Introduction to Amino Acid Metabolism

Essential versus Non-Essential Amino Acids

Inborn Errors in Amino Acid Metabolism

Nonessential Amino Acid Biosynthesis Amino Acid Catabolism
Glutamate and Aspartate
Alanine and the Glucose-Alanine Cycle
Cysteine
Tyrosine
Ornithine and Proline
Serine
Glycine
Aspartate/Asparagine and Glutamate/Glutamine Glutamine/Glutamate and Aspartate/Asparagine
Alanine
Arginine, Ornithine and Proline
Serine
Threonine
Glycine
Cysteine
Methionine
Valine, Leucine, Isoleucine
Phenylalanine and Tyrosine
Lysine
Histidine
Tryptophan

Introduction
All tissues have some capability for synthesis of the non-essential amino acids, amino acid remodeling, and conversion of non-amino acid carbon skeletons into amino acids and other derivatives that contain nitrogen. However, the liver is the major site of nitrogen metabolism in the body. In times of dietary surplus, the potentially toxic nitrogen of amino acids is eliminated via transaminations, deamination, and urea formation; the carbon skeletons are generally conserved as carbohydrate, via gluconeogenesis, or as fatty acid via fatty acid synthesis pathways. In this respect amino acids fall into three categories: glucogenic, ketogenic, or glucogenic and ketogenic. Glucogenic amino acids are those that give rise to a net production of pyruvate or TCA cycle intermediates, such as &豩-ketoglutarate or oxaloacetate, all of which are precursors to glucose via gluconeogenesis. All amino acids except lysine and leucine are at least partly glucogenic. Lysine and leucine are the only amino acids that are solely ketogenic, giving rise only to acetylCoA or acetoacetylCoA, neither of which can bring about net glucose production.

A small group of amino acids comprised of isoleucine, phenylalanine, threonine, tryptophan, and tyrosine give rise to both glucose and fatty acid precursors and are thus characterized as being glucogenic and ketogenic. Finally, it should be recognized that amino acids have a third possible fate. During times of starvation the reduced carbon skeleton is used for energy production, with the result that it is oxidized to CO2 and H2O.

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