Oxygenated complex of cytochrome bd from Escherichia coli : Stability and photolability

Cytochrome bd is one of the two terminal ubiquinol oxidases in the respiratory chain of Escherichia coli catalyzing reduction of O 2 to H 2 O. The enzyme is expressed under low oxygen tension; due to high affinity for O 2 it is isolated mainly as a stable oxygenated complex. Direct measurement of O 2 binding to heme d in the one‐electron reduced isolated enzyme gives K d (O 2 ) of ∼280 nM. It is possible to photolyse the heme d oxy‐complex by illumination of the enzyme for several minutes under microaerobic conditions; the light‐induced difference absorption spectrum is virtually identical to the inverted spectrum of O 2 binding to heme d .