Pyruvate:flavodoxin oxidoreductase

Pyruvate flavodoxin oxidoreductase Flavodoxin electron donor to nitrogenase... 

Not only are two molecules of ATP hydrolyzed to pump each electron, but the Fe-protein must receive electrons from a powerful (low E°) reductant such as reduced ferredoxin, reduced flavodoxin, or dithionite. Klebsiella pneumoniae contains a pyruvate flavodoxin oxidoreductase (Eq. 15-35) that reduces either flavodoxin or ferredoxin to provide the low potential electron donor.29 30 In some bacteria, e.g., the strictly aerobic Azotobacter, NADPH is the electron donor for reduction of N2. The Fe-protein is thought to accept electrons from a chain that includes at least the ordinary bacterial ferredoxin (Fd) and a special one-electron-accepting azotoflavin, a flavoprotein that is somewhat larger than the flavodoxins (Chapter 15) and appears to play a specific role in N2 fixation.31 In Clostridium and Rhizobium reduced ferredoxins generated by cleavage of pyruvate reduce nitrogenase directly.32... 

It should be noted that some anaerobic eubacteria (e.g. Escherichia coli, Clostridium acidurici, Chlorobium limicold) possess 2-oxoacid oxidoreductases, either in addition to, or instead of, the dehydrogenase complexes (ref. [37] and references therein). As in the archaebacteria, electrons are transferred to ferredoxin or flavodoxin and the enzymes are small oligomeric proteins. The evolutionary relationships of these enzymes converting pyruvate to acetyl-CoA will be discussed in section 6.2. 

Recently, Wahl and Orme-Johnson reported their studies on the characterization and mechanism of the pymvate flavodoxin (ferredoxin) oxidoreductase from Klebsiella pneumoniae and Clostridium thermoacetium (214). These oxi-doreductases appear to be closely related to that from C. acidiurici (215) in that the iron is present in two Fe4S4 clusters, which act as election acceptors in the catalytic mechanism. However the K. pneumoniae and C. thermoaceticum enzymes may be mechanistically distinct from the H. halobium oxidoreductase (213) in that free radical intermediates are not detected for the former enzymic reaction. EPR signals in the Klebsiella or C. thermoaceticum oxidoreductases are only observed in the fully reduced enzyme when the reductants dithionite or pyruvate and CoASH are present (214). The suggested mechanism for the pyruvate oxidoreductase from K. pneumoniae and C. thermoaceticum is initially similar to the mechanism for all TPP enzymes in that decarboxylation of pyra-vate leads to the formation of hydroxyethyl-TPP. Two one-electron transfers to each of the two Fe-S clusters occur on the binding of CoASH. However, the mechanism for the formation of acetyl-CoA from the hydroxyethyl-TPP intermediate and of the CoASH-induced electron transfers is not yet clear. 

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