Cyanide‐reactive sites in cytochrome bd complex from E. coli

Publisher Website
Google Scholar
Abstract
Cyanide reacts with cytochrome bd from E. coli in an ‘aerobically oxidized’ state (mainly, an oxygenated complex b 558 3+ b 595 3+ d 2+‐O2), bringing about (i) decomposition of the heme d 2+ oxycomplex (decay of the 648 nm absorption band) and (ii) extensive red shift in the Soret region accompanied by minor changes in the visible range assigned to ferric heme b 595. MCD spectra show that the Soret red shift is associated with heme b 558 3+ high‐to low‐spin transition. This is the first unambiguous demonstration that heme b 595 can bind exogenous ligands. No reaction of cyanide with b 558 is observed. In about 70% of the enzyme which forms the cyano complex, the spin‐state transition of b 595 decay of heme d oxycomplex match each other kinetically (k eff ca. 0.002 s−1 at 50 mM KCN, pH 8.1, 25°C). This points to an interaction between the two hemes. The concerted binding of cyanide to d 3+ and b 595 3+, perhaps as a bridging ligand, is probably rate‐limited by d 2+ oxycomplex autoxidation. In the remaining 30% of the isolated bd, there is a rapid phase of cyanide‐induced b 595 spin‐state transition which can be tentatively assigned to that proportion of the enzyme in which heme d is initially in the ferric rather than ferrous‐oxy form.