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8: Amino Acid Metabolism and Heritable Disorders of Degradation

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    Learning objectives

    1. Define essential, conditionally essential, and nonessential amino acids, and explain how certain nonessential amino acids become essential in certain conditions.
    2. Integrate amino acid synthesis with specific precursors from glycolysis, citric acid cycle, and the pentose phosphate pathway.
    3. Identify key roles of amino acids as substrates for the synthesis of specialized products including heme, GABA, carnitine, glutathione, serotonin, histamine, ubiquinone, melanin, creatine, and dopamine.
    4. Review the role of transamination in the interconversion of amino acids and connection to the urea cycle; see section 5.3.
    5. Distinguish the following disease states associated with inborn errors of metabolism, including (A) deficient enzyme, (B) inheritance pattern of the disease, and (C) relation of the deficiency to the buildup of secondary metabolites.
    6. The following is a list of diseases to focus on:
      • Phenylketonuria (phenylalanine metabolism),
      • Homocystinuria (methionine metabolism),
      • Maple syrup urine disease (metabolism of branched-chain amino acids), and
      • Alkaptouria (tyrosine metabolism).

    There are twenty amino acids required for metabolic homeostasis. Of the twenty amino acids, eleven are considered nonessential, meaning they can be synthesized by the body. With the exceptions of tyrosine and cysteine, the others can be synthesized from glucose and a nitrogen donor. The other nine amino acids are essential and must be supplied by the diet. As mentioned previously, in addition to supplying carbon for gluconeogenesis, amino acids play important roles in the synthesis of essential cellular components. Disruptions of many of these pathways can lead to clinical disorders, many of which are identified during newborn screenings. The synthesis of all amino acids will not be addressed in this section; rather the most clinically relevant pathways will be focused on.

    • 8.1: Amino acid metabolism and specialized products
      The metabolism of many amino acids largely relies on the availability of the cofactors pyridoxal phosphate (vitamin B6 or PLP), tetrahydrobiopterin (BH4), and tetrahydrofolate (TH4). It is important to recognize that deficiencies in these cofactors could present in a similar manner as enzymatic deficiencies of specific pathways.

    Thumbnail: Grey, Kindred, Chapter 8. 2021. CC BY 4.0.

    This page titled 8: Amino Acid Metabolism and Heritable Disorders of Degradation is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Renee J. LeClair (Virginia Tech Libraries' Open Education Initiative) .

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