Amino Acids
Proteins are the essential agents of biological function, and amino acids are the building blocks of proteins. The diversity of the thousands of proteins found in nature arises from the commonly occurring 20 amino acids. Proteins are polymers of amino acids, with each amino acid residue joined to its neighbor by a specific type of covalent bond. Proteins can be broken down (hydrolyzed) to their constituent amino acids the free amino acids derived from them. Of the over 300 naturally occurring amino acids, 20 constitute the monomer units of proteins. All 20 amino acids (Table 4.1) are biologically essential. Humans can synthesize 12 (nutritionally nonessential) of the 20 common amino acids from the amphibolic intermediates of glycolysis and of the citric acid cycle. Of the 12 nutritionally nonessential amino acids, nine are formed from amphibolicintermediates and three (cysteine, tyrosine and hydroxylysine) from nutritionallyessential amino acids.
List of essential and nonessential amino acids
Essential amino acids are “essential” not because they are more important to life than the others, but because the body does not synthesize them. They must be present in the diet or they will not be present in the body. In addition, the amino acids arginine, cysteine, glycine, glutamine, histidine, proline, serine and tyrosine are considered conditionally essential, meaning they are not normally required in the diet, but must be supplied exogenously to specific populations that do not synthesize them in adequate amounts.
Selenocysteine, while not normally considered an amino acid present in proteins, selenocysteine occurs at the active sites of several enzymes. Examples include the human enzymes thioredoxin reductase, glutathione peroxidase, and the deiodinase that converts thyroxine to triiodothyronine. Pyrrolysine sometimes considered “the 22nd amino acid”, is not listed here as it is not used by humans.
