Abstract: | Diazotrophic systems have developed a number of strategies to protect nitrogenase (N2ase; EC 1.18.6.1) from O2 excess and active-oxygen species (AOS). Protection against O2 excess is given by biochemical modifications of N2ase, increased rates of low-efficiency respiration, temporal segregation of N2 fixation and photosynthesis, physical barriers to O2 diffusion, and hemoglobins. On the other hand, AOS may originate from oxidation of N2ase components, ferredoxins, flavodoxins and hemoglobins; interaction among the AOS themselves, or between H2O2 and hemoglobins; and during reactions catalyzed by hydrogenase (EC 1.18.99.1), xanthine oxidase (EC 1.1.3.22) and uricase (EC 1.7.3.3). Active-oxygen species are scavenged enzymatically superoxide dismutase (EC 1.15.1.1), catalase (EC 1.11.1.6). peroxidase (EC 1.11.1.7), ascorbate peroxidase (EC 1.11.1.11)] or through non-enzymic reaction with low-molecular-weight compounds (ascorbate, α-tocopherol, glutathione). |