Characterization of ligand binding to the bifunctional key enzyme in the sialic acid biosynthesis by NMR: II. Investigation of the ManNAc kinase functionality

N-Acetylmannosamine (ManNAc) is the physiological precursors to all sialic acids that occur in nature. As variations in the sialic acid decoration of cell surfaces can profoundly affect cell-cell, pathogen-cell, or drug-cell interactions, the enzymes that convert ManNAc into sialic acid are attractive targets for the development of drugs that specifically interrupt sialic acid biosynthesis or lead to modified sialic acids on the surface of cells. The first step in the enzymatic conversion of ManNAc into sialic acid is phosphorylation, yielding N-acetyl-mannosamine-6-phosphate. The enzyme that catalyzes this conversion is the N-acetylmannosamine kinase (ManNAc kinase) as part of the bifunctional enzyme UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase. Here, we employed saturation transfer difference (STD) NMR experiments to study the binding of ManNAc and related ligands to the ManNAc kinase. It is shown that the configuration of C1 and C4 of ManNAc is crucial for binding to the enzyme, whereas the C2 position not only accepts variations in the attached N-acyl side chain but also tolerates inversion of configuration. Our experiments also show that ManNAc kinase maintains its functionality, even in the absence of Mg²⁺. From the analysis of the STD NMR-derived binding epitopes, it is concluded that the binding mode of the N-acylmannosamines critically depends on the N-acyl side chain. In conjunction with the relative binding affinities of the ligands obtained from STD NMR titrations, it is possible to derive a structure-binding affinity relationship. This provides a cornerstone for the rational design of drugs for novel therapeutic applications by altering the sialic acid decorations of cell walls.