| Abstract: | The histidine-containing phosphocarrier protein (HPr) of the phosphoenolpyruvate:sugar phosphotransferase system, when phosphorylated, contains a 1-phosphohistidinyl (1-P-histidinyl) residue (His-15). The properties of this 1-P-histidinyl residue were investigated by using phospho-HPr (P-HPr), P-HPr-1, and P-HPr-2. HPr-1 and HPr-2 are deamidated forms of HPr produced by boiling. In addition, HPr-1 produced during frozen storage was investigated. Both pH and temperature dependencies of the rate of hydrolysis of the phosphoryl group of the 1-P-histidinyl residue were investigated. The results show that the 1-P-histidinyl residue in HPr and HPr-1 has significantly different properties from free 1-P-histidine and that these differences are attributable to the active-site residues Glu-66 and Arg-17 and the pK of the imidazole group of the 1-P-histidinyl residue in P-HPr. The 1-P-histidinyl residue in P-HPr and P-HPr-1 shows a greater lability at physiological pH than the free amino acid. A proposal for the active site of P-HPr is made on the basis of these results and the recently obtained tertiary structure. In contrast, the hydrolysis properties of the 1-P-histidinyl residue in P-HPr-2 were similar to those obtained for either free 1-P-histidine or denatured P-HPr. The loss of activity that is associated with boiling HPr was shown to be due to HPr-2 formation as HPr-1 was found to be fully active. |
