Effects of pressure on visible spectra of complexes of myoglobin, hemoglobin, cytochrome c, and horse radish peroxidase.

The spectra of the ferric form of most heme proteins [metmyoglobin, methemoglobin, horse radish peroxidase (EC 1.11.1.7), and ferricytochrome c at pH 1.5] are converted from high-spin (open crevice) structure to low-spin (closed crevice) form under pressure. Pressures up to 8000 kg/cm2 (780 MPa) have no effect on the spectra of high-spin ferro- and ferricytochrome c, which have a closed crevice structure at pH 7.0. Spectra of deoxy-ferromyoglobin and deoxy-ferrohemoglobin are reduced in intensity, but pressure does not change the positions of the absorption maxima. Cyanide ion prevents pressure-induced spectral changes in metmyoglobin and methemoglobin up to 8000 kg/cm2. Carbon monoxide (with a high affinity for the ferro heme iron) has a similar effect on ferromyoglobin and ferrohemoglobin. The pressure required to cause spectral changes in the heme proteins falls in the order, cytochrome c (pH 7.0) greater than horse radish peroxidase greater than myoglobin greater than hemoglobin. We have calculated a volume change of --50 cm3/mol associated with the configurational change accompanying the reformation of the iron-methionine bond in cytochrome c at low pH.