ATR-FTIR spectroscopy and isotope labeling of the PM intermediate of Paracoccus denitrificans cytochrome c oxidase

The structure of the P-M intermediate of Paracoccus denitrficans cytochrome c oxidase was investigated by perfusion-induced attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy. Transitions from the oxidized to Pm state were initiated by perfusion with CO/oxygen buffer, and the extent of conversion was quantitated by simultaneously monitoring visible absorption changes. In prior work, tentative assignments of bands were proposed for heme a(3), a change in the environment of the protonated state of a carboxylic acid, and a covalently linked histidine-tyro sine ligand to Cu-B that has been found in the catalytic site. In this work, reduced minus oxidized difference spectra at pH 6.5 and 9.0 and P-M minus oxidized difference spectra at pH 9.0 were compared in unlabeled, universally N-15-labeled, and tyrosine-ring-d(4)-labeled proteins to improve these assignments. In the reduced minus oxidized difference spectrum, N-15 labeling resulted in large changes in the amide II region and a 9 cm(-1) downshift in a 1105 cm(-1) trough that is attributed to histidine. In contrast, changes induced by tyrosine-ring-d(4) labeling were barely detectable where the isotope-sensitive bands are expected. Both isotope substitutions had large effects on P-M minus oxidized difference spectra. A prominent trough at 1542 cm(-1) was shifted to 1527 cm(-1) with 15N labeling, and its magnitude was diminished with the appearance of a 1438 cm(-1) trough with tyrosine-ring-d(4) labeling. Both isotope substitutions also had large effects on a 1314 cm(-1) trough in the same spectra. These shifts indicate that the bands are linked to both a nitrogenous compound and a tyrosine, the most obvious candidate being the covalent histidine - tyrosine ligand Of Cu-B. Comparison with model material data suggests that the tyrosine hydroxyl group is protonated when the binuclear center is oxidized but deprotonated in the Pm intermediate. Positive bands at 1519 and 1570 cm(-1) were replaced with bands at 1504 and 1556 cm(-1), respectively, with tyrosine-ring-d(4) labeling, are characteristic of v(7a)(C-O) and v(C-C) bands of neutral phenolic radicals, and most likely reflect the formation of the neutral radical state of the histidine-tyrosine ligand in P-M.