Chapter 23

Protein hydrolysis is an exergonic process, yet the \(26 S\) proteasome is dependent on ATP hydrolysis for activity. (a) Although the exact function of the ATPase activity is not known, suggest some likely functions. (b) Small peptides can be hydrolyzed without the expenditure of ATP. How does this information concur with your answer to part \(a ?\)

Name the \(\alpha\) -ketoacid that is formed by the transamination of each of the following amino acids: (a) Alanine (b) Aspartate (c) Glutamate (d) Leucine (e) Phenylalanine (f) Tyrosine

The archaeal proteasome contains 14 identical active \(\beta\) subunits, whereas the eukaryotic proteasome has 7 distinct \(\beta\) subunits. What are the potential benefits of having several distinct active subunits?

Pyridoxal phosphate stabilizes carbanionic intermediates by serving as an electron sink. Which other prosthetic group catalyzes reactions in this way?

Propose a role for the positively charged guanidinium nitrogen atom in the cleavage of argininosuccinate into arginine and fumarate.

Glutamate is an important neurotransmitter whose levels must be carefully regulated in the brain. Explain how a high concentration of ammonia might disrupt this regulation. How might a high concentration of ammonia alter the citric acid cycle?

The urine of an infant gives a positive reaction with 2,4 -dinitrophenylhydrazine. Mass spectrometry shows abnormally high blood levels of pyruvate, \(\alpha\) -ketoglutarate, and the \(\alpha\) -ketoacids of valine, isoleucine, and leucine. Identify a likely molecular defect and propose a definitive test of your diagnosis.

Why should phenylketonurics avoid using aspartame, an artificial sweetener? (Hint: Aspartame is L-aspartylL-phenylalanine methyl ester.)

N-Acetylglutamate is required as a cofactor in the synthesis of carbamoyl phosphate. How might \(N\) -acetylglutamate be synthesized from glutamate?

Degradation signals are commonly located in protein regions that also facilitate protein-protein interactions. Explain why this coexistence of two functions in the same domain might be useful.

Within a few days after a fast begins, nitrogen excretion accelerates to a higher-than-normal level. After a few weeks, the rate of nitrogen excretion falls to a lower level and continues at this low rate. However, after the fat stores have been depleted, nitrogen excretion rises to a high level. (a) What events trigger the initial surge of nitrogen excretion? (b) Why does nitrogen excretion fall after several weeks of fasting? (c) Explain the increase in nitrogen excretion when the lipid stores have been depleted.