The protonation state of residues around the Qo binding site of the cyto-chrome bc1 complex from Paracoccus denitrificansand their interaction with bound quinone(s) was studied by a combined electrochemical and FTIR difference spectroscopic approach. Site-directed mutations of two groups of conserved residues were investigated: (a) acidic side chains located close to the surface and thought to participate in a water chain leading up to the heme bLedge, and (b) residues located in the vicinity of this site. .
