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.

The 20 catabolic pathways converge to form only six major products, all of which enter the citric acid cycle.

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
α-ketoglutarate, four to succinyl-CoA, two to fumarate, and two to oxaloacetate.

Parts or all of six amino acids are converted to pyruvate, which can be converted to either acetyl-CoA or oxaloacetate.

Ketogenic Amino Acids

The seven amino acids that are degraded entirely or in part to acetoacetyl-CoA and/or acetyl-CoA—phenylalanine, tyrosine, isoleucine, leucine, tryptophan, threonine, and lysine—can yield ketone bodies in the liver, where acetoacetyl-CoA is converted to acetoacetate and then to acetone and -hydroxybutyrate. These are the ketogenic amino acids. Their ability to form ketone bodies is particularly evident in uncontrolled diabetes mellitus, in which the liver produces large amounts of ketone bodies from both fatty acids and the ketogenic amino acids.

Glucogenic amino acids

The amino acids that are degraded to pyruvate, α-ketoglutarate, succinyl-CoA, fumarate, and/or oxaloacetate can be converted to glucose and glycogen. They are the glucogenic amino acids.

 

The division between ketogenic and glucogenic amino acids is not sharp; five amino acids—tryptophan, phenylalanine, tyrosine, threonine, and isoleucine—are both ketogenic and glucogenic. Catabolism of amino acids is particularly critical to the survival of animals with high-protein diets or during starvation. Leucine is an exclusively ketogenic amino acid that is very common in proteins. Its degradation makes a substantial contribution to ketosis under starvation conditions.