Cellular Accommodation of Nitrogenase

Abstract

The nutritional flexibility of most organisms permits them to utilize a variety of different nitrogen sources, and adaptation to a particular source usually requires only relatively modest modification in cellular physiology and biochemistry. However, when the source nutrient is dinitrogen, the changes involved in adaptation can be particularly far reaching, with profound effects not only on the physiology and biochemistry of organism, but on its structure as well. Perhaps most striking in this context are the adaptations of symbiotic systems, which produce complex anatomical structures, such as root nodules, whose sole purpose is to utilize N₂. Even among the simplest N₂-fixing organisms, the bacterial diazotrophs, significant modifications may occur. The large amount of nitrogenase produced is itself noteworthy, with the enzyme comprising 5% or more of readily extractable cell protein. Auxillary proteins are produced and the ATP demands of N₂ fixation may require different or accelerated energy metabolism. Moreover, the sensitivity of nitrogenase to O₂ may necessitate specialized metabolism or structures to protect the enzyme from O₂. In this chapter observations dealing with the impact of N₂ fixation on cell structure and function will be summarized and discussed.