# 7.7: Amino Acid Metabolism

The pathways for the synthesis and degradation of amino acids used in proteins are the most varied among the reactions synthesizing biological building blocks. We start with some terms. First, not all organisms can synthesize all the amino acids they need. Amino acids that an organism cannot synthesize (and therefore must have in their diets) are called essential amino acids. The remaining amino acids that the body can synthesize are called non-essential.

Figure 7.7.1: Glucogenic and Ketogenic Amino Acids

Amino acids are also divided according to the pathways involved in their degradation. There are three general categories. Ones that yield intermediates in the glycolysis pathway are called glucogenic and those that yield intermediates of acetyl-CoA or acetoacetate are called ketogenic. Those that involve both are called glucogenic and ketogenic.

An important general consideration in amino acid metabolism is that of transamination. In this process, an exchange of amine and oxygen between an amino acid and an alpha-ketoacid occurs (see below)

$\text{Alpha-ketoacid}+ \text{amino acid} \leftrightarrow \text{amino acid}+ \text{alpha-ketoacid}$

An example reaction follows

$\text{Pyruvate}+ \text{Aspartic acid} \leftrightarrow \text{Alanine}+ \text{Oxaloacetate}$

This reaction is catalyzed by an enzyme known as a transaminase. Amino acids, such as glutamate, can also gain nitrogen directly from ammonium ion, as shown below

$\text{Alpha-ketoglutarate} + \text{NH}_4^+ \leftrightarrow \text{Glutamate}$

This reaction can occur, for example, in nitrifying bacteria, and in places where ammonia waste is produced. Many amino acids can be synthesized from citric acid cycle intermediates. For example, synthesis of the non-essential amino acids occurs as follows: aspartic acid can be made by transamination of oxaloacetate. Glutamate comes from transamination of alpha-ketoglutarate. Pyruvate, as noted, is a precursor of alanine (via transamination). Amino acids that can be made from glutamate include glutamine (by addition of an additional ammonium ion), proline, and arginine, Asparagine is made from aspartate by addition of ammonium ion also. Serine is formed from 3-phosphoglycerate and is itself the precursor of both glycine and cysteine. Cysteine and serine are also made from methionine. Tyrosine is made by hydroxylation of phenylalanine.

### Contributors

Dr. Kevin Ahern and Dr. Indira Rajagopal (Oregon State University)