Effect of ligands on cytochrome d from Azotobacter vinelandii

Spectra of oxidized and reduced cytochrome d in particles of A. vinelandii were studied in the presence of the ligands CO, azide, and NH2OH under oxidizing, reducing, and turnover conditions. Under oxidizing conditions, spectral changes were observed on oxidized cytochrome d (absorption maximum at 648 nm) in the presence of CO and NH2OH showing a shift of the maximum to shorter wavelengths (639 and 645 nm, respectively) and a broadening of the half-band width. Under reducing conditions, spectral changes were observed on reduced cytochrome d (absorption maximum at 631 nm) in the presence of CO (absorption maximum at 636 nm), NO, NO2-, and NH2OH (absorption maximum at 642 nm in the presence of dithionite). The spectral changes of cytochrome d in the presence of NH2OH or with dithionite and NO2- were ascribed to the formation of the NO-cytochrome d compound. Under turnover conditions CO, NH2OH, and azide cause a spectral shift of the absorption maximum of cytochrome d from 648 nm to 636, 645, and 655 nm, respectively. With NH2OH and azide a broadening of the half-band width of 7 and 6 nm, respectively, was also observed. The spectral changes caused by CO and NH2OH were interpreted as a binding of the ligands to cytochrome d changing its conformation from the oxidized state absorbing at 648 nm into a more stable liganded form. Since azide does not affect the spectral bands of oxidized and reduced cytochrome d, the spectral change during turnover in the presence of azide were ascribed to a preferential binding of azide to enzymically active conformation of cytochrome d (cytochrome dx).