The redox centers in the tungsten-containing formate dehydrogenase from Clostridium thermoaceticum were examined by potentiometric titration and electron paramagnetic resonance spectroscopy. At low temperature two overlapping iron-sulfur signals which correlated with enzymatic activity were observed with formal potentials near-400 mV vs. SHE. Based on their temperature dependences, one signal is assigned to a reduced Fe2S2cluster and one to a reduced Fe4S4cluster. Quantitation of signal intensity suggests two Fe2S2and two Fe4S4clusters per formate dehydrogenase molecule. Another signal (g= 2.101, 1.980, 1.950) present in low concentrations at more negative potentials was observable up to 200°K and is not attributed to any iron-sulfur cluster. The possible orgin of this signal is analyzed using ligand field theory, and the redox behavior is considered with respect to possible ligation at the active site. © 1987.
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