In enzymology, a GDP-L-fucose synthase () is an enzyme that catalyzes the chemical reaction
Thus, the three substrates of this enzyme are GDP-4-dehydro-6-deoxy-D-mannose, NADPH, and H<sup>+</sup>, whereas its two products are GDP-L-fucose and NADP<sup>+</sup>.
This enzyme belongs to the family of oxidoreductases, specifically those acting on the CH-OH group of donor with NAD<sup>+</sup> or NADP<sup>+</sup> as acceptor. The systematic name of this enzyme class is GDP-L-fucose:NADP<sup>+</sup> 4-oxidoreductase (3,5-epimerizing). This enzyme is also called GDP-4-keto-6-deoxy-D-mannose-3,5-epimerase-4-reductase. This enzyme participates in fructose and mannose metabolism.
It has been reported that some cases of multiple sclerosis that present the HLA variant DRB3, present also autoimmunity against GDP-L-fucose synthase. The same report points out that the autoimmune problem could derive from the gut microbiota.