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Which amino acid has this one-letter code?

Amino acids reference chart

All 20 standard proteinogenic amino acids, each with its three-letter code, one-letter code, and side-chain class. Filter by side-chain class to narrow the list.

Showing 20 of 20 amino acids

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The 20 standard amino acids

Amino acids are the building blocks of proteins. This page lists all 20 standard proteinogenic amino acids — from Glycine to Glutamate — that the genetic code specifies and cells string together to build every protein. Each entry shows the amino acid's full name, its three-letter code (Gly, Ala, His), its one-letter code (G, A, H), and its side-chain class. The non-standard residues selenocysteine and pyrrolysine are intentionally left out so the chart matches the classic 20-amino-acid reference taught in biochemistry courses.

Three-letter and one-letter codes

Every amino acid has two standard abbreviations. The three-letter code (such as Ser, Thr, Cys) is common in textbooks and structural biology, while the one-letter code (S, T, C) packs sequences into compact strings for bioinformatics and database work. The side-chain class — nonpolar/aliphatic, aromatic, polar uncharged, basic (positively charged), or acidic (negatively charged) — groups amino acids by the chemistry of their R group, which drives how each residue behaves inside a folded protein. Use the class filter above the chart to see just one group at a time.

How the quiz works

Each round shows one one-letter code and four possible amino acid names. Pick the name you think matches; the quiz marks it right or wrong, reveals the correct answer with its three-letter code and side-chain class, and keeps a running score, current streak, and best streak for the session. Press Next amino acid for a new card or Reset score to start over. Every code and answer choice is selected with secure browser randomness, entirely in your browser. For a single random amino acid instead of a quiz, try the Random Amino Acid generator.