The Escherichia coli Phosphoenolpyruvate-Dependent Phosphotransferase System: Observation of Heterogeneity in the Amino Acid Composition of HPr

Resonances of the aromatic protons of tyrosine have been observed in the proton nuclear magnetic resonance (1H NMR) spectrum of purified HPr from Escherichia coli. Analysis of the NMR spectrum of native HPr suggests that the tyrosine is located in a single position in the secondary structure and that this position is on the interior of the molecule inaccessible to solvent. Previous reports suggested that E. coli HPr contained no tyrosine. In contrast, we find, by amino acid analysis and ultraviolet and NMR spectroscopy, that E. coli HPr does contain tyrosine but at a subintegral level of 0.5 ± 0.1 mol of tyrosine per mol of HPr.