Fig. 4.

Oxygen dependence of cluster conversion measured by optical absorbance. (A) Reconstituted [4Fe-4S] FNR (5 μM) was mixed with aliquots of buffer B containing varying concentrations of dissolved oxygen at 25°C. Loss of the [4Fe-4S] cluster was monitored at 420 nm as a function of time (in gray). Double- 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). A least-squares linear fit (black line) of the data is drawn in.