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-acetylmannosamine-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 Mg(2+). 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.

Conformational Analysis of Chirally Deuterated Tunicamycin as an Active Site Probe of UDP-N-Acetylhexosamine:Polyprenol-P N-Acetylhexosamine-1-P Translocases

Tunicamycins are potent inhibitors of UDP-N-acetyl-D-hexosamine:polyprenol-phosphate N-acetylhexosamine-1-phosphate translocases (D-HexNAc-1-P translocases), a family of enzymes involved in bacterial cell wall synthesis and eukaryotic protein N-glycosylation. Structurally, tunicamycins consist of an 11-carbon dialdose core sugar called tunicamine that is N-linked at C-1' to uracil and O-linked at C-11' to N-acetylglucosamine (GlcNAc). The C-11' O-glycosidic linkage is highly unusual because it forms an alpha/beta anomeric-to-anomeric linkage to the 1-position of the GlcNAc residue. We have assigned the (1)H and (13)C NMR spectra of tunicamycin and have undertaken a conformational analysis from rotating angle nuclear Overhauser effect (ROESY) data. In addition, chirally deuterated tunicamycins produced by fermentation of Streptomyces chartreusis on chemically synthesized, monodeuterated (S-6)-[(2)H(1)]glucose have been used to assign the geminal H-6'a, H-6'b methylene bridge of the 11-carbon dialdose sugar, tunicamine. The tunicamine residue is shown to assume pseudo-D-ribofuranose and (4)C(1) pseudo-D-galactopyranosaminyl ring conformers. Conformation about the C-6' methylene bridge determines the relative orientation of these rings. The model predicts that tunicamycin forms a right-handed cupped structure, with the potential for divalent metal ion coordination at 5'-OH, 8'-OH, and the pseudogalactopyranosyl 7'-O ring oxygen. The formation of tunicamycin complexes with various divalent metal ions was confirmed experimentally by MALDI-TOF mass spectrometry. Our data support the hypothesis that tunicamycin is a structural analogue of the UDP-D-HexNAc substrate and is reversibly coordinated to the divalent metal cofactor in the D-HexNAc-1-P translocase active site.

Synthesis of new glycosides by transglycosylation of N-acetylhexosaminidase from Serratia marcescens YS-1

Serratia marcescens YS-1, a chitin-degrading microorganism, produced mainly N-acetylhexosaminidase. The purified enzyme had an optimal pH of approximately 8-9 and remained stable at 40 degrees C for 60 min at pH 6-8. The optimum temperature was around 50 degrees C, and enzyme activity was relatively stable below 50 degrees C. YS-1 N-acetylhexosaminidase hydrolyzed p-nitrophenyl beta-N-acetylgalactosamide by 28.1% relative to p-nitrophenyl beta-N-acetylglucosamide. The N-acetylchitooligosaccharides were hydrolyzed more rapidly, but the cellobiose and chitobiose of disaccharides that had the same beta-1,4 glycosidic bond as di-N-acetylchitobiose were not hydrolyzed. YS-1 N-acetylhexosaminidase efficiently transferred the N-acetylglucosamine residue from di-N-acetylchitobiose (substrate) to alcohols (acceptor). The ratio of transfer to methanol increased to 86% in a reaction with 32% methanol. N-Acetylglucosamine was transferred to the hydroxyl group at C1 of monoalcohols. A dialcohol was used as an acceptor when the carbon number was more than 4 and a hydroxyl group existed on each of the two outside carbons. Sugar alcohols with hydroxyl groups in all carbon positions were not proper acceptors.