The 20 Standard Amino Acids
Amino acids are in our food and dietary supplements, but they are not important for humans only. They supply the components necessary to create proteins, which are essential for all life. Think of it this way: if proteins are the building, amino acids are the bricks. So, if every living thing requires proteins and, therefore, amino acids, then it is crucial to understand these “bricks” that are necessary to grow your model research organism.
This article provides an overview of the 20 standard amino acids and explains what amino acids are, what they do in organisms, the different categories of amino acids, and how you might use them in culture media. Also, you will find a reference table of the 20 standard amino acids.
What are Amino Acids?
All living organisms have amino acids in their cells. Each cell can contain up to a billion amino acids (BNID 100089). Amino acids are the individual units that together build proteins, which are large molecules that perform many different activities in living cells—more on that below. Without them, we would not survive. When looking at the dry weight of an E. coli bacterial cell, proteins make up over 50% of the cell (BNID 104954). Thymosin β, one of the smallest identified human protein, contains only 44 amino acids and has a wide range of activities in cell proliferation, migration, and differentiation, while one of the largest, called Titin (BNID 101653), contains 33,423 amino acids and is part of the assembly and function of muscles.
All amino acids contain an amino group (―NH2), a carboxyl group (―COOH), and an organic R group (or side chain) (Figure 1). Each amino acid has a different side chain that determines the chemical nature of the amino acid (e.g., polar vs. non-polar, positively vs. negatively charged, aromatic vs. non-aromatic). Amino acids are linked together to form peptide chains through a reaction between the amino group of one amino acid and the carboxyl group of another, resulting in the release of water.
How Many Amino Acids are There?
Figure 1. General structure of an amino acid.
How are Protein Sequences Defined?
The protein coding regions of DNA are transcribed to RNA, which are then translated to proteins. Amino acids are encoded by 3 adjacent nucleotides, called codons, specified in the messenger RNA. For example, the nucleotide triplets CAU and CAC code for the amino acid histidine (Table 1; download PDF). Depending on the organism, certain codons are preferred due to a phenomenon called codon usage bias. So, one organism may use CAU for histidine significantly more often than they use CAC, and another organism may do the opposite.
Table 1. The 20 standard amino acids, abbreviations, and associated nucleotide codons. Download PDF.
|Amino Acid||Abbreviation||Single-Letter Abbreviation||Codons*|
|Alanine||Ala||A||GCU, GCC, GCA, GCG|
|Arginine||Arg||R||CGU, CGC, CGA, CGG, AGA, AGG|
|Aspartic acid||Asp||D||GAU, GAC|
|Glutamic Acid||Glu||E||GAA, GAG|
|Glycine||Gly||G||GGU, GGC, GGA, GGG|
|Isoleucine||Ile||I||AUU, AUC, AUA|
|Leucine||Leu||L||UUA, UUG, CUU, CUC, CUA, CUG|
|Proline||Pro||P||CCU, CCC, CCA, CCG|
|Serine||Ser||S||UCU, UCC, UCA, UCG, AGU, AGC|
|Threonine||Thr||T||ACU, ACC, ACA, ACG|
|Valine||Val||V||GUU, GUC, GUA, GUG|
|Stop||—||—||UAG, UGA, UAA|
* A = Adenine, C = Cytosine, G =Guanine, U = Uracil
† There are also codons that indicate the start and stop of protein translation. The ones shown are the most common codons.
What Do Amino Acids Do Besides Create Proteins?
Not only do amino acids help build proteins, but they are also involved in other processes in organisms. They play roles as metabolites and nucleic acid, hormone, and heme precursors, regulate gene expression and cell signaling, and function as neurotransmitters (messengers for nerves).
Categories of Amino Acids
Amino acids are also classified as essential, non-essential, or conditionally essential. Essential amino acids must be obtained from an outside source because organisms cannot produce them on their own. Nine amino acids are essential in humans: phenylalanine, valine, tryptophan, threonine, isoleucine, methionine, histidine, leucine, and lysine. When organisms are capable of producing a certain amino acid, those amino acids are considered non-essential. Some amino acids, however, can become limited and are sometimes considered essential—those amino acids are categorized as conditionally essential.
Amino Acids in Culture Media
Because amino acids help organisms grow and some are essential for different organisms, culture media often contain amino acid or protein fragment mixtures such as peptones, casamino acids, or yeast extracts (see orange box for amino acid profiles). Alternatively, defined quantities of amino acids can be added if you are creating a chemically-defined medium. When specific amino acids are omitted, such as tryptophan, leucine, or histidine, for yeast growth, cloning, or metabolism experiments, you can add these amino acid solutions back individually to your growth media when necessary.
Using Amino Acids in Your Research
Amino acids provide a crucial resource for all living organisms. They are the components that make up proteins, which are necessary for organisms to function. In different organisms, amino acids may be essential, non-essential, or conditionally essential. So, obtaining essential and conditionally essential amino acids from an outside source is necessary. Due to these requirements, you may need to supply amino acids to your organism of study through culture media.