An alternative enzyme to ferredoxin in accepting an electron from Photosystem I is flavodoxin, which also participates in a broad range of other electron transfer reactions. The prosthetic group in flavodoxin is a flavin mononucleotide (FMN- . Interestingly, the use of flavodoxin is always preferred in iron limited environments [6, 7] -however, replacement of ferredoxin with flavodoxin has only been recognized in a small number of published papers. In contrast to FNR structures with ferredoxin, cocrystallization of flavodoxin with FNR has not been successful. Therefore, the best knowledge is about the structures of only flavodoxin and FNR alone. A homology model has been built for the purpose of visualization. The highest resolution structures of flavodoxin are at 1.2 to 1.35 A, from Anabaena [8] and Desulfuvibrio [9]. Depicted in Figure 15.3 is the flavodoxin from Anabaena (PDB accession code 1OBO). In this cartoon representation, the 5 alpha helices are like poles in a picket fence in relationship to the 5 beta sheets in each monomer. Two monomers form the asymmetric unit. The newest kinetic studies, performed with Anabaena for flavodoxin mediated electron transfer to Photosystem I, are from Goni and coworkers, 2008 [10] .

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