Enzymatic synthesis using glycosyltransferases is a powerful approach to building polysaccharides with high efficiency and selectivity. Sugar nucleotides are fundamental donor molecules in enzymatic glycosylation reactions by Leloir-type glycosyltransferases. The applications of these donors are restricted by their limited availability. In this protocol, N-acetylglucosamine (GlcNAc)/N-acetylgalactosamine (GalNAc) are phosphorylated by N-acetylhexosamine 1-kinase (NahK) and subsequently pyrophosphorylated by N-acetylglucosamine uridyltransferase (GlmU) to give UDP-GlcNAc/GalNAc. Other UDP-GlcNAc/GalNAc analogues can also be prepared depending on the tolerance of these enzymes to the modified sugar substrates. Starting from L-fucose, GDP-fucose is constructed by one bifunctional enzyme L-fucose pyrophosphorylase (FKP) via two reactions.