Rubredoxin from the green sulfur bacterium Chlorobium tepidum functions asan electron acceptor for pyruvate ferredoxin oxidoreductase

Rubredoxin (Rd) from the moderately thermophilic green sulfur bacterium Chl orobium tepidum was found to function as an electron acceptor for pyruvate ferredoxin oxidoreductase (PFOR), This enzyme, which catalyzes the conversi on of pyruvate to acetyl-CoA and CO2, exhibited an absolute dependence upon the presence of Rd, However, Rd was incapable of participating in the pyru vate, synthase or CO2 fixation reaction of C. tepidum PFOR, for which two d ifferent reduced ferredoxins are employed as electron donors, These results suggest a specific functional role for Rd in pyruvate oxidation and provid e the initial indication that the two important physiological reactions cat alyzed by PFOR/pyruvate synthase are dependent on different electron carrie rs in the cell, The W-visible spectrum of oxidized Rd, with a monomer molec ular weight of 6500, gave a molar absorption coefficient at 492 nm of 6.89 mM(-1) cm(-1) with an A(492)/A(280) ratio of 0.343 and contained one iron a tom/molecule. Further spectroscopic studies indicated that the CD spectrum of oxidized C. tepidum Rd exhibited a unique absorption maximum at 385 nm a nd a shoulder at 420 nm, The EPR spectrum of oxidized Rd also exhibited unu sual anisotropic resonances at g = 9.675 and g = 4.322, which is composed o f a narrow central feature with broader shoulders to high and low field, Th e midpoint reduction potential of C. tepidum Rd was determined to be -87 mV , which is the most electronegative value reported for Rd from any source.