Glycine is the lightest of the 20 natural amino acids; its side chain[?] is a hydrogen atom. Because there is a second hydrogen atom at the α carbon, glycine is not optically active.
Because glycine has such a small side chain, it can fit into many places where no other amino acid can. For example, only glycine can be the internal amino acid of a collagen helix.
Glycine isn't part of any enzyme's active center[?], but it is very evolutionarily stable at certain positions of some enzymes (for example, in cytochrome c, myoglobin, and hemoglobin), because mutations that change it to an amino acid with a larger side chain could break the enzyme's structure.
Most proteins contain only small quantities of glycine. A notable exception is collagen, which is about one-third glycine.