a) tryptophan

(b) histidine

(c) carbamoyl phosphate

(d) glucosamine 6-phosphate

(e) AMP

(f) CTP

(g) alanine

(h) glycine

15. How does covalent modification contribute to the regulation of glutamine synthetase in E. coli?

16. Which of the following answers complete the sentence correctly? Glutathione (a) cycles between oxidized and reduced forms in the cell.

(b) is involved in the detoxification of H2O2 and organic peroxides.

(c) donates amide groups from its gamma-glutamyl residue during biosynthetic reactions.

(d) contains an Se atom.

17. Which of the following statements about nitric oxide (NO) is correct?

(a) It is used as an inhalation anesthetic.

(b) It is a long-lived signal molecule.

(c) It is produced from asparagine.

(d) Its synthesis requires NADPH and O2.

(e) Its synthesis requires ATP and NH +

4 .

18. Which of the following are intermediates or precursors in the synthesis of heme?

(a) alfa-aminolevulinic acid

(e) glycine

(b) bilirubin

(f) succinyl CoA

(c) porphobilinogen

(g) Fe2+

(d) biliverdin

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19. The essential amino acids are those that cannot be synthesized de novo in humans. Given an abundance of other amino acids in the diet, the α-keto acid analogs that correspond to the essential amino acids can substitute for these compounds in the diet. If l5N-labeled alanine is supplied in the diet, many other amino acids in the body will contain at least a small amount of the label within 48 hours. What enzymes are primarily responsible for this observation?

20. In muscle, glutamine synthetase is very active, catalyzing the formation of glutamine from glutamate and ammonia at the expense of a molecule of ATP In the liver, the rate of formation of glutamine is very low, but a high level of glutaminase activity, which generates ammonia and glutamate, is observed. How would you explain the difference in the levels of enzyme activity in these two organs?

21. Consider three forms of bacterial glutamine synthetase: GS, the deadenylylated form; GS-(AMP) 1, a form with one AMP unit per 12 subunits; and GS-(AMP)12, the fully adenylylated form.

(a) Which of these forms is most sensitive to feedback inhibition by several of the final products of glutamine metabolism, such as tryptophan or histidine? Why is it important that the activity of the most sensitive form not be completely inhibited by tryptophan?

(b) Which form has the lowest KM for ammonia?

(c) Why is it important that adenylyl transferase not carry out adenylylation and deadenylylation of glutamine synthetase at the same time?

(d) Glutamine synthetase in mammals is not subject to the same type of complex regulation that is seen in bacteria. Why?

22. The diagram below outlines the biosynthesis of a compound that is required for the oxidation of fatty acids in the mitochondrion.

(a) Name compound D and briefly explain its role in fatty acid metabolism.

(b) Name compound A. Why is it considered essential in human diets?

(c) Three molecules of compound B are required for the formation of compound C. Its synthesis depends on the availability of an essential amino acid. Name that aminoacid and then name compound B and compound C.

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