Energetics of Biological Nitrogen Fixation: Determination of the Ratio of Formation of H2 to NH4+ Catalysed by Nitrogenase of Klebsiella pneumoniae in vivo

Abstract

N2 fixation (Nif)-derepressed mutants of K. pneumoniae consumed, under optimum conditions, 7.5-8.5 mol glucose/mol N2 fixed. The nitrogenase system of these mutants catalyzed the production of about 1.3 mol H2/mol N2 reduced. Almost 1/3 of the energy as ATP and reductant used by nitrogenase in vivo may be lost in H2 production, since an ATP/2e [electron] ratio of .apprx. 4 was obtained. Nitrogenase-catalyzed H2 production was not substantially suppressed by increasing the partial pressure of N2 from 0.2 atm (20 kPa[kilopascal]) to 1 atm (101 kPa). In the absence of N2, H2 production catalyzed by nitrogenase increased about 3-fold. Nitrogenase-catalyzed H2 production is of major importance in the overall efficiency of biological N2 fixation in vivo.