Serine hydroxymethyltransferase

Serine hydroxymethyltransferase (SHMT) is an enzyme which plays an important role in cellular one-carbon pathways by catalyzing the reversible, simultaneous conversions of L -serine to glycine (retro-aldol cleavage) and 5,6,7,8-tetrahydrofolate to 5,10-methylenetetrahydrofolate (hydrolysis). This reaction provides the largest part of the one-carbon units available to the cell.

Isoforms
Bacteria such as Escherichia coli and Bacillus stearothermophilus have versions of this enzyme and there appear to be two isoforms of SHMT in mammals, one in the cytoplasm (cSHMT) and another in the mitochondria (mSHMT). Plants may have an additional SHMT isoform within chloroplasts

Other reactions
As well as its primary role in folate metabolism, SHMT also catabolyzes other reactions that may be biologically significant, including the conversion of 5,10-methenyltetrahydrofolate to 10-formyltetrahydrofolate. When coupled with C1-tetrahydrofolate synthase and tetrahydropteroate, cSHMT also catalyzes the conversion of formate to serine.

Role in Smith-Magenis syndrome
Smith-Magenis syndrome (SMS) is a rare disorder that manifests as a complex set of traits including facial abnormalities, unusual behaviors, and developmental delay. It results from an interstital deletion within chromosome 17p11.2, including the cSHMT gene and a small study showed SHMT activity in SMS patients was ~50% of normal. Reduced SHMT would result in less glycine which could affect the nervous system by acting as an agonist to the NMDA receptor and this could be a mechanism behind SMS.