The Structure of a Clostridial Flavodoxin, an Electron-transferring Flavoprotein. III. An Interpretation of an Electron-Density Map at a Nominal Resolution of 3.25 A

Within the last five years, several low molecular weight flavoproteins have been isolated from microorganisms. The first of these proteins was purified from Clostridium pasteurianum grown in limiting concentrations of iron (Knight and Hardy, 1966) and was named flavodoxin since it substituted for the non-heme iron protein, ferredoxin, in ferredoxin-dependent reactions (Knight and Hardy, 1966; 1967). Subsequently flavodoxins have been isolated from other strict anaerobes including Peptostreptococcus elsdenii (Mayhew and Massey, 1969), Clostridium MP, a strain closely related to C. pasteurianum (Mayhew, 1971a), and two species of Desulfovibrio (Dubourdieu and Le Gall, 1970), and also from E. coli grown under aerobic conditions (Vetter and Knappe, 1971). All the flavodoxins contain a single FMN and no other prosthetic groups or metals have been detected (Knight and Hardy, 1966; Mayhew and Massey, 1969). They can replace ferredoxins as electron carriers in the phosphoroclastic reaction of pyruvate, the fixation of nitrogen, the photosynthetic...