The dense glycocalyx surrounding the protozoan parasite Leishmania is an essential virulence factor. It protects the parasite from hostile environments in the sandfly vector and mammalian host and supports steps of development and invasion. Therefore, new therapeutic concepts concentrate on disturbing glycocalyx biosynthesis. Deletion of genes involved in the metabolism of galactose and mannose have been shown to drastically reduce Leishmania virulence. Here we report the identification of Leishmania major (L. major) UDP-glucose pyrophosphorylase (UGP). UGP catalyses the formation of UDP-glucose from glucose-1-phosphate and UTP. This activation step enables glucose to enter metabolic pathways and is crucial for the activation of galactose. UDP-galactose is made from UDP-glucose by nucleotide-donor transfer to galactose-1-phosphate or by epimerisation of the glucose moiety. Isolated in a complementation cloning approach, the activ-ity of L. major UGP was proven in vitro. Moreover, purified protein was used to inves-tigate enzyme kinetics, quaternary organisa-tion and binding of ligands. While sequestra-tion by oligomerisation is a known regulatory mechanism for eukaryotic UGPs, the recombinant as well as native L. major UGP migrated as monomer in size exclusion chromatography and in accord with this showed simple Michaelis-Menten kinetics towards all substrates. In saturation transfer difference (STD)-NMR studies we clearly demonstrated that the molecular geometry at position 4 of glucose is responsible for substrate specificity. Furthermore, the gamma-phosphate group of UTP is essential for binding and for induction of the open conformation, which then allows entry of glucose-1-phosphate. Our data provide the first direct proof for the ordered bi-bi mechanism suggested in earlier studies.