Kinetic resolution of a tryptophan-radical intermediate in the reaction cycle of Paracoccus denitrificans cytochrome c oxidase

The catalytic mechanism, electron transfer coupled to proton pumping, of heme-copper oxidases is not yet fully understood. Microsecond freeze-hyperquenching single turnover experiments were carried out with fully reduced cytochrome aa(3) reacting with O-2 between 83 mu s and 6 ms. Trapped intermediates were analyzed by low temperature UV-visible, X-band, and Q-band EPR spectroscopy, enabling determination of the oxidation-reduction kinetics of Cu-A, heme a, heme a(3), and of a recently detected tryptophan radical (Wiertz, F. G. M., Richter, O. M. H., Cherepanov, A. V., MacMillan, F., Ludwig, B., and de Vries, S. (2004) FEBS Lett. 575, 127-130). Cu-B and heme a3 were EPR silent during all stages of the reaction. Cu-A and heme a are in electronic equilibrium acting as a redox pair. The reduction potential of Cu-A is 4.5 mV lower than that of heme a. Both redox groups are oxidized in two phases with apparent half-lives of 57 mu s and 1.2 ms together donating a single electron to the binuclear center in each phase. The formation of the heme a3 oxoferryl species PR (maxima at 430 nm and 606 nm) was completed in similar to 130 mu s, similar to the first oxidation phase of Cu-A and heme a. The intermediate F (absorbance maximum at 571 nm) is formed from PR and decays to a hitherto undetected intermediate named F-W*. F-W* harbors a tryptophan radical, identified by Q-band EPR spectroscopy as the tryptophan neutral radical of the strictly conserved Trp-272 (Trp-272*). The Trp-272* populates to 4-5% due to its relatively low rate of formation (t(1/2) = 1.2 ms) and rapid rate of breakdown (t(1/2) = 60 mu s), which represents electron transfer from Cu-A/ heme a to Trp-272*. The formation of the Trp-272* constitutes the major rate-determining step of the catalytic cycle. Our findings show that Trp-272 is a redox-active residue and is in this respect on an equal par to the metallo-centers of the cytochrome c oxidase. Trp-272 is the direct reductant either to the heme a3 oxoferryl species or to Cu-B(2+). The potential role of Trp-272 in proton pumping is discussed.