Redox control of fast ligand dissociation from Escherichia coli cytochrome bd.

Bacterial bd-type quinol oxidases, such as cytochrome bd from Escherichia coli, contain three hemes, but no copper. In contrast to heme-copper oxidases and similarly to globins, single electron-reduced cytochrome bd forms stable complexes with O2, NO and CO at ferrous heme d. Kinetics of ligand dissociation from heme d2+ in the single electron- and fully-reduced cytochrome bd from E. coli has been investigated by rapid mixing spectrophotometry at 20 C. Data show that (i) O2 dissociates at 78 s-1, (ii) NO and CO dissociation is fast as compared to heme-copper oxidases and (iii) dissociation in the single electron-reduced state is hindered as compared to the fully-reduced enzyme. Presumably, rapid ligand dissociation requires reduced heme b595. As NO, an inhibitor of respiratory oxidases, is involved in the immune response against microbial infection, the rapid dissociation of NO from cytochrome bd may have important bearings on the patho-physiology of enterobacteria.