Interconversion of serine and glycine. (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + glycine + H2O = (6S)-5,6,7,8-tetrahydrofolate + L-serine One-carbon metabolism; tetrahydrofolate interconversion. Homotetramer. In eukaryotes there are two forms of the enzymes: a cytosolic one and a mitochondrial one. Has antigenic properties. Belongs to the SHMT family.