The catalytic activity and molecular aspects of Thiobacillus novellus cytpchrome c oxidase were affected by ATP. The steady-state kinetics in the oxidation of ferrocytochrome c by the oxidase varied with the presence or absence of ATP; the [S]±v curve of the reaction was sigmoid in the absence of ATP whereas it was a Michaelis± Menten-type hyperbola in the presence of 700 mm ATP. The oxidase was a dimer of the minimal structural subunit consisting of one molecule each of two subunits in the presence of Tween 20 and in the absence of ATP. The dimer dissociated into monomers in the presence of 700 mm ATP. The trough at 452 nm seen in the second derivative absorption spectrum of the CO compound of the oxidase in the absence of ATP, a characteristic of the cytochrome a component of cytochrome aa 3, dissappeared in the presence of 700 mm ATP. However, ADP, AMP, GTP, CTP and UTP had little affect on both the [S]±v curve and the molecular mass of the oxidase when used in place of ATP.

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