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10.4: Amino Acid Synthesis

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  • In humans, only half of the standard amino acids (Glu, Gln, Pro, Arg, Asp, Asn, Ala, Gly, Ser, Tyr, Cys) can be synthesized, and are thus classified the nonessential amino acids.  

    Most amino acids are synthesized from α-ketoacids or α-hydroxy acids (3-phosphoglycerate), and later transaminated from another amino acid (usually glutamate). The enzyme involved in this reaction is an aminotransferase. Glutamate is usually the amino group donor for this reaction: α-ketoacid + glutamate ⇄ amino acid + α-ketoglutarate


    Glutamate itself is regenerated by the amination of α-ketoglutarate, catalyzed by Glutamate dehydrogenase:

    Glutamate dehydrogenase.png


    The carbon skeletons used for the synthesis of amino acids are intermediates of the glycolysis pathway and the citric acid cycle (see table below)

    Source of carbon skeleton used for the synthesis of nonessential amino acids
    Intermediates of glycolysis
    pyruvate is used for the synthesis of  glycine, serine, cysteine
    3-phosphoglycerate is used for the synthesis of  alanine
    Intermediates of citric acid cycle
    α-ketoglutarate is used for the synthesis of  glutamate, glutamine, proline, arginine
    oxalacetate is used for the synthesis of  aspartate, asparagine

    Tyrosine is another amino acid that depends on an essential amino acid as a precursor. In this case, phenylalanine hydroxylase oxidizes phenylalanine to produce tyrosine:

    phenylalanine hydroxylase.png

    Phenylketonuria is a genetic disorder that results in low levels of the enzyme phenylalanine hydroxylase. This results in the buildup of dietary phenylalanine to potentially toxic levels. Untreated, PKU can lead to intellectual disability, seizures, behavioral problems, and mental disorders. It may also result in a musty smell and lighter skin.

    In general, the synthesis of essential amino acids, usually in microorganisms, is much more complex than for the nonessential amino acids and is best left to a full-fledged biochemistry course.