DYNAMICS OF THE BINUCLEAR CENTER OF THE QUINOL OXIDASE FROM ACIDIANUS AMBIVALENS

We have investigated the kinetic and thermodynamic properties of carbon monoxide binding to the fully reduced quinol oxidase (cytochrome aa3) from the hyperthermophilic archaeon Acidianus ambivalens. After flash photolysis of CO from heme a3, the complex recombines with an apparent rate constant of ∼3 s-1, which is much slower than with the bovine cytochrome c oxidase (∼80 s-1). Investigation of the CO-recombination rate as a function of the CO concentration shows that the rate saturates at high CO concentrations, which indicates that CO must bind transiently to CuB before binding to heme a3. With the A. ambivalens enzyme the rate reached 50% of its maximum level (which reflects the dissociation constant of the CuB(CO) complex) at ∼13 μM CO, which is a concentration ∼103 times smaller than for the bovine enzyme (∼11 mM). After CO dissociation we observed a rapid absorbance relaxation with a rate constant of ∼1.4 × 104 s-1, tentatively ascribed to a heme-pocket relaxation associated with release of CO afte...