Pressure Effects on the Intramolecular Electron Transfer Reactions in Hemoproteins

Abstract

The activation volumes (ΔV ≠) for intramolecular electron transfer (ET) reactions in Ru-modified cytochrome b 5 (Ru-cytb 5) and Zn-porphyrin substituted myoglobins (Ru-ZnMb) were determined to investigate the pressure effects on the ET pathway; this provided us with new insights into the fluctuation—controlled ET reaction mechanism in proteins. Ru-cytb 5, in which the Ru complex was attached at His26, exhibited a large negative activation volume for the ET reaction, although the ET reactions in previous Rucytb 5 mutants were almost insensitive to pressurization [14]. The pathway analysis revealed that our Ru-cytb 5 system has a long ‘through-space’ process on the electron pathway, while the pathway for the previous system consisted of only a ‘through-bond’ process with covalent bonds or included a single short ‘through-space’ process. The pressure effects on the ET reaction, therefore, depend highly on the flexibility of the pathway, a flexible ‘through-space’ or rigid ‘through-bond’ process, and the ET reaction mediated by the ‘through-space’ process would be more susceptible to pressurization, implying that the structural fluctuation would preferentially affect the ‘through-space’ ET reaction. To confirm the prominent effects of pressure on the ‘through-space’ ET reaction, we also examined the pressure dependence of Ru-ZnMbs, the ET pathways of which have some flexible and long ‘through-space’ processes. In spite of the same donor—acceptor (D—A) pair, three Ru-ZnMbs, for which D—A distances for the ET reactions are 12.7 (His48Mb), 15.5 (His83Mb) and 19.3 A (His81Mb), showed different activation volumes for the ET reactions: —1.6 (His83Mb), +3.7 (His81Mb), +6.5 cm3 mol−1 (His48Mb). Since the Marcus theory indicates that the acceleration and deceleration of the ET reaction rates in RuZnMbs would correspond to the shortening and stretching of the D—A distance, respectively, some of the flexible ‘through-space’ processes in Ru-ZnMbs would reduce the D—A distance and the others would increase the distance by pressurization. In other words, the structural fluctuation affecting the ‘through-space’ ET process is not isotropic in protein, and local fluctuations count as one of the factors regulating the protein ET reactions.