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There are many different types of amino acids, which act as the basic structural units of proteins and serve many other important biological functions. They are grouped into several categories based on several different factors, such as the mechanisms through which they are synthesized and whether or not they can be acquired through one's diet. All amino acids contain an amine group, or a compound that has a nitrogen with a lone pair of electrons. They also have a carboxylic acid group, which are acids involving a compound attached to a carbon atom, an oxygen atom, and a hydroxide molecule. Different types are distinguished by a side chain that is unique to each amino acid.
Different types of amino acids are commonly grouped into standard and nonstandard amino acids. Standard amino acids are produced through the normal biological processes through which genetic information contained in DNA is transcribed to RNA and translated into amino acids. There are 20 amino acids that are produced through such standard biological processes while another two standard amino acids are produced through more specialized synthesis processes, bringing the total number of standard amino acids to 22.
Nonstandard amino acids, on the other hand, are not produced through the normal means of transcription and translation. They are typically modified versions of standard amino acids; they undergo some form of post-translational modification that takes place after the normal phases of protein production. Proteins that have highly specific functions or that need to bind to specific molecules often contain nonstandard amino acids, as the 22 standard amino acids are not specialized enough.
Different types of amino acids are also grouped based on the dietary needs of various animals, usually humans. Essential amino acids, such as phenylalanine, cannot be synthesized by the body so they must be consumed as part of one's diet to maintain health. Nonessential amino acids, such as alanine, can be synthesized by the body so they do not necessarily need to be included in one's diet. The terms "essential" and "non-essential" do not refer to their importance; both types are necessary. It merely refers to the fact that one is an essential part of a person's diet while the other is not.
Amino acids can be divided into categories based on many different traits. One trait commonly used in biochemistry is amino acid structure. Different amino acids are grouped together based on certain atoms or molecules or structural arrangements that they contain. Two different structural categories are aromatic amino acids and sulfur containing amino acids.