The role of diffusion in oxygen protection of nitrogenase in nodules of Alnus rubra

Abstract

Endophyte respiration in actinorhizal nodules has two main functions: to provide ATP and reductant for nitrogenase and associated enzymes and to protect nitrogenase from inactivation by oxygen. Combined with a diffusion path which limits the flow of oxygen, the consumption of oxygen by respiration maintains a very low internal [Formula: see text]. We have used a continuous-flow root-nodule gas-exchange system to investigate the properties of such a diffusion path in intact, attached nodules of Alnus rubra infected with Frankia strain ArI3. When nodules were exposed to gas mixtures containing argon, oxygen, and acetylene, the resulting curves of ethylene production versus acetylene concentration showed diffusion-limited enzyme kinetics. Consequently, ethylene production at very low acetylene concentrations (<0.2% v/v) was used to characterize the diffusion path that both acetylene and oxygen must traverse to reach their active sites in the endophyte. The permeability of this path changed markedly with temperature and can account for the shape of the temperature response curve for total nitrogenase activity. Owing to the temperature-dependent barrier, energy production remains limited by oxygen diffusion, and nitrogenase can function over a wide temperature range at a relatively constant energy cost without inactivation by excess oxygen.