Noninvasive auto-photoreduction used as a tool for studying structural changes in heme-copper oxidases by FTIR spectroscopy

Photobiochemical production of carbon monoxide by Thermus thermophilus ba3 -cytochrome c oxidase

We report the photobiochemical production of carbon monoxide by a terminal ba3 -cytochrome c oxidase from T. thermophilus HB8. FTIR and time-resolved step-scan FTIR spectroscopies were combined to probe this process and also monitor the concomitant binding of the produced gas to other intact ba3 molecules forming the ba3 -CO complex. The activation of this mechanism by ba3 -oxidase under visible excitation raises the question as to whether such a mechanism is physiologically relevant to the extreme environment in which it operates.

Photoreactions of cytochrome C oxidase

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.