N-15 and H-1 NMR study of histidine containing protein (HPr) from Staphylococcus carnosus at high pressure

The pressure-induced changes in N-15 enriched HPr from Staphylococcus carno sus were investigated by two-dimensional (2D) heteronuclear NMR spectroscop y at pressures ranging from atmospheric pressure up to 200 MPa. The NMR exp eriments allowed the simultaneous observation of the backbone and side-chai n amide protons and nitrogens. Most of the resonances shift downfield with increasing pressure indicating generalized pressure-induced conformational changes. The average pressure-induced shifts for amide protons and nitrogen s are 0.285 ppm GPa(-1) at 278 K and 2.20 ppm GPa(-1), respectively. At 298 K the corresponding values are 0.275 and 2.41 ppm GPa(-1). Proton and nitr ogen pressure coefficients show a significant but rather small correlation (0.31) if determined for all amide resonances. When restricting the analysi s to amide groups in the P-pleated sheet, the correlation between these coe fficients is with 0.59 significantly higher. As already described for other proteins, the amide proton pressure coefficients are strongly correlated t o the corresponding hydrogen bond distances, and thus are indicators for th e pressure-induced changes of the hydrogen bond lengths. The nitrogen shift changes appear to sense other physical phenomena such as changes of the lo cal backbone conformation as well. Interpretation of the pressure-induced s hifts in terms of structural changes in the HPr protein suggests the follow ing picture: the four-stranded beta-pleated sheet of HPr protein is the lea st compressible part of the structure showing only small pressure effects. The two long helices a and c show intermediary effects that could be explai ned by a higher compressibility and a concomitant bending of the helices. T he largest pressure coefficients are found in the active center region arou nd His15 and in the regulatory helix b which includes the phosphorylation s ite Ser46 for the HPr kinase. This suggests that this part of the structure occurs in a number of different structural states whose equilibrium popula tions are shifted by pressure. In contrast to the surrounding residues of t he active center loop that show large pressure effects, Ile14 has a very sm all proton and nitrogen pressure coefficient. It could represent some kind of anchoring point of the active center loop that holds it in the right pla ce in space, whereas other parts of the loop adapt themselves to changing e xternal conditions.