Thermal inactivation of reduced ferredoxin (flavodoxin):NADP+ oxidoreductase from Escherichia coli.

Ferredoxin (flavodoxin):NADP+ oxidoreductase (FNR) is an essential enzyme that supplies electrons from NADPH to support flavodoxin-dependent enzyme radical generation and enzyme activation. FNR is a monomeric enzyme that contains a non-covalently bound FAD cofactor. We report that reduced FNR from Escherichia coli is subject to inactivation due to unfolding of the protein and dissociation of the FADH(2) cofactor at 37 degrees C. The inactivation rate is temperature-dependent in a manner that parallels the thermal unfolding of the protein and is slowed by binding of ferredoxin or flavodoxin. Understanding factors that minimize inactivation is critical for utilizing FNR as an accessory protein for S-adenosyl-L-methionine-dependent radical enzymes and manipulating FNR as an electron source for biotechnology applications.