Amino Acid Metabolism and Genetic Disorders: Key Facts

The amino acid pool is supplied by three sources:
- Amino acids provided by the degradation of body proteins
- Amino acids derived from dietary proteins
- Synthesis of nonessential amino acids from simple intermediates of metabolism
In healthy, well-fed individuals, the input to the amino acid pool is balanced by the output; that is, the amount of amino acids contained in the pool is constant.
In healthy adults, the total amount of protein in the body remains constant because the rate of protein synthesis is just sufficient to replace the protein that is degraded. This process is called protein turnover.

The digestion of proteins begins in the stomach.
Select true sentences:
- Pepsin is an acid-stable endopeptidase.
A disease of malabsorption resulting from immune-mediated damage to the small intestine in response to ingestion of gluten, a protein found in wheat and other grains, is called Celiac disease (celiac sprue).

Select false sentence:
- AST is more specific than ALT for liver disease, but the latter is more sensitive because the liver contains larger amounts of ALT.
When energy levels are low in the cell, amino acid degradation by glutamate dehydrogenase is high.

Urea is produced by the liver.
Reactions of the urea cycle occur both in the mitochondria and in the cytosol.
Formation of carbamoyl phosphate by carbamoyl phosphate synthetase is driven by the cleavage of 2 molecules of ATP.
Ammonia incorporated into carbamoyl phosphate is provided primarily by the oxidative deamination of glutamate by mitochondrial glutamate dehydrogenase.
Amino acids in the urea cycle with no codons (no protein) are ornithine and citrulline.
Synthesis of argininosuccinate is driven by the cleavage of 1 molecule of ATP.
Fumarate produced in the urea cycle is hydrated to malate. The malate can be transported into the mitochondria via the malate shuttle and reenter the tricarboxylic acid cycle.
Fill the gaps in the stoichiometry of the urea cycle:
- Aspartate + NH₃ + CO₂ + 3 ATP → urea + fumarate + 2ADP + AMP + 2 P_i + PP_i + 3H₂O

Replace several sources of ammonia:
- From glutamine
- From bacterial action in the intestine
- From amines
- From purines and pyrimidines
The two major causes of hyperammonemia are liver disease and inherited deficiencies of enzymes in the urea cycle.
Which one of the following statements about the urea cycle is correct?
- Urinary urea is increased by a diet rich in protein.

Glucogenic amino acids are alanine and aspartate.
Which one of the following statements is false?
- Asparaginase increases the level of asparagine in the plasma and, therefore, deprives cancer cells of a required nutrient.
Glutamine is converted to glutamate and ammonia by the enzyme glutaminase. Glutamate is converted to α-ketoglutarate by:
- Transamination
- Through oxidative deamination by glutamate dehydrogenase
Amino acids that form pyruvate are alanine, serine, glycine, cysteine, and threonine.

Phenylketonuria (PKU) is caused by a deficiency of phenylalanine hydroxylase and accumulation of phenylalanine.
Albinism refers to a group of conditions in which a defect in tyrosine metabolism results in a deficiency in the production of melanin.
Parkinson’s disease, a neurodegenerative movement disorder, is due to insufficient dopamine production as a result of the idiopathic loss of dopamine-producing cells in the brain.

Replace the examples:
- Of purine – adenine (A); guanine (G)
- Of pyrimidine – cytosine (C); thymine (T), and uracil (U)
The rate of DNA synthesis in a culture of cells could be most accurately determined by measuring the incorporation of which of the following radioactive compounds? Thymidine
Deoxyribonucleosides include deoxyadenosine.

Gout is a disorder characterized by high levels of uric acid – the end product of purine catabolism.