Exploring the mechanism of binding of UDP-galactopyranose to UDP-galactopyranose mutase by STD-NMR spectroscopy and molecular modeling.

UDP-galactopyranose mutase (UGM) is the key enzyme involved in the biosynthesis of Galf. In this study, reliable structural binding modes of the natural substrate, UDP-Galp, and inhibitor, UDP, in the UGM active site were provided with the combined use of STD-NMR spectroscopy, molecular modeling, and CORCEMA-ST calculations. UDP-Galp and UDP exhibited similar binding epitopes recognized by UGM. However, the relative binding affinities of the ligands changed dramatically upon reduction of UGM, as explored by competitive STD-NMR experiments. UDP-Galp competes with UDP for binding to UGM, especially when UGM is in its reduced state. Docking studies for predicting the binding mode within the active site of the two monomers in UGM explored the possibility that the mobile loop might act as a gateway for substrate binding, and the structure of the binding cleft in monomer A might be a closer approximation of the substrate-bound active site than monomer B. Important information regarding the critical interactions of UGM with UDP-Galp has been obtained.