Effect of ligands on cytochromed fromAzotobacter vinelandii

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Abstract
Spectra of oxidized and reduced cytochromed in particles ofA. 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 cytochromed (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 cytochromed (absorption maximum at 631 nm) in the presence of CO (absorption maximum at 636 nm), NO, NO− 2, and NH2OH (absorption maximum at 642 nm in the presence of dithionite). The spectral changes of cytochromed in the presence of NH2OH or with dithionite and NO− 2 were ascribed to the formation of the NO-cytochromed compound. Under turnover conditions CO, NH2OH, and azide cause a spectral shift of the absorption maximum of cytochromed 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 cytochromed 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 cytochromed, the spectral change during turnover in the presence of azide were ascribed to a preferential binding of azide to enzymically active conformation of cytochromed (cytochromed x).

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