Effect of the Redox-Dependent Ionization State of the Heme Propionic Acid Side Chain on the Entropic Contribution to the Redox Potential of Pseudomonas aeruginosa Cytochrome c551

The thermodynamic properties of the redox potentials (E-m) of Pseudomonas aeruginosa cytochrome c(551) (PA) and its mutants possessing a variety of pK(a) values for the heme 17-propionic acid side chain, which ranged from similar to 5 to similar to 8, have been investigated to elucidate the role of ionization of the heme side chain in the E-m control. Since the pK(a) values of the heme 17-propionic acid side chains of the oxidized and reduced forms of PA are 5.9 +/- 0.2 and 7.0 +/- 0.2, respectively [Takayama, S. J., Mikami, S., Terui, N., Mita, H., Hasegawa, J., Sambongi, Y., and Yamamoto, Y. (2005) Biochemistry 44, 5488-5494], the ionization state of the heme 17-propionic acid side chain at physiological pH depends on the oxidation state of the protein. This redox-dependent ionization state of the heme 17-propionic acid side chain was found to have a large effect on the entropic contribution (Delta S) to the F, value. The magnitude of the E-m control through the Delta S value due to the redox-dependent ionization state of the heme 17-propionic acid side chain was shown to be about 170 mV and hence is considerably larger than that through the enthalpic contribution (Delta H) to the E-m value due to stabilization of the cationic ferriheme in the oxidized protein through partial neutralization of its positive charge by the heme 17-propionate side chain, i.e., about 60 mV [Takayania, S. J., Mikanil, S., Terui. N., Mita, I-L, Hasegawa, J., Sambongi, Y., and Yamamoto, Y. (2005) Biochemistry 44, 5488-5494]. The present study revealed that the heme 17-propionic acid side chain of the protein plays a pivotal role in the E-m control of the protein.