Pathways of Amino Acid Degradation

The pathways of amino acid catabolism, taken together, normally account for only 10% to 15% of the human body's energy production; these pathways are not nearly as active as glycolysis and fatty acid oxidation. Flux through these catabolic routes also varies greatly, depending on the balance between requirements for bio-

synthetic processes and the availability of a particular amino acid. The 20 catabolic pathways converge to form only six major products, all of which enter the citric acid cycle (Fig. 18-15). From here the carbon skeletons are diverted to gluconeogenesis or ketogenesis or are completely oxidized to CO2 and H2O.

All or part of the carbon skeletons of seven amino acids are ultimately broken down to acetyl-CoA. Five amino acids are converted to a-ketoglutarate, four to succinyl-CoA, two to fumarate, and two to oxaloacetate. Parts or all of six amino acids are converted to pyru-vate, which can be converted to either acetyl-CoA or oxaloacetate. We later summarize the individual pathways for the 20 amino acids in flow diagrams, each leading to a specific point of entry into the citric acid cycle. In these diagrams the carbon atoms that enter the citric acid cycle are shown in color. Note that some amino acids appear more than once, reflecting different fates for different parts of their carbon skeletons. Rather than examining every step of every pathway in amino acid catabolism, we single out for special discussion some enzymatic reactions that are particularly noteworthy for their mechanisms or their medical significance.

Some Amino Acids Are Converted to Glucose, Others to Ketone Bodies

The seven amino acids that are degraded entirely or in part to acetoacetyl-CoA and/or acetyl-CoA—phenylala-nine, tyrosine, isoleucine, leucine, tryptophan, threo-nine, and lysine—can yield ketone bodies in the liver,

Leucine Lysine

Phenylalanine Ketone Tryptophan bodies Tyrosine

Leucine Lysine

Phenylalanine Ketone Tryptophan bodies Tyrosine

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Acetoacetyl-CoA

Isoleucine Methionine Threonine Valine

Lysine Ketogenic Glucogenic

Isoleucine Methionine Threonine Valine

Phenylalanine Tyrosine

^ Glucose

Isoleucine Leucine Threonine Tryptophan

Glucogenic Ketogenic

Phenylalanine Tyrosine

^ Glucose

Isoleucine Leucine Threonine Tryptophan

Alanine Cysteine Glycine Serine

Threonine Asparagine Tryptophan Aspartate

Glucogenic Ketogenic

FIGURE 18-15 Summary of amino acid catabolism. Amino acids are grouped according to their major degradative end product. Some amino acids are listed more than once because different parts of their carbon skeletons are degraded to different end products. The figure shows the most important catabolic pathways in vertebrates, but there are minor variations among vertebrate species. Threonine, for instance, is degraded via at least two different pathways (see Figs 18-19, 18-2 7), and the importance of a given pathway can vary with the organism and its metabolic conditions. The glucogenic and ketogenic amino acids are also delineated in the figure, by color shading. Notice that five of the amino acids are both glucogenic and ketogenic. The amino acids degraded to pyruvate are also potentially ketogenic. Only two amino acids, leucine and lysine, are exclusively ketogenic.

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Responses

  • john
    What are the seven degradation amino acids?
    8 years ago
  • addison
    How we can say that isoleucine is glucogenic and ketogenic?
    7 years ago
  • Anette
    Can acetyl CoA be a Glucogenic amino acids?
    7 years ago
  • merja
    What are the specific points of entry for the pathways of amino acids?
    7 years ago

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