The pressure-induced changes in 15N
enriched HPr from Staphylococcus carnosus were
investigated by two-dimensional (2D) heteronuclear NMR
spectroscopy at pressures ranging from atmospheric pressure
up to 200 MPa. The NMR experiments allowed the simultaneous
observation of the backbone and side-chain 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 nitrogens 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 nitrogen pressure coefficients
show a significant but rather small correlation (0.31)
if determined for all amide resonances. When restricting
the analysis to amide groups in the β-pleated sheet,
the correlation between these coefficients is with 0.59
significantly higher. As already described for other proteins,
the amide proton pressure coefficients are strongly correlated
to the corresponding hydrogen bond distances, and thus
are indicators for the pressure-induced changes of the
hydrogen bond lengths. The nitrogen shift changes appear
to sense other physical phenomena such as changes of the
local backbone conformation as well. Interpretation of
the pressure-induced shifts in terms of structural changes
in the HPr protein suggests the following picture: the
four-stranded β-pleated sheet of HPr protein is the
least compressible part of the structure showing only small
pressure effects. The two long helices a and c show intermediary
effects that could be explained by a higher compressibility
and a concomitant bending of the helices. The largest pressure
coefficients are found in the active center region around
His15 and in the regulatory helix b which includes the
phosphorylation site Ser46 for the HPr kinase. This suggests
that this part of the structure occurs in a number of different
structural states whose equilibrium populations are shifted
by pressure. In contrast to the surrounding residues of
the active center loop that show large pressure effects,
Ile14 has a very small proton and nitrogen pressure coefficient.
It could represent some kind of anchoring point of the
active center loop that holds it in the right place in
space, whereas other parts of the loop adapt themselves
to changing external conditions.