2Materials and Surface Science Institute, University of Limerick, Ireland
* To whom correspondence should be addressed.
Received September 8, 2009; Revision received October 9, 2009
Cytochrome c oxidase of the ba3-type from Thermus thermophilus does not interact with cyanide in the oxidized state and acquires the ability to bind heme iron ligands only upon reduction. Cyanide complexes of the reduced heme a3 in cytochrome ba3 and in mitochondrial aa3-type cytochrome oxidase are similar spectroscopically, but the a32+–CN complex of cytochrome ba3 is strikingly tight. Experiments have shown that the Kd value of the cytochrome ba3 complex with cyanide in the presence of reductants of the enzyme binuclear center does not exceed 10–8 M, which is four to five orders of magnitude less than the Kd of the cyanide complex of the reduced heme a3 of mitochondrial cytochrome oxidase. The tightness of the cytochrome ba3 complex with cyanide is mainly associated with an extremely slow rate of the ligand dissociation (koff ≤ 10–7 sec–1), while the rate of binding (kon ~ 102 M–1·sec–1) is similar to the rate observed for the mitochondrial cytochrome oxidase. It is proposed that cyanide dissociation from the cytochrome ba3 binuclear center might be hindered sterically by the presence of the second ligand molecule in the coordination sphere of CuB2+. The rate of cyanide binding with the reduced heme a3 does not depend on pH in the neutral area, but it approaches linear dependence on H+ activity in the alkaline region. Cyanide binding appears to be controlled by protonation of an enzyme group with pKa = 8.75.
KEY WORDS: cytochrome oxidase ba3, cyanide, hemoproteins, oxygen reducing center, Thermus thermophilus