Electrochemical and Spectroscopic Studies of Iron Porphyrin Nitrosyls and their Reduction Products
American Chemical Society
The enzymatic reduction of nitrate to ammonia is catalyzed by a class of enzymes called the assimilatory nitrite reductases. These enzymes, commonly found in plants, contain siroheme and a 4Fe-4S cluster in their native form. Sulfite reductases, which are also able to carry out the reduction of nitrite to ammonia, are large, complex enzymes, but they can be dissociated into a functioning subunit that has a single siroheme and a 4Fe-4S cluster. The catalytic reduction of nitrite proceeds via the formation of a siroheme-nitrosyl complex, which is then reduced to ammonia. The nitrosyl complex has been observed experimentally and is the major species present during turnover. While no other intermediates have been observed in the operating enzyme, hydroxylamine can be reduced by the enzyme, though at a slower rate than nitrite, and is thought to be an intermediate in the reduction. The details of the reduction are not known, and it has been the aim of several research groups to characterize intermediates in the reduction and to elucidate the reduction mechanism using model porphyrin complexes.