Glycine is the lightest of the 20 natural amino acids. It's [side chain]? is a hydrogen atom. Because there is a second hydrogen atom at α carbon, glycine is not optically active.
Because it has so small side chain, in can fit into many places where no other amino acid can. For example only glycine can be internal amino acid of [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.
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