The post-reactive structures of Leishmania major UDP-sugar pyrophosphorylase provide insights into the product release mechanism

Biosynthesis of the nucleotide sugars UDP-glucose (UDP-Glc) and UDP-galactose (UDP-Gal) is intimately connected and essential for the viability of trypanosomatid parasites. In the genus Leishmania, it is controlled by the UDP-glucose pyrophosphorylase (UGP) and UDP-sugar pyrophosphorylase (USP). In contrast to UGP, USP has a broad substrate specificity and may generate several UDP-sugars in vitro, including UDP-Glc and UDP-Gal. This enzyme, present in protozoan parasites (including Leishmania species and Trypanosoma cruzi) and in plants, most likely plays a role in salvaging monosaccharides. In order to gain a detailed mechanistic understanding of USPs, we determined high-resolution X-ray structures of Leishmania major USP (LmUSP) in post-reactive states. Several positions of the byproduct pyrophosphate (PPi) were identified and revealed a product release channel in the forward reaction, as well as the geometries of post-reactive Michaelis product complexes. The conformational changes of functional loops (hinge loop-1, hinge loop-2, and the nucleotide-binding loop) showed dynamic effects accompanying the product release process. Structural information about the post-reactive states of LmUSP also includes the metastable binding position of a magnesium (Mg2+) ion in the active site. The proposed product release mechanism was substantiated by molecular dynamics simulations and can serve as a model for other UDP-sugar pyrophosphorylases.