FIG. 3.

O₂ consumption in RsL-catalyzed oxidation of NHA at pH 5.5. Plotted against the initial NHA concentration [NHA] are the initial O₂ consumption rate (as the output voltage change rate) (A) and the final O₂ consumed (as the final output voltage change) (B). For graph clarity, two sets of data, obtained with 0.05 and 0.3 mM NHA, are omitted from panel B. Their symbols overlap with those for 0 and 0.33 mM NHA (shown), and their values were included in the graph fitting. In panel A, the solid curve shows the fit to the Michaelis-Menten equation {v = −V_max[NHA]/(K_m + [NHA])} with a K_m of 2.0 ± 0.5 mM and a V_max of 0.39 ± 0.03 V min⁻¹ or 0.19 ± 0.01 mM min⁻¹ (corresponding to a k_cat of 150 ± 10 min⁻¹ (mean ± standard deviation). In panel B, the horizontal broken line represents the voltage change (0.58 ± 0.03 V, averaged over the data obtained with 3.3, 5.0, and 6.7 mM NHA) corresponding to maximal O₂ consumption. Its cross point with the other broken line (voltage change, 0.5 × [NHA]; r², 0.92), obtained by fitting the data obtained with 0, 0.05, 0.30, and 0.33 mM NHA, yielded a saturating [NHA] of 1.2 mM. By dividing 1.2 mM by 0.28 mM, the dissolved [O₂] in water, we estimated an oxidation stoichiometry number of 4.1.