What is the true nitrogenase reaction? A guided approach

Only diazotrophic bacteria, called Rizhobia, living as symbionts in the root nodules of leguminous plants and certain free-living prokaryotic cells can fix atmospheric N-2. In these microorganisms, nitrogen fixation is carried out by the nitrogenase protein complex. However, the reduction of nitrogen to ammonia has an extremely high activation energy due to the stable (unreactive) NN triple bond. The structural and functional features of the nitrogenase protein complex, based on the stepwise transfer of eight electrons from reduced ferredoxin to the nitrogenase, coupled to the hydrolysis of 16 ATP molecules, to fix one N-2 molecule into two NH3 molecules, is well understood. Yet, a number of different nitrogenase-catalyzed reactions are present in biochemistry textbooks, which might cause misinterpretation. In this article, we show that when trying to balance the reaction catalyzed by the nitrogenase protein complex, it is important to show explicitly the 16 H+ released by the hydrolysis of the 16 ATP molecules needed to fix the atmospheric N-2 (c) 2015 by the International Union of Biochemistry and Molecular Biology, 43(3):142-144, 2015.