Amino acids
Amino acids are organic compounds that play a crucial role in the biochemistry of life. Each amino acid consists of an amine group, a carboxylic acid group, and a unique side chain, which distinguishes one amino acid from another. These compounds are the building blocks of proteins, which are essential macromolecules necessary for various biological functions. While there are hundreds of known amino acids, only twenty-two are classified as proteinogenic, meaning they are coded for in the genetic material of living organisms. The majority of these proteinogenic amino acids are encoded by specific sequences in DNA known as codons, which are crucial for protein synthesis.
The side chains of amino acids can influence their properties, including whether they are hydrophilic or hydrophobic, and can affect the overall function of the proteins they compose. Additionally, two unusual amino acids, selenocysteine and pyrrolysine, are included among the proteinogenic amino acids due to their unique coding mechanisms. Understanding amino acids and their role in protein structure and function is fundamental to comprehending biological processes and the diversity of life.
On this Page
Subject Terms
Amino acids
An amino acid is an organic compound that contains both an amine group and a carboxylic acid group. Chains of amino acids form proteins, which are necessary for the existence of life. However, of the hundreds of different amino acids known to exist, only twenty are genetically coded for in living organisms, with an additional two that occur naturally but are produced by biosynthetic means. These twenty-two amino acids are known as “proteinogenic amino acids,” while the rest are categorized as “non-proteinogenic.”
Overview
Each amino acid is made up of an amine functional group (NH2), a carboxylic acid functional group (COOH, sometimes written as CO2H), and a unique side chain, also called an “R group” and often represented in diagrams as —R. These three groups plus a single hydrogen atom all surround a central carbon atom.
The side chains are what differentiate amino acids from one another and determine the characteristics of each one. For example, the pH of the side chain determines whether the amino acid is acidic, basic, or neutral. Similarly, an amino acid with a polar side chain will be hydrophilic (attracted to water molecules), while one with a nonpolar side chain will be hydrophobic (repelled by water molecules). The side chain of the simplest amino acid, glycine, is a single hydrogen atom, while those of other amino acids may be far more complex.
Over 140 different amino acids have been found in natural proteins, and hundreds more exist in other forms. However, only twenty-two amino acids are proteinogenic, meaning they are genetically coded for in living organisms as the basic building blocks for protein synthesis. All other amino acids are considered non-proteinogenic. While many non-proteinogenic amino acids do appear in proteins, they are created by chemical interactions that take place after the protein has been synthesized.
The instructions for protein synthesis are encoded in the DNA of all organisms. The genes that make up DNA are composed of organic molecules called “nucleotides,” which combine in groups of three to form codons. Generally, each codon represents a particular amino acid. Thus, each sequence of codons represents a specific protein to be synthesized, as proteins are simply chains of amino acids. Certain codons, known as “stop codons” or “termination codons,” signal the end of the particular protein being synthesized. The codons in DNA are mirrored by those in messenger RNA (mRNA), and mRNA in turn directs the process of protein synthesis.
Of the twenty-two proteinogenic amino acids, twenty are represented by specific codons, while the remaining two, selenocysteine and pyrrolysine, are special cases. Selenocysteine occurs naturally in all three domains of life—eukaryotes, bacteria, and archaea—though not in every organism, while pyrrolysine is only found in a small number of archaea and bacteria. Both are coded for by codons that typically function as termination codons, selenocysteine by the codon UGA and pyrrolysine by UAG. However, the mechanisms by which these codons are reinterpreted differ significantly from one another.
Bibliography
Ambrogelly, Alexandre, Sotiria Palioura, and Dieter Söll. “Natural Expansion of the Genetic Code.” Nature Chemical Biology 3.1 (2007): 29–35. Print.
“Amino Acids, Peptides, and Proteins.” Handbook of Biochemistry and Molecular Biology. 4th ed. Ed. Roger L. Lundblad and Fiona M. MacDonald. Boca Raton: CRC, 2010. 3–159. Print.
Clark, Jim. “Introducing Amino Acids.” Chemguide. Clark, 2004. Web. 1 Oct. 2013.
Hughes, Andrew B., ed. Amino Acids, Peptides and Proteins in Organic Chemistry. 5 vols. Weinheim: Wiley, 2009–12. Print.
Jabukowski, Henry. “Structure and Properties of Amino Acids.” UC Davis BioWiki. U of California, Davis, n.d. Web. 1 Oct. 2013.
Koolman, Jan, and Klaus-Heinrich Roehm. Color Atlas of Biochemistry. 3rd ed. Stuttgart: Verlag, 2013. Print.
Ling, Zhe-Nan. "Amino Acid Metabolism in Health and Disease." Signal Transduction and Targeted Therapy, vol. 8, no. 345, 13 Sept. 2023, doi.org/10.1038/s41392-023-01569-3. Accessed 12 Feb. 2025.
Meierhenrich, Uwe. Amino Acids and the Asymmetry of Life: Caught in the Act of Formation. Berlin: Springer, 2008. Print.
Ragsdale, Stephen W. “Biochemistry: How Two Amino Acids Become One.” Nature 471.7340 (2011): 583–84. Print.
Reddy, Michael K. "Amino Acids." Britannica, 2 Feb. 2025, www.britannica.com/science/amino-acid. Accessed 12 Feb. 2025.
Tropp, Burton E. Molecular Biology: Genes to Proteins. 4th ed. Sudbury: Jones, 2012. Print.
Wu, Guoyao. Amino Acids: Biochemistry and Nutrition. Boca Raton: CRC, 2013. Print.