The effect of electron carriers and other ligands on oxygen stability of clostridial hydrogenase.

The effects of various electron carriers, a substrate (H2) and a reversible inhibitor (CO) on the rate of irreversible oxygen inactivation of clostridial hydrogenase (ferredoxin: H+ oxidoreductase, EC 1.18.3.1) have been studied kinetically. Some electron carriers (e.g., clostridial ferredoxin and methyl viologen) greatly stabilize the enzyme, some (FAD, FMN) drastically reduce its stability, while others (benzyl viologen and methylene blue) only slightly alter the stability. Competitive experiments indicate that stabilizers and destabilizers do not compete with each other for binding with the active center of hydrogenase. Hydrogen and CO do not affect the rate of the oxygen inactivation. On the basis of the results obtained herein and kinetic data on hydrogenase catalysis from the literature, it is concluded that the active center of this hydrogenase comprises at least three different independent subsites. The first one (presumably an iron atom of the iron-sulfur cluster) binds H2 and CO and does not contribute to the oxygen stability. The second one binds stabilizers like methyl viologen while the third one binds destabilizers like FMN and FAD.