FIG 4.

Oxygen reductase activity of wild-type Cbb₃-1 from P. stutzeri ZoBell. Each data point (turnover number) represents the mean value ± standard deviation (SD), which is calculated from at least five independent measurements. (A) pH dependence of the cbb₃-CcO activity was measured with 3 mM ascorbate, 1 mM TMPD, and 8.3 nM cbb₃-CcO by varying the pH from 5.8 to 8.7 and in the presence of 100 mM NaCl in the reaction buffer (50 mM Tris-HCl, 50 μM EDTA, 0.02% [wt/vol] DDM). The indicated pH was measured after the addition of ascorbate and TMPD, prior to the reaction initiation. (B) Dependence of the cbb₃-CcO activity on the concentration of NaCl (0 to 500 mM), measured under the same conditions as mentioned above, while maintaining the pH at 7.5. (C) Dependence of the cbb₃-CcO activity on the molar ratio of ascorbate to TMPD (2:1 to 10:1), measured in the presence of 100 mM NaCl at pH 7.5. (D) Dependence of the cbb₃-CcO activity on the concentration of TMPD. The dotted line shows a nonlinear regression fit of the experimental data according to the Michaelis-Menten equation.