Presteady-state and steady-state kinetic properties of human cytochrome c oxidase. Identification of rate-limiting steps in mammalian cytochrome c oxidase.
Human cytochrome c oxidase was purified in a fully active form from heart and skeletal muscle. The enzyme was selectively solubilised with octylglucoside and KCl from submitochondrial particles followed by ammonium sulphate fractionation. The presteady-state and steady-state kinetic properties of the human cytochrome c oxidase preparations with either human cytochrome c or horse cytochrome c were studied spectrophotometrically and compared with those of bovine heart cytochrome c oxidase. The interaction between human cytochrome c and human cytochrome c oxidase proved to be highly specific. It is proposed that for efficient electron transfer to occur, a conformational change in the complex is required, thereby shifting the initially unfavourable redox equilibrium. The very slow presteady-state reaction between human cytochrome c oxidase and horse cytochrome c suggests that, in this case, the conformational change does not occur. The proposed model was also used to explain the steady-state kinetic parameters under various conditions. At high ionic strength (I = 200 mM, pH 7.4), the kcat was highly dependent on the type of oxidase and it is proposed that the internal electron transfer is the rate-limiting step. The kcat value of the 'high-affinity' phase, observed at low ionic strength (I = 18 mM, pH 7.4), was determined by the cytochrome c/cytochrome c oxidase combination applied, whereas the Km was highly dependent only on the type of cytochrome c used. Our results suggest that, depending on the cytochrome c/cytochrome c oxidase combination, either the dissociation of ferricytochrome c or the internal electron transfer is the rate-limiting step in the 'high-affinity' phase at low ionic strength. The 'low-affinity' kcat value was not only determined by the type of oxidase used, but also by the type of cytochrome c. It is proposed that the internal electron-transfer rate of the 'low-affinity' reaction is enhanced by the binding of a second molecule of cytochrome c.
Van Kuilenburg
A B
AB
E. C. Slater Institute for Biochemical Research, University of Amsterdam, The Netherlands.
Gorren
A C
AC
Dekker
H L
HL
Nieboer
P
P
Van Gelder
B F
BF
Muijsers
A O
AO
D000818:Animals; D002417:Cattle; D003574:Cytochrome c Group / Q000378:metabolism; D003576:Electron Transport Complex IV / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D006736:Horses; D006801:Humans; D007700:Kinetics; D009994:Osmolar Concentration; D011487:Protein Conformation
D003574:Cytochrome c Group; D003576:Electron Transport Complex IV