Amino Acid Metabolism

By Yolanda Smith, B.Pharm.Reviewed by Susha Cheriyedath, M.Sc.

Amino acid metabolism is an important process that occurs within the human body to assist in numerous biological reactions. This article will cover the role of glutamate, transamination reactions, and various types of amino acids such as glycogenic, ketogenic, and mixed amino acids.

Glutamate and Transamination Reactions

The four most common amino acids in the human body are glutamate, aspartate, alanine, and glutamine and each has major metabolic functions and roles in the body.

Glutamate has a similar chemical structure to 2-oxoglutarate, which is an intermediate substance in the Krebs cycle of the body. Glutamate and 2-oxoglutarate exist in equilibrium and can be converted by transaminases or glutamate dehydrogenase. Glutamate can also be converted to glutamine, which is the most common free amino acid in human blood plasma and can be a carrier of nitrogen in the body.

Glutamate is the most prevalent and has important roles to play in bodily functions. Of note when it comes to metabolism, glutamate has a central part in the breakdown of amino acids. For example, the glutamate pool is essential for excreting nitrogen from dietary protein from the body.

Transamination is a reaction that involves the conversion of an amino acid to the corresponding keto acid. In this type of reaction, the amino group from an amino acid swaps to another keto acid so that there is a new pairing of amino acid and keto acid. There is no net loss or gain of nitrogen in this type of reaction. Transamination reactions are reversible with an equilibrium constant that is close to 1.

Glycogenic, Ketogenic, and Mixed Amino Acids

The majority of amino acid carbon skeletons are degraded to intermediates of the Krebs cycle after transamination. As a result, they can increase the blood glucose levels via the gluconeogenic pathway.

These amino acids were traditionally known as “glycogenic” amino acids because it was observed that they had a tendency to worsen diabetic glycosuria for this reason:

• Alanine: made from and degraded to pyruvate
• Arginine: made from and degraded to glutamate
• Asparagine: made from and degraded to aspartate
• Aspartate: made from and degraded to oxaloacetate
• Cysteine: an essential amino acid that can be made from methionine and degraded to pyruvate
• Glutamate: made from and degraded to oxoglutarate
• Glutamine: made from and degraded to glutamate
• Glycine: made from serine with multiple pathways of degradation
• Histidine: an essential amino acid that can be degraded to glutamate
• Methionine: an essential amino acid that can be degraded to propionyl-CoA
• Proline: made from and degraded to glutamate
• Serine: made from phosphoglycerate and degraded to pyruvate
• Threonine: an essential amino acid with unknown degradation products
• Valine: an essential amino acid that can be degraded to propionyl-CoA

“Ketogenic” amino acids tend to worsen diabetic ketoacidosis and are typically degraded to acetoacetate or acetyl-CoA. Leucine is an example of this type, which is an essential amino acid that can be degraded to acetyl-CoA.

“Mixed” amino acids can be degraded both to amino acids of the Krebs cycle and to acetyl-CoA, with characteristics of both glycogenic and ketogenic amino acids.

• Isoleucine: an essential amino acid that can be degraded to acetyl-CoA and propionyl-CoA
• Lysine: an essential amino acid with unknown degradation products
• Phenylalanine: an essential amino acid that can be degraded to tyrosine
• Tryptophan: an essential amino acid with unknown degradation products
• Tyrosine: an essential amino acid that can be made from phenylalanine and degraded to fumarate and acetoacetate.

References

• https://www.khanacademy.org/test-prep/mcat/biomolecules/fat-and-protein-metabolism/v/overview-of-amino-acid-metabolism
• https://www.slideshare.net/senchiy/amino-acids-metabolism-new-12281450
• http://fblt.cz/en/skripta/ii-premena-latek-a-energie-v-bunce/12-metabolismus-aminokyselin/
• https://themedicalbiochemistrypage.org/amino-acid-metabolism.php
• http://www.bmb.leeds.ac.uk/illingworth/metabol/amino.htm