A Brief Guide to the Twenty Common Amino Acids

20 Common Amino Acids v3

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The proteins that make up living organisms are huge molecules, but they’re composed of tinier building blocks, known as amino acids. There are over 500 amino acids found in nature, yet, of these, the human genetic code only directly codes for 20. Every protein in your body is made up of some linked combination of these amino acids – this graphic shows the structure of each, as well as giving a little information on the notation used to represent them.

Broadly, these twenty amino acids can be sorted into two groups: essential and non-essential. Non-essential amino acids are those which the human body is capable of synthesising, whereas essential amino acids must be obtained from the diet. The non-essential amino acids are alanine, arginine, asparagine, aspartate, cysteine, glutamic acid, glutamine, glycine, proline, serine and tyrosine; some of these can also be termed ‘conditionally essential’, meaning that they may be needed from the diet during illness or as a result of health problems. This sub-category includes arginine, glycine, cysteine, tyrosine, proline, and glutamine. The essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.

Amino acids can’t be stored by the body in the same manner as fat and starch, so it’s important that we obtain those that we cannot synthesise from our diet. Failure to do so can lead to inhibition of protein synthesis in the body, which can have a wide range of subsequent health effects. Amino acids are obtained from the breakdown of protein in the diet, so a diet deficient in protein can impact on essential amino acid intake.

Because the proteins formed by amino acids can be incredibly large molecules, it’d be very time consuming and difficult to draw out the chemical structure of them in the same way we do for smaller molecules. For this reason, the common amino acids that make up proteins are given codes that can be used to represent them when they occur in molecules, to make describing the structure of proteins easier. Both three letter and one letter codes exist; the origin of the one letter codes was due to the requirement, back when computers were older and clunkier, to reduce the size of files being used to describe the sequences of amino acids making up proteins. These one letter codes were developed by Dr. Margaret Oakley Dayhoff, considered a pioneer in the field of bioinformatics (using software and information systems to store, organise, and interpret biological data).

Although this chart shows the 20 amino acids the human genetic code directly codes for, there has been some debate over whether or not another amino acid should be classified as the 21st. Selenocysteine is an amino acid which is found in a small number of human proteins; unlike the 20 pictured here, however, it is not coded for directly, but in a special manner. Yet another, pyrrolysine, is coded for in a similar manner, and considered the 22nd amino acid.

(Note: Another manner in which the amino acids can be divided up is based upon their physical properties. You can see a summary of this method of categorising the amino acids here.)


You can also download a version of the graphic which shows DNA codons for each of the amino acids, as well as the structures at physiological (body) pH.


The graphic in this article is licensed under a  Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. See the site’s content usage guidelines.

References & Further Reading

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  14. While this is a beautiful summary of much important information about amino acids, there is an important inacccuracy – the structures of all the amino acids show the N- and C-terminal functional groups in their uncharge states. Based on their pKa values, this cannot be this way – rather all amino acids exist as zwitterions, with the carboxyl group deprotonated (and negative with a pKa less than 7) and the amino group protonated (and positive with a pKa of the protonated amine >7).

    • Hi Amy, thanks for your comment. It’s possible you missed the link to the version of the post with the structures of the amino acids at physiological pH? In the UK education system knowledge of the uncharged structures is also required, which is why the original version shows these.

      Hope that clarifies things for you!

      • Thank you – I had missed that link – I appreciate your comment on the “uncharged structures,” but wonder why one would want to consider those structures as they cannot exist in that format naturally because of their pKa values (unless you consider the gas phase or non-aqueous solvents?). I would love any insight you could provide as to the rationale for learning structures in that format.

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