Photoreactions of Cytochrome c Oxidase

Abstract

The photoreduction of oxidized bovine heart cytochrome c oxidase (CcO) by visible and UV radiation was investigated in the absence and presence of external reagents. In the former case, the quantum yields for direct photoreduction of heme A (heme a + heme a(3)) were 2.6 +/- 0.5 x 10(-3), 4 +/- 1 x 10(-4), and 4 +/- 2 x 10(-6) with pulsed laser irradiation at 266, 355 and 532 nm, respectively. Within experimental uncertainty, the quantum yields did not depend on pulse energy, implying that the mechanism is monophotonic. Irradiation with 355 nm light resulted in spectral changes similar to those produced independently by reduction with dithionite, whereby the low-spin heme a and Cu(A) are reduced first. Extended illumination at 355 and 532 nm yielded substantial amounts of reduced heme a(3). Heme decomposition was noted with 266 nm light. In the presence of formate and cyanide ions, which bind at the binuclear heme a(3)/copper center in CcO, irradiation at 355 nm caused selective reduction of only the low-spin heme a and Cu(A). The addition of ferrioxalate ion dramatically increased the efficiency of cytochrome c oxidase photoreduction. The quantum efficiency for heme A reduction was found to be near unity, significantly greater than for other known methods of photoreduction. The active reductant is most likely ferrous iron, and its reduction of the enzyme is thermodynamically driven by the reformation of ferrioxalate in the presence of excess oxalate ion. Other metalloenzymes with redox potentials similar to those of cytochrome c oxidase should be amenable to indirect photoreduction by this method.

Flash-Photolysis of Fully Reduced and Mixed-Valence CO-Bound Rhodobacter sphaeroides Cytochrome c Oxidase: Heme Spectral Shifts

“…The spectrum at each time point was an average of 40 individual spectra, each produced by a single laser pulse. As we have shown recently (22), insignificant photoreduction of heme a occurs upon single laser pulse photolysis at 532 nm. The data matrix was analyzed by SVD, and the time-dependent v-vectors were fitted to a sum of exponential functions, followed by calculation of the wavelength-dependent amplitudes, i.e., the b-spectra.…”

Review of light parameters and photobiomodulation efficacy: dive into complexity

“…38 When applying light to cells, mitochondria are the initial sites of light absorption and CCO (particularly, the CuA and CuB metal centers) are believed to be the photoacceptors. 39 Photon absorption results in setting in motion a cascade of reactions known as cellular signaling pathways leading to NO dissociation, ROS production, and increased ATP synthesis. 9 The number of mitochondria in cells varies widely and it is strongly correlated with the metabolic requirements of the cell (how many chemical reactions the cell has to carry out) and may range from a few to thousands of individual organelles.…”

Photoinduced Proton‐Coupled Electron Transfers in Biorelevant Phenolic Systems

“…To our knowledge, few photochemical studies were conducted on CcO (see for example [163][164][165][166][167]) and none of them explicitly treated the PCET mechanism. The complete elucidation of the CcO mechanism on a molecular scale is still a challenge in view of the system and the possibility of several mechanistic options for the coupled electron and proton transfers (151,168,169).…”