On the mechanism of inhibition of cytochrome c oxidase by nitric oxide

The mechanism of inhibition of cytochrome (cyt) c oxidase by nitric oxide (NO) has been investigated by stopped flow transient spectroscopy and singular value decomposition analysis. Following the time course of cyt c oxidation at different O2/NO ratios, we observed that the onset of inhibition: (i) is fast and at a high NO concentration is complete during the first turnover; (ii) is sensitive to the O2/NO ratio; and (iii) is independent of incubation time of the oxidized enzyme with NO. Analysis of the reaction kinetics and computer simulations support the conclusion that inhibition occurs via binding of NO to a turnover intermediate with a partially reduced cyt a3-Cu(B) binuclear center. The inhibited enzyme has the optical spectrum typical of NO bound to reduced cyt a3. Reversal of inhibition in the presence of O2 does not involve a direct reaction of O2 with NO while bound at the binuclear center, since recovery of activity occurs at the rate of NO dissociation (k = 0.13 s-1), as determined in the absence of O2 using hemoglobin as a NO scavenger. We propose that removal of NO from the medium is associated with reactivation of the enzyme via a relatively fast thermal dissociation of NO from the reduced cyt a3-Cu(B) center.