Formate dehydrogenase from Clostridium pasteurianum

Formate dehydrogenase from Clostridiun pasteurianum catalyses the interconversion of CO2, and formate. It is a complex enzyme, containing molybdenum, iron, acid‐labile sulfur and pterin. Redox potentiometry and EPR analysis reveal two spectrally and thermodynamically distinct iron‐sulfur clusters, with g 1.87, 1.95 and 2.05 (Em8 ‐318 mV) and g 1.92 and 2.05 (Em8 ‐372 mV), both present at approx. 0.3 spins per enzyme. There is also a free radical signal centered near g 2.005 which we attribute to the pterin, perhaps in its anionic semiquinone form. This signal disappears on oxidation (Em8 ‐150 mV), but is not reduced further at pH 8. It is present at approx. 0.8 spins per enzyme. No EPR signals attributable to the molybdenum were detected.