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FCR activity of inner envelope of chloroplast vesicles isolated from Fe-deficient plants shows clear biphasic kinetics. Using NADPH, biphasic saturation kinetics are measured with the first saturation found at KM = 0.0046 mM Fe(III)-EDTA, and the second saturation found at KM = 0.0557 mM Fe(III)-EDTA. In the presence of NADH, the saturation kinetics is monophasic, the sole saturation reaction rate is significantly lower, and the KM value of the reaction is significantly higher with 0.0834 mM Fe(III)-EDTA compared to the second saturation measured with NADPH. In vesicles isolated from Fe-sufficient plants, the affinity of the enzyme does not change with KM = 0.0042mM Fe(III)-EDTA. The second, low-affinity saturation point, KM = 0.155 mM Fe(III)-EDTA is significantly higher than that in Fe-sufficient plants, with a reaction rate that is also somewhat higher than in the envelope of chloroplast membranes of Fe-sufficient plants |
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