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Zusammenfassung:
The Staphylococcus aureus phosphocarrier protein HPr and its phospho derivatives, prepared enzymatically as well as chemically, have been investigated by 1H and 31P nuclear magnetic resonance (NMR). To obtain sufficient experimental data to interpret the protein NMR spectra phosphohistidine compounds were prepared by phosphoamidate‐dependent phosphorylation of histidine.
The assignment of the C‐2 and the C‐4 proton peaks of the imidazole rings of the 1H NMR spectrum as well as the assignment of the phosphorus signals of the 31P NMR spectrum had to be carried out with a mixture of the phosphohistidine compounds (1‐phosphohistidine, 3‐phosphohistidine, 1,3‐bisphosphohistidine) since the isolation and chromatography of phosphohistidines is hampered by their sensitivity towards hydrolysis.
The following pK values of the imidazole residues of phosphohistidines were determined by the titration behaviour of the suitable 1H and 31P NMR signals: histidine, 6.0; 1‐phosphohistidine, 7.04; 3‐phosphohistidine, 6.2. For the imidazole residue in HPr and its phospho derivatives (P‐HPr) the following pK values were obtained by observing the C‐2 and C‐4 imidazole protons: HPr, 6.0; P‐HPr(enzymatic), 8.3; P‐HPr(chemical), 6.9.
The correlation between the pK values of the model phosphohistidines with the protein HPr and its phospho derivatives allowed us to localize the position of the phospho group in the intact P‐HPr molecule. In P‐HPr(enzymatic) the phospho group is linked to the N‐1 of the only imidazole residue of the protein. In P‐HPr(chemical) which is no longer able to act as phosphocarrier, the phospho group is attached to N‐3 of the imidazole residue.
The chemical shift of the 31P NMR signal of P‐HPr(enzymatic) showed only good agreement with the 31P shift of 1‐phosphohistidine if the protein spectrum was recorded above the pH value required for the denaturation of HPr (pH 11.5).
