FOR THE RECORD Tautomeric state and pK , of the phosphorylated active site histidine in the N - terminal domain of enzyme I of the Escherichia coli phosphoeno 1 pyruvate : sugar phosphotransferase system
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چکیده
The phosphorylated form of the N-terminal domain of enzyme I of the phosphoeno1pyruvate:sugar phosphotransferase system of Escherichia coli has been investigated by one-bond and long-range IH-I5N correlation spectroscopy. The active site His 189 is phosphorylated at the Ne2 position and has a pK, of 7.3, which is one pH unit higher than that of unphosphorylated His 189. Because the neutral form of unphosphorylated His 189 is in the N61-H tautomer, and its Ne2 atom is solvent inaccessible and accepts a hydrogen bond from the hydroxyl group of Thr 168, both protonation and phosphorylation of His 189 must be accompanied by a change in the side-chain conformation of His 189, specifically from a x2 angle in the g + conformer in the unphosphorylated state to the gconformer in the phosphorylated state.
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