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

Cyanide reacts with cytochrome bd from E. coli in an 'aerobically oxidized' state (mainly, an oxygenated complex b558(3+) b595(3+) d(2+)-O2), bringing about (i) decomposition of the heme d2+ 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 b595. MCD spectra show that the Soret red shift is associated with heme b595(3+) high-to low-spin transition. This is the first unambiguous demonstration that heme b595 can bind exogenous ligands. No reaction of cyanide with b558 is observed. In about 70% of the enzyme which forms the cyano complex, the spin-state transition of b595 decay of heme d oxycomplex match each other kinetically (keff ca. 0.002 s-1 at 50 mM KCN, pH 8.1, 25 degrees C). This points to an interaction between the two hemes. The concerted binding of cyanide to d3+ and b595(3+), perhaps as a bridging ligand, is probably rate-limited by d2+ oxycomplex autoxidation. In the remaining 30% of the isolated bd, there is a rapid phase of cyanide-induced b595 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.