Highly selective ligand binding by methylophilus methylotrophus cytochrome c"

Cytochrome c′′ (cyt c′′) from Methylophilus methylotrophus is unusual insofar as the heme has two axial histidine ligands in the oxidized form but one is detached when the protein is reduced. Despite cyt c′′ having an axial site available for binding small ligands, we show here that only NO binds readily to the ferrous cyt c′′. Binding of CO, as well as CN^-, on the other hand requires considerable structural reorganization, or reduction of the disulfide bridge close to the heme. Standard free energies for the binding of NO and CO reveal high selectivity of the ferrous cyt c′′ for NO, indicating its putative physiological role. In this work, we characterize in detail the kinetics of NO binding and the structural features of the Fe²⁺-NO adduct by stopped-flow and resonance Raman spectroscopy, respectively.