A novel nitrogenase: superoxide-dependent nitrogen fixation.

Streptomyces thermoautotrophicus is able to fix dinitrogen and harbors a completely unusual nitrogen-fixing system that differs from “conventional” nitrogenases. The nitrogen-fixing system of S. thermoautotrophicus is comprised of three proteins, a heterotrimeric molybdenum-containing dinitrogenase (St1), a homodimeric manganese-containing superoxide oxidoreductase (St2), and a heterotrimeric molybdenum-containing carbon monoxide dehydrogenase (CODH or St3). All three proteins of the dinitrogen-fixing system of S. thermoautotrophicus are located in the cytoplasmic fraction and are stable towards oxygen. The fixation of nitrogen is coupled to the oxidation of CO. CODH oxidizes CO and transfers the electrons released to O 2 , thereby producing superoxide. The Mn-superoxide oxidoreductase reoxidizes O 2 - to O 2 and transfers the electrons to a Mo-dinitrogenase which in turn reduces dinitrogen to ammonium. In contrast to the electronic coupling in “conventional” nitrogenases, via ferredoxin/flavodoxin, superoxide oxidoreductase, and dinitrogenase establish a molecular coupling via O 2 - . Superoxide is produced by CO dehydrogenase (St3 protein) through the oxidation of CO and the transfer of the electrons to O 2 . Subsequently, the superoxide is reoxidized by a superoxide oxidoreductase (St2 protein) that delivers the electrons to a dinitrogenase.