Fig. 5.

Oxygen dependence of the rate of Fe²⁺ release from [4Fe-4S] FNR upon oxidation. (A) Reconstituted [4Fe-4S] FNR (2 μM) was mixed with aliquots of buffer B containing varying concentrations of dissolved oxygen at 25°C. Release of Fe²⁺ was monitored with Ferene (100 μM) at 593 nm (in gray). Single- exponential function fits of the data are indicated by solid black lines. Arrows indicate the direction of response with increasing initial oxygen concentrations. (B) Plot of the first observed (first order) rate constants obtained from the data in A and similar experiments as a function of oxygen concentration (filled circles, left ordinate). A least-squares linear fit (black line) of the data is drawn in. The dependence of Fe²⁺ recovered per [4Fe-4S] by Ferene is plotted as a function of the oxygen concentration (open triangles, right ordinate).