Carboxyl group protonation upon reduction of the Paracoccus denitrificanscytochrome coxidase: direct evidence by FTIR spectroscopy

The redox reactions of the cytochrome coxidase from Paracoccus denitrificanswere investigated in a thin‐layer cell designed for the combination of electrochemistry under anaerobic conditions with UVIVIS and IR spectroscopy. Quantitative and reversible electrochemical reactions were obtained at a surface‐modified electrode for all cofactors as indicated by the optical signals in the 400–700 nm range. Fourier transform infrared (FTIR) difference spectra of reduction and oxidation (reduced‐minus‐oxidized and oxidized‐minus‐reduced, respectively) obtained in the 1800‐1000 cm−1range reveal highly structured band features with major contributions in the amide I (1620–1680 cm−1) and amide II (1580‐1520 cm−1) range which indicate structural rearrangements in the cofactor vicinity. However, the small amplitude of the IR difference signals indicates that these conformational changes are small and affect only individual peptide groups. In the spectral region above 1700 cm−1, a positive peak in the reduced state (1733 cm−1) and negative peak in the oxidized state (1745 cm−1) are characteristic for the formation and decay of a COOH mode upon reduction. The most obvious interpretation of this difference signal is proton uptake by one Asp or Gin side chain carboxyl group in the reduced state and deprotonation of another Asp or Glu residue. Moreover, both residues could well be coupled as a donor‐acceptor pair in the proton transfer chain. An alternative interpretation is in terms of a protonated carboxyl group which shifts to a different environment in the reduced state. The relevance of this first direct observation of protein protonation changes in the cytochrome coxidase for vectorial proton transfer and the catalytic reaction is discussed.