Serine hydroxymethyltransferase (SHMT) is a pyridoxal phosphate (PLP) dependent enzyme and belongs to the aspartate aminotransferase superfamily (fold type I) [PMID:10828359]. The pyridoxal-P group is attached to a lysine residue around which the sequence is highly conserved in all forms of the enzyme [PMID:8305478]. The enzyme carries out interconversion of serine and glycine using PLP as the cofactor. SHMT catalyses the transfer of a hydroxymethyl group from N5, N10- methylene tetrahydrofolate to glycine, resulting in the formation of serine and tetrahydrofolate. Both eukaryotic and prokaryotic SHMT enzymes form tight obligate homodimers and the mammalian enzyme forms a homotetramer [PMID:10828359, PMID:11877399]. PLP dependent enzymes were previously classified into alpha, beta and gamma classes, based on the chemical characteristics (carbon atom involved) of the reaction they catalysed. The availability of several structures allowed a comprehensive analysis of the evolutionary classification of PLP dependent enzymes, and it was found that the functional classification did not always agree with the evolutionary history of these enzymes. Structure and sequence analysis has revealed that the PLP dependent enzymes can be classified into four major groups of different evolutionary origin: aspartate aminotransferase superfamily (fold type I), tryptophan synthase beta superfamily (fold type II), alanine racemase superfamily (fold type III), D-amino acid superfamily (fold type IV) and glycogen phophorylase family (fold type V) [PMID:8112347, PMID:7748903].

In vertebrates, glycine hydroxymethyltransferase exists in a cytoplasmic and a mitochondrial form whereas only one form is found in prokaryotes.