Hydrogenase I of Clostridium pasteurianum functions as a novel selenite reductase

Clostridium pasteurianum's hydrogenase I, an important constitutive metabolic enzyme, has been shown to function as a ‘novel selenite reductase’. Selenite reductase activity was found to co-purify with hydrogenase I activity; the fold purification and specific activities for these two activities paralleled each other throughout the purification steps. The highly purified hydrogenase I apparent K m for the selenite substrate was 0.2 mM. The stoichiometry for the enzymatic reduction of SeO 3 staggered2− to Se 0 via H 2 oxidation, was determined to be 2.3:1 (H 2:Se 0), very close to the theoretical ratio of 2:1 for this reduction reaction. Known electron carriers required for hydrogenase I activity were also found to couple its selenite reductase activity, the most efficient one being ferredoxin. The purified hydrogenase I not only reduced selenite but also tellurite, and its selenite activity was completely inhibited by O 2 and CuSO 4, potent inhibitors of hydrogenase I activity.