In enzymology, a GDP-mannose 3,5-epimerase (EC 5.1.3.18) is an enzyme that catalyzes the chemical reaction GDP-mannose GDP-L-galactose + GDP-L-gulose - initial reaction overview (updated in 2020, see below) Hence, this enzyme has one substrate, GDP-mannose, and two products, and GDP-D-mannose GDP-L-galactose (C3,5-epimer) + GDP-L-gulose (C5-epimer) + GDP-D-altrose (C3-epimer) - reaction overview update in 2020