Electron transport components from methanogenic bacteria: The ferredoxin from Methanosarcinabarkeri (strain Fusaro)

A ferredoxin has been isolated from the methanogenic organism $\text{Methanosarcina}\text{barkeri}$ (strain Fusaro). The protein appears to be constituted by two identical subunits of molecular weight approx. 6000 daltons. The UV-visible spectrum of the protein is characterized by two broad absorption peaks centered at 410 and 300 nm and an absorbance ratio $\text{A}{}_{410}\text{A}{}_{300}\text{= 0.8}$. The molar extinction coefficients at 410 and 300 nm are 36,500 and 45,625 M^?1 cm^?1, respectively. The amino acid compsition of $\text{M.}\text{barkeri}$ ferredoxin shows a preponderance of acidic residues and lacks five amino acids. The protein contains 8 cysteine residues and approx. 7 iron atoms and 7–8 acid-labile sulfide groups per molecule which are indicative of the presence of two iron-sulfur clusters in the molecule. The N-terminal sequence shows a high degree of homology with the sequences of ferredoxins from $\text{Clostridium}\text{pasteurianum}\text{,}\text{Desulfovibrio}\text{gigas}$ and $\text{Desulfovibrio}\text{africanus}\text{.}\text{M.}\text{barkeri}$ ferredoxin functions as an electron carrier in the pyruvate dehydrogenase system. Its possible role in a variety of electron transfer reactions is discussed.