On the Reduction of Aliphatic and Aromatic Nitro Compounds byClostridia,the Role of Ferredoxin and its Stabilization

Abstract

Crude extracts of several Clostridium spp. reduced aromatic and aliphatic nitro compounds in the presence of H2. Using different clostridia, the uptake of H2 by p-nitrobenzoate was .apprx. 5-10 times faster than by 2-nitroethanol. Structurally different aliphatic nitro compounds showed rates which differed by a factor of < 3. Hydrogenase from C. kluyveri and ferredoxins from Clostridium sp. La 1 and spinach were purified. Combinations of the hydrogenase with each of the ferredoxins catalyzed H2 uptake by nitro compounds. Clostridial flavodoxin also transferred electrons onto nitro compounds. Nitroaryl and nitroalkyl compounds behaved differently with ferredoxin. The 1st reduction step (1-electron transfer) of p-nitrobenzoate led to the nitro radical anion which was detected by EPR. Nitro alkanes seemed to form an unstable radical which decomposed partially to form nitrite. 2-(N-hydroxyimino)- and 2-(N-hydroxyamino)ethanol, a nitrogen radical of 2-(N-hydroxyamino)ethanol, and glycol and 1,4-butanediol were detected as intermediates and side products during the reduction of 2-nitroethnaol to 2-aminoethanol. While the hydrogenase from C. kluyveri was not affected by any reduction intermediate, the ferredoxin from Clostridium sp. La 1 was inactivated by nitrite in a few minutes. Ferrous and sulfide ions in concentrations substoichiometric to that of nitrite stabilized and reactivated the ferrodoxin in the presence of 2-mercaptoethanol. A mechanism for the reduction of aliphatic nitro compounds catalyzed by hydrogenase and ferrodoxin is proposed.