The major class of hydrogenases that facilitate energy input via H_2 into respiratory metabolism are the NiFe uptake-type hydrogenases. Although important in bacterial physiology in general, these uptake hydrogenases play a specific role in maintaining the energetic efficiency of symbiotic nitrogen fixation. This is because H_2 is an ample energy source readily available to diazotrophs while fixing N_2, as it is made available due to its inherent production by nitrogenase. Indeed, the energetic (ATP) and reductant input into nitrogenases can in some instances favor H_2 production over NH_3 production. This situation can result in over 50 percent of the reductant through nitrogenase being in H_2 rather than in ammonia. In cases where energy is the limiting factor for N_2 fixation, the ability to use H_2 would seem to be an important attribute for an efficient system.
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