A. Giuffre et al., Electron transfer kinetics of caa(3) oxidase from Bacillus stearothermophilus: A hypothesis for thermophilicity, BIOPHYS J, 76(1), 1999, pp. 438-442
The O-2 reaction and the reverse electron transfer of the thermophilic caa,
terminal oxidase of Bacillus stearothermophilus have been studied by laser
flash-photolysis. The results show that both reactions, although studied a
t a temperature of 20 degrees C, far from the optimal temperature of > 60 d
egrees C for caa(3), follow a kinetic behavior essentially identical to tha
t observed with the electrostatic complex between mammalian cyt c and cyt c
oxidase. In the O-2 reaction cyt a and cyt a(3) are very quickly oxidized;
cyt a is then re-reduced via Cu,, whereas cyt c oxidation is apparently ra
te-limited by the oxidation of Cu-A, Upon photodissociation of the mixed va
lence-CO caa(3), reverse electron transfer from the binuclear center to cyt
a(3+) (tau(1) = 3 mu s) and Cu-A(2+) (tau(2) = 64 mu s) is observed, while
cyt c is not reduced by any detectable level.
These results seem to rule out accounting for enzymatic thermophilicity by
altered kinetics of intramolecular electron transfer involving the cyt cent
er in the reduced configuration, which is very fast. On the basis of these
results and previous data, we propose that thermophilicity involves an incr
eased activation barrier for the reduction of cyt a(3)-Cu-B in the configur
ation typical of the oxidized site.