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convention for amino acid configuration refers not to the optical activity of the amino acid itself but rather to the optical activity of the isomer of glyceraldehyde from which that amino acid can, in theory, be synthesized (-glyceraldehyde is levorotatory).

In alternative fashion, the (S) and (R) designators are used to indicate the absolute stereochemistry.

The other two ("non-standard" or "non-canonical") are selenocysteine (present in many prokaryotes as well as most eukaryotes, but not coded directly by DNA), and pyrrolysine (found only in some archea and one bacterium).

In the form of proteins, amino acid residues form the second-largest component (water is the largest) of human muscles and other tissues.

Beyond their role as residues in proteins, amino acids participate in a number of processes such as neurotransmitter transport and biosynthesis.

Almost all of the amino acids in proteins are (S) at the α carbon, with cysteine being (R) and glycine non-chiral.

Cysteine has its side chain in the same geometric position as the other amino acids, but the R/S terminology is reversed because of the higher atomic number of sulfur compared to the carboxyl oxygen gives the side chain a higher priority, whereas the atoms in most other side chains give them lower priority.

About 500 naturally occurring amino acids are known (though only 20 appear in the genetic code) and can be classified in many ways.