Structural studies of histidine-containing phosphocarrier protein from Enterococcus faecalis.

Based on the complete sequential assignment of the 1H-NMR spectrum by multidimensional NMR techniques the secondary structure and the local geometry of the active site of histidine-containing phosphocarrier protein (HPr) from Enterococcus faecalis were elucidated. We present a comparative analysis of the active site in the seven known structures of HPr from different organisms determined by NMR or X-ray crystallography. In catalysis, HPr is phosphorylated at the ring N61 of His15. No general agreement exists in literature regarding the structure of the active-centre loop. In the crystal structure of HPr from E. faecalis, a torsion strain of the backbone at position 16 was observed, which was assumed to be important to the catalytic mechanism. Coupling constants were determined in order to calculate phi angles to establish whether there are strained torsion angles in HPr from E. faecalis in the solution state. The evaluation of data obtained indicate a stable and well-defined structure of HPr from E. faecalis, with an overall fold similar to that found in HPr from other bacteria. We find that in the active-site region there are relatively large variations in local geometry between the evaluated structures. In HPr from E. faecalis, a particularly detailed view of the phosphate-binding His15 and residues in close spatial proximity was obtained by determination of coupling constants obtained from the double-quantum-filtered COSY spectrum. Our data indicate that in aqueous solution, in the dominant conformational state there is no torsion strain of the backbone at position 16, as observed in the crystal state. The maximum population of a strained conformation in solution can be estimated to be smaller than 23%. The analysis of the data suggests that the active-centre loop is able to adopt different conformations in solution. A similar observation was made for HPr from E. faecalis phosphorylated at its regulatory site (Ser46). 31P-NMR shows that phosphorylated HPr exists in two conformational substates with nearly equal populations.