Peroxide complex of cytochrome bd: kinetics of generation and stability

Hydrogen peroxide reacts with the isolated fully oxidized cytochrome bd from Escherichia coli bringing about spectral changes characterized by increased absorption at 680 nm, disappearance of a charge transfer band at 740 nm and a red shift in the Soret band. Only one type of spectral changes is observed throughout the entire range of H2O2 concentration studied, 5 - 5000 microM. The absorption changes are consistent with peroxide binding to heme d and do not show any evidence for reaction with heme b-595. The spectral response saturates at increased H2O2 concentration with apparent Kd of 30 microM and is reversed by catalase. Stopped-flow measurements show the reaction to be first order with respect to H2O2 with a second order rate constant Kon = 600M-1s-1. Decay of the H2O2-induced spectral changes upon addition of catalase (k approximately 0.001 s-1) is about 20-fold slower than expected for dissociation of peroxide from the complex with heme d assuming a simple reversible binding of H2O2 with Kd and Kon values give above (Koff = Kon). We suggest that the reaction of H2O2 with cytochrome bd may be in fact irreversible, the initial binding followed by a cleavage of the O-O bond and formation of the oxoferryl complex of heme d. Upon removal of excess peroxide, the oxoferryl compound could decay being reduced to the ferric state by endogenous reductants.