The Enzyme Horseradish Peroxidase Is Less Compressible at Higher Pressures

AbstractFluorescence line-narrowing (FLN) spectroscopy at 10K was used to study the effect of high pressure through the prosthetic group in horseradish peroxidase (HRP), which was Mg-mesoporphyrin (MgMP) replacing the heme of the enzyme. The same measurement was performed on MgMP in a solid-state amorphous organic matrix, dimethyl sulfoxide (DMSO). Series of FLN spectra were registered to determine the (0, 0) band shape through the inhomogeneous distribution function (IDF). In the range of 0–2GPa a red-shift of the IDF was determined, and yielded the isothermal compressibility of MgMP-HRP as 0.066GPa−1, which is significantly smaller than that found earlier as 0.106 GPa−1 by fine-tuning the pressure in the range up to 1.1MPa. The vibrational frequencies also shifted with pressure increase, as expected. The compressibility in the DMSO matrix was smaller, 0.042GPa−1, both when the pressure was applied at room temperature before cooling to 10K, or at 10K. At 200K or above, the bimodal (0, 0) band shape in DMSO showed a population conversion under pressure that was not observed at or below 150K. A significant atomic rearrangement was estimated from the volume change, 3.3±0.7cm3/mol upon conversion. The compressibility in proteins and in amorphous solids seems not to significantly depend on the temperature and in the protein it decreases toward higher pressures