Probing the Nitrite Reductase Activity of Heme-Copper Respiratory Oxidase

Abstract

Heme-copper oxidases, in particular cytochrome ba 3, is capable of reducing NO to N2O and oxidizing CO to CO2, although with a low activity. Despite the very low (20%) overall sequence homology to other structurally known cytochrome c oxidases, the four redox centers of cytochrome ba 3 display high similarity with those of other oxidases. Extensive studies, at ambient temperatures, have shown that the heme a 3-CuB binuclear center of fully reduced (FR) and mixed valence (MV) cytochrome ba 3 is subjected to peculiar thermodynamic and kinetic properties, with significant deviations, relative to the mesophilic aa 3-type oxidases. These properties include an increased hydrophobicity, a high affinity of CuB for exogenous ligands such as CO (K > 104 M-1) and a slow intramolecular ligand transfer to heme a 3 (k = 8 s-1). As a result, cytochrome ba 3 is the only documented oxidase where the binding of CO to CuB is exergonic and accounts for 25-30% of the total enzyme concentration. Furthermore, the proximal and the distal environment of heme a 3 shows a high structural flexibility evidenced from the multiple heme a 3 Fe2+-CO conformers detected in both the FR and MV forms of the enzyme. Time-resolved step-scan FTIR studies showed that after photolysis of CO from heme a 3 2+ the ligand is almost quantitatively transferred to CuB 1+ and the transiently formed CuB 1+-CO complex exhibits an unusual long lifetime of 20 ms (pD 8.50). In addition, the midpoint redox potentials of hemes b and a 3 have been reported in three different studies being inverted, with values of 210/213 and 430/285 mV (versus Normal Hydrogen Electrode (NHE)), respectively. Resonance Raman and FTIR studies of the ligand binding properties of heme-copper oxidases will be presented.References. Discrete Ligand Binding and Electron Transfer Properties of ba 3-Cytochrome c Oxidase from Thermus thermophilus: Evolutionary Adaption to Low Oxygen and ...C Koutsoupakis, T Soulimane, C. Varotsis Accounts of chemical research 52 (5), 1380-139, 2019. The structure of a ferrous heme-nitro species in the binuclear heme a3/CuB center of ba 3-cytochrome c oxidase as determined by resonance Raman spectroscopy Loullis, MR Noor, T Soulimane, E Pinakoulaki Chemical Communications 51 (2), 286-289, 2015.