Redox-coupled proton transfer in the active site of cytochrome cbb₃

Cytochrome cbb₃ is a distinct member of the superfamily of respiratory heme-copper oxidases, and is responsible for driving the respiratory chain in many pathogenic bacteria. Like the canonical heme-copper oxidases, cytochrome cbb₃ reduces oxygen to water and couples the released energy to pump protons across the bacterial membrane. Homology modeling and recent electron paramagnetic resonance (EPR) studies on wild type and a mutant cbb₃ enzyme [V. Rauhamäki et al. J. Biol. Chem. 284 (2009) 11301-11308] have led us to perform high-level quantum chemical calculations on the active site. These calculations bring molecular insight into the unique hydrogen bonding between the proximal histidine ligand of heme b₃ and a conserved glutamate, and indicate that the catalytic mechanism involves redox-coupled proton transfer between these residues. The calculated spin densities give insight in the difference in EPR spectra for the wild type and a recently studied E383Q-mutant cbb₃-enzyme. Furthermore, we show that the redox-coupled proton movement in the proximal cavity of cbb₃-enzymes contributes to the low redox potential of heme b₃, and suggest its potential implications for the high apparent oxygen affinity of these enzymes.