Question 1

Which of the following statements best describes the fate of nitrogen from amino acid catabolism in humans?

Accepted Answer

Most nitrogen is converted to urea in the liver and excreted in the urine.

Question 2

During protein turnover, which process is responsible for the selective degradation of damaged or short-lived proteins in the cytosol?

Accepted Answer

Ubiquitin-proteasome system

Question 3

A patient presents with elevated plasma levels of alanine aminotransferase (ALT) and aspartate aminotransferase (AST). What does this most likely indicate?

Accepted Answer

Liver cell damage

Question 4

Which amino acid is the primary carrier of ammonia from muscle to liver during fasting, participating in the glucose-alanine cycle?

Accepted Answer

Alanine

Question 5

What is the role of pyridoxal phosphate in transamination reactions?

Accepted Answer

It serves as a coenzyme that temporarily holds the amino group.

Question 6

Which enzyme catalyzes the first committed step of the urea cycle, and what is its essential allosteric activator?

Accepted Answer

Carbamoyl phosphate synthetase I; N-acetylglutamate

Question 7

A deficiency in which urea cycle enzyme is most commonly associated with congenital hyperammonemia?

Accepted Answer

Ornithine transcarbamylase

Question 8

Which of the following best explains why amino acids cannot be stored in the body?

Accepted Answer

There is no specialized storage form for amino acids; excess are degraded.

Question 9

Which process is responsible for the non-selective degradation of extracellular proteins?

Accepted Answer

Lysosomal hydrolases (heterophagy)

Question 10

In the urea cycle, which molecule donates the second nitrogen atom that is incorporated into urea?

Accepted Answer

Aspartate

Question 11

Which of the following equations correctly represents the overall stoichiometry of the urea cycle?

Accepted Answer

$$CO_2$$ + $$NH_3 + 3$$ATP + aspartate + 2H_2O \rightarrow urea + 2ADP + 2Pi + AMP + PP_i + fumarate$$

Question 12

Which amino acid is NOT a substrate for transamination reactions?

Accepted Answer

Lysine

Question 13

A patient with a defect in the intestinal and renal transport system for cystine, ornithine, arginine, and lysine (COAL) is most likely suffering from which condition?

Accepted Answer

Cystinuria

Question 14

Which of the following best describes the function of glutamine in nitrogen metabolism?

Accepted Answer

It serves as a nontoxic storage and transport form of ammonia.

Question 15

Which of the following statements about the glucose-alanine cycle is correct?

Accepted Answer

Alanine transports ammonia from muscle to liver, where it is converted to urea.

Question 16

Which of the following is a consequence of hyperammonemia?

Accepted Answer

Neurological symptoms such as confusion and coma

Question 17

Which step in amino acid catabolism is catalyzed by glutamate dehydrogenase?

Accepted Answer

Oxidative deamination of glutamate to a-ketoglutarate and ammonia

Question 18

Which of the following is true regarding the regulation of the urea cycle?

Accepted Answer

N-acetylglutamate is an essential activator of carbamoyl phosphate synthetase I.

Question 19

Which of the following best explains the role of the N-end rule in protein degradation?

Accepted Answer

It influences the half-life of a protein based on its N-terminal amino acid.

Question 20

Which of the following is a multi-step process that integrates transamination, oxidative deamination, and the urea cycle for nitrogen disposal?

Accepted Answer

Conversion of alanine to pyruvate, then to glucose, and finally to urea in the liver

Amino Acids: Nitrogen Disposal and Metabolism

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