Amino acid | Definition, Structure, & Facts (2024)

chemical compound

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What is an amino acid?

An amino acid is an organic molecule that is made up of a basic amino group (−NH₂), an acidic carboxyl group (−COOH), and an organic R group (or side chain) that is unique to each amino acid.
The term amino acid is short for α-amino [alpha-amino] carboxylic acid.
Each molecule contains a central carbon (C) atom, called the α-carbon, to which both an amino and a carboxyl group are attached. The remaining two bonds of the α-carbon atom are generally satisfied by a hydrogen (H) atom and the R group.
Amino acids function as the building blocks of proteins. Proteins catalyze the vast majority of chemical reactions that occur in the cell. They provide many of the structural elements of a cell, and they help to bind cells together into tissues.

carboxylic acidLearn more about carboxylic acids.

proteinLearn more about proteins.

What are the 20 amino acid building blocks of proteins?

In the human body, there are 20 amino acids that function as building blocks of proteins.
Nine of these amino acids are considered essential—they must be consumed in the diet—while five are considered nonessential in that they can be made by the human body. The remaining six protein-building amino acids are conditional, being essential only at certain life stages or in certain disease states.
The essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
The nonessential amino acids are alanine, asparagine, aspartic acid, glutamic acid, and serine.
Conditional amino acids include arginine, cysteine, glutamine, glycine, proline, and tyrosine.
Some authorities recognize a 21st amino acid, selenocysteine, which is derived from serine during protein biosynthesis.

Read more below:Building blocks of proteins

What is the difference between standard and nonstandard amino acids?

Amino acids typically are classified as standard or nonstandard, based on the polarity, or distribution of electric charge, of the R group (side chain).
The 20 (or 21) amino acids that function as building blocks of proteins are classified as standard.
Nonstandard amino acids basically are standard amino acids that have been chemically modified after they have been incorporated into a protein (posttranslational modification); they can also include amino acids that occur in living organisms but are not found in proteins. Among the latter is γ-carboxyglutamic acid, a calcium-binding amino acid residue found in the blood-clotting protein prothrombin.
The most important posttranslational modification of amino acids in eukaryotic organisms (including humans) is phosphorylation, in which a phosphate molecule is added to the hydroxyl portion of the R groups of serine, threonine, and tyrosine. Phosphorylation serves a critical role in the regulation of protein function and cell signaling.

What are some industrial uses of amino acids?

In addition to their role as protein building blocks in living organisms, amino acids are used industrially in numerous ways. The first report of the commercial production of an amino acid was in 1908. It was then that the flavouring agent monosodium glutamate (MSG) was prepared from a type of large seaweed. This led to the commercial production of MSG, which is now produced using a bacterial fermentation process with starch and molasses as carbon sources. Glycine, cysteine, and D,L-alanine are also used as food additives, and mixtures of amino acids serve as flavour enhancers in the food industry.

Amino acids are used therapeutically for nutritional and pharmaceutical purposes. For example, treatments with single amino acids are part of the medical approach to control certain disease states. Examples include L-dihydroxyphenylalanine (L-dopa) for Parkinson disease; glutamine and histidine to treat peptic ulcers; and arginine, citrulline, and ornithine to treat liver diseases.

Building blocks of proteins

Proteins are of primary importance to the continuing functioning of life on Earth. Proteins catalyze the vast majority of chemical reactions that occur in the cell. They provide many of the structural elements of a cell, and they help to bind cells together into tissues. Some proteins act as contractile elements to make movement possible. Others are responsible for the transport of vital materials from the outside of the cell (“extracellular”) to its inside (“intracellular”). Proteins, in the form of antibodies, protect animals from disease and, in the form of interferon, mount an intracellular attack against viruses that have eluded destruction by the antibodies and other immune system defenses. Many hormones are proteins. Last but certainly not least, proteins control the activity of genes (“gene expression”).

This plethora of vital tasks is reflected in the incredible spectrum of known proteins that vary markedly in their overall size, shape, and charge. By the end of the 19th century, scientists appreciated that, although there exist many different kinds of proteins in nature, all proteins upon their hydrolysis yield a class of simpler compounds, the building blocks of proteins, called amino acids. The simplest amino acid is called glycine, named for its sweet taste (glyco, “sugar”). It was one of the first amino acids to be identified, having been isolated from the protein gelatin in 1820. In the mid-1950s scientists involved in elucidating the relationship between proteins and genes agreed that 20 amino acids (called standard or common amino acids) were to be considered the essential building blocks of all proteins. The last of these to be discovered, threonine, had been identified in 1935.

Chirality

All the amino acids but glycine are chiral molecules. That is, they exist in two optically active asymmetric forms (called enantiomers) that are the mirror images of each other. (This property is conceptually similar to the spatial relationship of the left hand to the right hand.) One enantiomer is designated d and the other l. It is important to note that the amino acids found in proteins almost always possess only the l-configuration. This reflects the fact that the enzymes responsible for protein synthesis have evolved to utilize only the l-enantiomers. Reflecting this near universality, the prefix l is usually omitted. Some d-amino acids are found in microorganisms, particularly in the cell walls of bacteria and in several of the antibiotics. However, these are not synthesized in the ribosome.

Acid-base properties

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Another important feature of free amino acids is the existence of both a basic and an acidic group at the α-carbon. Compounds such as amino acids that can act as either an acid or a base are called amphoteric. The basic amino group typically has a pKa between 9 and 10, while the acidic α-carboxyl group has a pKa that is usually close to 2 (a very low value for carboxyls). The pKa of a group is the pH value at which the concentration of the protonated group equals that of the unprotonated group. Thus, at physiological pH (about 7–7.4), the free amino acids exist largely as dipolar ions or “zwitterions” (German for “hybrid ions”; a zwitterion carries an equal number of positively and negatively charged groups). Any free amino acid and likewise any protein will, at some specific pH, exist in the form of a zwitterion. That is, all amino acids and all proteins, when subjected to changes in pH, pass through a state at which there is an equal number of positive and negative charges on the molecule. The pH at which this occurs is known as the isoelectric point (or isoelectric pH) and is denoted as pI. When dissolved in water, all amino acids and all proteins are present predominantly in their isoelectric form. Stated another way, there is a pH (the isoelectric point) at which the molecule has a net zero charge (equal number of positive and negative charges), but there is no pH at which the molecule has an absolute zero charge (complete absence of positive and negative charges). That is, amino acids and proteins are always in the form of ions; they always carry charged groups. This fact is vitally important in considering further the biochemistry of amino acids and proteins.