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Introduction
Essential amino acids, also known as indispensable amino acids, are amino acids that humans and other vertebrates cannot synthesize from metabolic intermediates. These amino acids must be supplied from an exogenous diet because the human body lacks the metabolic pathways required to synthesize these amino acids.<1><2> In nutrition, amino acids are classified as either essential or non-essential. These classifications resulted from early studies on human nutrition, which showed that specific amino acids were required for growth or nitrogen balance even when there is an adequate amount of alternative amino acids.<3> Although variations are possible depending on the metabolic state of an individual, the general held thought is that there are nine essential amino acids, including phenylalanine, valine, tryptophan, threonine, isoleucine, methionine, histidine, leucine, and lysine. The mnemonic PVT TIM HaLL ("private Tim Hall") is a commonly used device to remember these amino acids as it includes the first letter of all the essential amino acids. In terms of nutrition, the nine essential amino acids are obtainable by a single complete protein. A complete protein, by definition, contains all the essential amino acids. Complete proteins usually derive from animal-based sources of nutrition, except for soy.<4><5> The essential amino acids are also available from incomplete proteins, which are usually plant-based foods. The term "limiting amino acid" is used to describe the essential amino acid present in the lowest quantity in a food protein relative to a reference food protein like egg whites. The term "limiting amino acid" may also refer to an essential amino acid that does not meet the minimal requirements for humans.<6>
Fundamentals
Amino acids are the basic building blocks of proteins, and they serve as the nitrogenous backbones for compounds like neurotransmitters and hormones. In chemistry, an amino acid is an organic compound that contains both an amino (-NH2) and carboxylic acid (-COOH) functional group, hence the name amino acid. Proteins are long chains or polymers of a specific type of amino acid known as an alpha-amino acid. Alpha-amino acids are unique because the amino and carboxylic acid functional groups are separated by only one carbon atom, which is usually a chiral carbon. In this article, we will solely focus on the alpha-amino acids that make up proteins.
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Proteins are chains of amino acids that assemble via amide bonds known as peptide linkages. The difference in the side-chain group or R-group is what determines the unique properties of each amino acid. The uniqueness of different proteins is then determined by which amino acids it contains, how these amino acids are arranged in a chain, and further complex interactions the chain makes with itself and the environment. These polymers of amino acids are capable of producing the diversity seen in life.
There are approximately 20,000 unique protein encoding genes responsible for more than 100,000 unique proteins in the human body. Although there are hundreds of amino acids found in nature, only about 20 amino acids are needed to make all the proteins found in the human body and most other forms of life. These 20 amino acids are all L-isomer, alpha-amino acids. All of them, except for glycine, contain a chiral alpha carbon. And all these amino acids are L-isomers with an R-absolute configuration except for glycine (no chiral center) and cysteine (S-absolute configuration, because of the sulfur-containing R-group). It bears mentioning that the amino acids selenocysteine and pyrrolysine are considered the 21st and 22nd amino acids, respectively. They are more recently discovered amino acids that may become incorporated into protein chains during ribosomal protein synthesis. Pyrroloysine has functionality in life; however, humans do not use pyrrolysine in protein synthesis. Once translated, these 22 amino acids may also be modified via a post-translational modification to add further diversity in generating proteins.<8>
The 20 to 22 amino acids that comprise proteins include: