Since only GDP-L-gulose (the C5-epimer of GDP-D-mannose) was found in the reaction mixture, it was postulated that the enzyme performs the C5-epimerization prior to the C3-epimerization. However, GDP-D-altrose was recently found as a reaction product, which means that both reaction routes can occur: C5-prior-to-C3 and C3-prior-to-C5.
This also means that the GDP-mannose 3,5-epimerase has three reaction products, namely the main product GDP-L-galactose (C3,5-epimer) and two sideproducts GDP-L-gulose (C5-epimer) + GDP-D-altrose (C3-epimer).
This enzyme belongs to the family of isomerases, specifically those racemases and epimerases acting on carbohydrates and derivatives. The systematic name of this enzyme class is GDP-mannose 3,5-epimerase. Other names in common use include GDP-D-mannose:GDP-L-galactose epimerase, guanosine 5'-diphosphate D-mannose:guanosine 5'-diphosphate, GM35E,[1] and L-galactose epimerase. This enzyme participates in ascorbate and aldarate metabolism.
Structural studies
As of late 2007, 4 structures have been solved for this class of enzymes, with PDB accession codes 2C54, 2C59, 2C5A, and 2C5E.
Hebda PA, Behrman EJ, Barber GA (1979). "The guanosine 5'-diphosphate D-mannose: guanosine 5'-diphosphate L-galactose epimerase of Chlorella pyrenoidosa. Chemical synthesis of guanosine 5'-diphosphate L-galactose and further studies of the enzyme and the reaction it catalyzes". Arch. Biochem. Biophys. 194 (2): 496–502. doi:10.1016/0003-9861(79)90644-1. PMID443816.
