An Approach Based on Quantum Chemistry Calculations and Structural Analysis of a [2Fe-2S*] Ferredoxin That Reveal a Redox-Linked Switch in the Electron-Transfer Process to the Fd-NADP+ Reductase

[2Fe-2S*] ferredoxins act as electron carriers in photosynthesis by mediating the transfer of electrons from photosystem I to various enzymes such as ferredoxin:NADP+:reductase (FNR). We have analyzed by density functional theory the possible variations of the electronic properties of the [2Fe-2S*] ferredoxin, from the cyanobacterium Anabaena, depending on the redox-linked structural changes observed by X-ray diffraction at atomic resolution (Morales, R.; et al. Biochemistry 1999, 38, 15764−15773). The present results point out a specific and concerted role of Ser47, Phe65, and Glu94 located at the molecule surface, close to the iron−sulfur cluster. These residues were already known to be crucial for efficient electron transfer to FNR (e.g., Hurley, J. K.; et al. Biochemistry 1997, 36, 11100−11117). Our calculations suggest that the Glu94 carboxylate negative charge regulates the electron charge delocalization between the Ser47 CO group and the Phe65 aromatic ring, depending on the redox state. The Glu94 ...