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Cytochrome  bd -I is one of the three proton motive force-generating quinol oxidases in the O 2 -dependent respiratory chain of  Escherichia coli . It contains one low-spin haem ( b  558 ) and the two high-spin haems ( b  595  and  d ) as the redox-active cofactors. In order to examine the flash-induced intraprotein reverse electron transfer (the so-called ''electron backflow''), CO was photolyzed from the ferrous haem  d  in one-electron reduced ( b  558  3+  b  595  3+  d  2+ -CO) cytochrome  bd -I, and the fully reduced ( b  558  2+  b  595  2+  d  2+ -CO) oxidase as a control. In contrast to the fully reduced cytochrome  bd -I, the transient spectrum of one-electron reduced oxidase at a delay time of 1.5 μs is clearly different from that at a delay time of 200 ns. The difference between the two spectra can be modeled as the electron transfer from haem  d  to haem  b  595  in 3–4% of the cytochrome  bd -I population. Thus, the interhaem electron backflow reaction induced by photodissociation of CO from haem  d  in one-electron reduced cytochrome  bd -I comprises two kinetically different phases: the previously unnoticed fast electron transfer from haem  d  to haem  b  595  within 0.2–1.5 μs and the slower well-defined electron equilibration with τ ~16 μs. The major new finding of this work is the lack of electron transfer at 200 ns.

Evidence for Fast Electron Transfer between the High-Spin Haems in Cytochrome bd-I from Escherichia coli