Further physicochemical studies on the complex formation between iron-sulfur proteins and flavoproteins from spinach...

Physicochemical experiments were performed in order to get further evidence of the formation of complexes between iron-sulfur proteins and flavoproteins from spinach chloroplast and beef adrenal cortex electron-transfer systems. With both spinach and adrenal iron-sulfur protein-flavoprotein systems, characteristic difference circular dichroism (CD) spectra between the sum of CD spectra of the respective proteins and the CD spectrum of a mixture with a molar ratio of one to one were obtained in the visible and far-ultraviolet regions. By differential scanning calorimetry (DSC) measurements, ferredoxin and ferredoxin-NADP reductase were found to be stabilized against heat treatment upon mixing: (1) the apparent denaturation temperatures of ferredoxin and the reductase increased by about 18 and 5 degree C, respectively, and (2) the activation energy of the denaturation reaction increased by about 50%. These results lead to the conclusion that the iron-sulfur protein and flavoprotein from a tightly bound complex, and suggest that the complex formation alters both the environments of the iron-sulfur and flavin chromophores and the contents of the ordered structures, resulting in stabilization of the proteins.