The first amino acid was found in asparagus, and named after it
Nature makes more than 500 amino acids, but life builds its proteins from just 22 of them. Chemists began finding these molecules in 1806, starting with a compound pulled from asparagus juice. The last of the twenty common ones, threonine, was not identified until 1935, well into the age of modern biochemistry.
Asparagine came out of asparagus, extracted by the Paris chemists Louis-Nicolas Vauquelin and Pierre Jean Robiquet. Discoveries trickled in afterwards: cystine in 1810, glycine and leucine in 1820, and cysteine, the single unit that pairs up to make cystine, only in 1884. William Cumming Rose, who found threonine, also worked out which amino acids the body cannot make for itself and how much of each is needed for growth. Charles-Adolphe Wurtz saw in 1865 that these substances formed one chemical family, though English speakers had no name for it until 1898.
The idea of how they fit together came in 1902, when Emil Fischer and Franz Hofmeister each proposed that proteins are long chains of amino acids, the amino group of one bonded to the carboxyl group of the next. Fischer called the resulting chain a peptide. Every protein-building amino acid shares the same core: a central alpha carbon carrying an amino group, a carboxyl group, a hydrogen atom and a distinctive side chain.
That central carbon gives most of them a handedness. Apart from glycine, whose side chain is just hydrogen, each exists in mirror-image forms, and life's proteins use only the left-handed L versions. Right-handed D forms do turn up in bacterial cell walls, some antibiotics and the brain signal D-serine. In water, amino acids carry a positive and a negative charge at once, a so-called zwitterion that is neutral overall.
Side chains decide how a protein folds. Oily, nonpolar ones cluster in the middle of water-soluble proteins, and that clustering is the main force driving folding. Five carry charges at neutral pH and tend to sit on the surface, where opposite charges can form salt bridges. Cysteine links to other cysteines through sulfur bonds, proline kinks the backbone into a ring, and glycine is unusually flexible. After muscles' water, these residues make up the next largest share of our tissues.
Source: Amino acid
