Energetics of biological nitrogen fixation: determination of the ratio of formation of H2 to NH4+ catalysed by nitrogenase of Klebsiella pneumoniae in vivo.

Nitrogen fixation (Nif)-derepressed mutants of Klebsiella pneumoniae consumed, under optimum conditions, 7.5 to 8.5 mol glucose per mol N2 fixed. The nitrogenase system of these mutants catalysed the production of about 1.3 mol H2 per mol N2 reduced. Almost one-third of the energy as ATP and reductant used by nitrogenase in vivo may be lost in H2 production, since an ATP/2e ratio of approximately 4 was obtained. Nitrogenase-catalysed H2 production was not substantially suppressed by increasing the partial pressure of N2 from 0.2 atm (20 kPa) to 1 atm (101 kPa). In the absence of N2, H2 production catalysed by nitrogenase increased about threefold. It is concluded that nitrogenase-catalysed H2 production is of major importance in the overall efficiency of biological N2 fixation in vivo.