Amino Acid Metabolism Disorders

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Where is Protein Metabolized?

• Protein is metabolized mainly in muscle and liver.

Urea Cycle:

• Occurs partially in mitochondria and partially in cytoplasm of hepatocyte.

• Urea is formed and enters the blood for entry into kidney.

• Defect causes: elevated blood glutamine, hyperammonemia, and decreased BUN.

• Two diseases result from deficiencies of mitochondrial enzymes in urea cycle: carbamoyl phosphate synthetase (causes no increase in uracil or orotic acid), and ornithine transcarbamylase (increased uracil and orotic acid in blood and urine).

Amino acids are stripped off their alpha-amino group and metabolized for energy.

Maple Syrup Urine Disease:

• Urine smells like maple syrup.

• Mental retardation, ketosis, coma, abnormal muscle tone, death.


• Pale skin and white-blond hair.

• High-levels of phenylalanine.

• Treatment: decreased intake of phenylalanine.

• Pregnant women with phenylalanine in their blood expose fetus to high risk of neurological developmental disorders.


• Urine turns black upon exposure to air.

• Deficiency of homogentisate oxidase.

Homocysteinemia or Homocystinuria:

• Accumulation of homocystine in blood.

• Findings: DVT, thromboembolism, lens dislocation, mental retardation, myocardial infarction before age 20.

• Treatment: decreased intake of methionine.

• Treatment: folate, vitamin B12, and vitamin B6.

Folate Deficiency:

• Megaloblastic anemia.

• Homocystinemia with risk for CV disease.

Folate Deficiency Causes:

• Pregnancy, alcoholism, and malnutrition.

Vitamin B12 (Cobalamin) Deficiency:

• Megaloblastic anemia.

• Progressive peripheral neuropathy.

• Homocystinemia with risk for CV disease.

• Methylmalonic aciduria.

Vitamin B12 Deficiency Causes:

• Pernicious anemia, chronic pancreatitis, malnutrition, vegan, and D. lactum infection.


• Formed from folate.

• Needed for RNA and DNA synthesis.

Heme Synthesis:

• Occurs in almost all tissues.

• Rate-limiting enzyme: delta-ALA; repressed by heme.

• ALA dehydrase, ferrochelatase: inhibited by Lead.

• Acute intermittent porphyria: deficiency of uroporphyrinogen-I synthase.

Vitamin B6 Deficiency:

• Decreased protoporphyrin, decreased ALA, increased ferritin, increased serum iron.

Iron Deficiency:

• Increased protoporphyrin, normal ALA, decreased ferritin, decreased serum iron.

Lead Poisoning:

• Increased protoporphyrin, increased ALA, increased ferritin, increased serum iron. Headache, memory loss, diarrhea, abdominal pain, lead lines in gums, lead deposits in abdomen, neuropathy.

Iron Transport and Storage:

• Ferroxidase: oxidizes Fe2+ to Fe3+ for transport and storage.

• Transferring: carries Fe3+ in blood.

• Ferritin: oxidizes Fe2+ to Fe3+.

• Hemosiderin: prevents excess Fe3+ from escaping into blood, where it is toxic.

Bilirubin Metabolism:

• Spleen: heme converted to biliverdin converted to bilirubin.

• Blood: bilirubin-albumin formed.

• Liver: bilirubin converted to bilirubin diglucuronide.

• Intestine: urobilinogen/bile pigments excreted in feces.

Additional Readings:

Basic Biochemistry

1. Nucleic Acid Structure and Organization
2. DNA Replication and Repair
3. Transcription and RNA Processing
4. Genetic Code, Mutations, and Translation
5. Genetic Regulation
6. Recombinant DNA
7. Amino Acids, Proteins, Enzymes
8. Hormones
9. Vitamins
10. Energy Metabolism
11. Glycolysis and Pyruvate Dehydrogenase
12. Citric Acid Cycle and Oxidative Phosphorylation
13. Glycogen, Gluconeogenesis, and Hexose Monophosphate Shunt
14. Lipid Synthesis and Storage
15. Lipid Mobilization and Catabolism
16. Amino Acid Metabolism Disorders
17. Purine and Pyrimidine Metabolism
18. Electron Transport
19. Citric Acid Cycle and Glyoxylate Cycle
20. Glycolysis
21. Pyruvate Metabolism
22. Mitochondrial ATP formation
23. Gluconeogenesis
24. Glycogen Metabolism
25. Nitrogen Fixation (Metabolism) reactions, and Heme Metabolism
26. Amino Acid Metabolism
27. What is Medium Chain Acyl-CoA Dehydrogenase Deficiency (MCADD)?

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