The role of the conserved tryptophan272 of the Paracoccus denitrificans cytochrome c oxidase in proton pumping

The catalytic mechanism of heme-copper oxidases - electron transfer coupled to proton pumping - is not yet fully understood. Single turnover experiments in which fully reduced cytochrome aa3 from Paracoccus denitrificans reacts with O2 using the microsecond freeze-hyperquenching sampling technique enabled trapping of transient catalytic intermediates and analysis by low temperature UV-Visible, X-band and Q-band EPR spectroscopy. Our recent findings (Wiertz et al. (2007) J. Biol. Chem. 282, 31580-31591), which show that the strictly conserved W272 is a redox active residue are reviewed here. The W272 forms a tryptophan neutral radical in the transition F → FW⁎ → OH in which the novel intermediate FW⁎ harbors the tryptophan radical. The potential role of W272 in proton pumping is highlighted.