Chemical Formula: NH2CH2COOH
Isoelectric point: 6.06
Because it has such a small side chain, in can fit into many places where no other amino acid can. For example, only glycine can be the internal amino acid of of a collagen helix.
Glycine isn't part of any enzyme's active center, but it is very evolutionary stable at certain positions of some enzymes (for example cytochrome c, myoglobin and hemoglobin) because mutations that change it to amino acid with bigger side chain could break enzyme's structure.
Most proteins contain only small quantities of glycine. Notable exception is collagen, which in about third part consists of glycine.