Reconstitution of Azotobacter vinelandii ferredoxin I as a {2[4Fe‐4S]1+/2+} protein

As normally isolated, Azotobacter vinelandii ferredoxin I (Fd I) is a {[4Fe‐4S],[3Fe‐3S]} protein: (7Fe)Fd I. We report that anaerobic reconstitution of Fd I from its apoprotein yields a protein whose spectra are distinct from those of (7Fe)Fd I and typical of bacterial ferredoxins. We identify this new form of Fd I as a {2[Fe‐4S]} protein: (8Fe)Fd I. (8Fe)Fd I is unstable in air and decomposes to give a ∼ 10% yield of (7Fe)Fd I. These results increase the probability that (8Fe)Fd I is the form of Fd I occurring in vivo and that (7Fe)Fd I results from oxidative degradation during purification.