All proteinogenic amino acids share the same backbone: a central carbon (the alpha carbon) bonded to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen and a variable side chain, called the R group. The only thing that separates leucine from lysine or aspartate is that side chain.
Based on the chemistry of their side chain they are grouped into hydrophobic (alanine, valine, leucine, isoleucine, phenylalanine), polar uncharged (serine, threonine, asparagine, glutamine), acidic (aspartate, glutamate), basic (lysine, arginine, histidine) and special cases such as glycine, proline or cysteine, which can form disulfide bridges.
In sequences they are written with a three-letter code (Gly-His-Lys) or a one-letter code (GHK). Except for glycine, all of them exist in two mirror forms, L and D; the body builds its proteins almost exclusively from the L form.
Amino acid composition explains why a peptide oxidises (methionine, cysteine), deamidates (asparagine) or is poorly soluble near a certain pH. Reading the sequence lets you anticipate how to handle it.
Related terms
Learn it in depth at the Peptide University
- From amino acid to peptide · Module 1
- Classification and solid-phase synthesis · Module 1
- Chemical modifications: from the natural sequence to the analog · Module 1
- Related impurities and counterions · Module 7
- Neuroimmunomodulation: VIP, α-MSH, KPV and ARA-290 · Module 12
Definition for educational and scientific purposes. It is not medical advice or a recommendation for use. NeoPeptidos products are sold labelled for research use only (RUO).