Nutrition Guide

Amino acids are the basic structural building units of proteins. They form short polymer chains called peptides or longer chains called either polypeptides or proteins. These polymers are linear and unbranched. The process of making proteins is called translation and involves the step-by-step addition of amino acids to a growing protein chain by a ribozyme that is called a ribosome.[13] The order in which the amino acids are added is read through the genetic code from an mRNA template, which is a RNA copy of one of the organism’s genes. Twenty amino acids are encoded by the standard genetic code and are called proteinogenic or standard amino acids.[3]

As amino acids have both an amine and a carboxylic acid functional group and are therefore both acid and base at the same time.[3] At a certain compound-specific pH known as the isoelectric point, the number of protonated ammonium groups with a positive charge and deprotonated carboxylate groups with a negative charge are equal, resulting in a net neutral charge[10] These ions are known as a zwitterion, which comes from the German word Zwitter meaning “hybrid”.[11] Amino acids are zwitterions in solid phase and in polar solutions such as water and depending on the pH, but not in the gas phase.[12] Zwitterions have minimal solubility at their isolectric point and amino acids are often isolated by precipitation from water after adjusting the pH to their isolectric point.

In the structure shown at the top of the page, R represents a side chain specific to each amino acid. The carbon atom next to the carbonyl group is called the ?–carbon and amino acids with a side chain bonded to this carbon are referred to as alpha amino acids. These are the most common form found in nature. In the alpha amino acids, the ?–carbon is a chiral carbon atom (with the exception of glycine).[3] In amino acids that have a carbon chain attached to the ?–carbon, as in lysine on the right, the carbons are labeled in order as ?, ?, ?, ?, and so on.[4] In some amino acids, the amine group is attached to the ? or ?-carbon, and these are therefore referred to as beta or gamma amino acids.

Amino acids are usually classified by the properties of their side chain into four groups. The side chain can make them behave like a weak acid, a weak base, a hydrophile if they are polar, and hydrophobe if they are nonpolar.[3] The chemical structures of the 20 standard amino acids, along with their chemical properties, are catalogued in the list of standard amino acids.

The phrase “branched-chain amino acids” or BCAA is sometimes used to refer to the amino acids having aliphatic side chains that are non-linear; these are leucine, isoleucine, and valine. Proline is the only proteinogenic amino acid whose side group links to the ?-amino group and, thus, is also the only proteinogenic amino acid containing a secondary amine at this position.[3] Proline has sometimes been termed an imino acid, but this is not correct in the current nomenclature.[5]

Alpha-amino acids are the building blocks of proteins. Amino acids combine in a condensation reaction, that is, through dehydration synthesis, that releases water and the new “amino acid residue” that is held together by a peptide bond. Proteins are defined by their unique sequence of amino acid residues; this sequence is the primary structure of the protein. Just as the letters of the alphabet can be combined to form an almost endless variety of words, amino acids can be linked in varying sequences to form a vast variety of proteins.[2]

Twenty standard amino acids are used by cells in protein biosynthesis, and these are specified by the general genetic code.[2] These 20 amino acids are biosynthesized from other molecules, but organisms differ in which ones they can synthesize and which ones must be provided in their diet. The ones that cannot be synthesized by an organism are called essential amino acids.

This article is about the class of chemicals. For the structures and properties of the standard proteinogenic amino acids, see List of standard amino acids.

The general structure of an ?-amino acid, with the amino group on the left and the carboxyl group on the right

In chemistry, an amino acid is a molecule containing both amine and carboxyl functional groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent.[1] In the alpha amino acids, the amino and carboxylate groups are attached to the same carbon, which is called the ?–carbon. The various alpha amino acids differ in which side chain (R group) is attached to their alpha carbon. They can vary in size from just a hydrogen atom in glycine through a methyl group in alanine to a large heterocyclic group in tryptophan.

Amino acids are critical to life, and have a variety of roles in metabolism. One particularly important function is as the building blocks of proteins, which are linear chains of amino acids. Amino acids are also important in many other biological molecules, such as forming parts of coenzymes, as in S-adenosylmethionine, or as precursors for the biosynthesis of molecules such as heme. Due to this central role in biochemistry, amino acids are very important in nutrition.

The amino acids are commonly used in food technology and industry. For example, monosodium glutamate is a common flavor enhancer that gives foods the taste called umami. Beyond the amino acids that are found in all forms of life, amino acids are also used in industry, with the production of biodegradable plastics, drugs and chiral catalysts being particularly important applications.