Every peptide and protein is built from the same set of 20 standard amino acids. Their order determines what a peptide does. Here’s a quick reference to the building blocks.
- 20 standard amino acids make up nearly all peptides and proteins.
- Each has a three-letter and one-letter code.
- They differ by their side chain, which gives each one its character.
The building blocks
All 20 share the same backbone — an amine group, a central carbon, and an acid group — and differ only in their side chain. That side chain decides whether an amino acid is water-loving, water-fearing, charged, or neutral.
| Group | Examples (1-letter) |
|---|---|
| Nonpolar / hydrophobic | Ala (A), Val (V), Leu (L), Ile (I), Phe (F) |
| Polar / uncharged | Ser (S), Thr (T), Asn (N), Gln (Q) |
| Positively charged | Lys (K), Arg (R), His (H) |
| Negatively charged | Asp (D), Glu (E) |
| Special cases | Gly (G), Pro (P), Cys (C) |
Why the order matters
The same 20 letters can spell countless “words.” A peptide’s specific sequence — the order of its amino acids — gives it a unique shape and role. That’s why two peptides of the same length can behave completely differently.
Learn how sequences are written in Peptide Sequences & One-Letter Codes, or start with Peptides vs Amino Acids.
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