Calorimetry of Nitrogenase-Mediated Reductions in Detached Soybean Nodules

摘要

Heat evolved by isolated soybean (Glycine max cv Clark) nodules was measured to estimate more directly the metabolic cost associated with the symbiotic N2 fixation system. A calorimeter constructed by modifying standard laboratory equipment allowed measurement on 1 gram of detached nodules under a controlled gas stream. Simultaneous gas balance and heat output determinations were made.There was major heat output by nodules for all of the nitrogenase substrates tested (H⁺, N2, N2O, and C2H2) further establishing the in vivo energy inefficiency of biological N2 fixation. Exposure to a short burst of 100% O2 partially inactivated nitrogenase to permit calculations of heat evolved per mole of substrate reduced. The specific rate of heat evolution for H⁺ reductions was 171 ± 6 kilocalories per mole H2 evolved in an Ar-O2 atmosphere, that for N2 fixation was 784 ± 26 kilocalories per mole H2 evolved and N2 fixed, and that for C2H2 reduction was 250 ± 12 kilocalories/mole C2H4 formed. When the appropriate thermodynamic parameters are taken into account for the different substrates and products, a ΔH′ of −200 kilocalories per mole 2e⁻ is shown to be associated with active transfer of electrons by the nitrogenase system. These values lead to a calculated N₂ fixation cost of 9.5 grams glucose per gram N₂ fixed or 3.8 grams C per gram N₂, which is in close agreement with earlier calculations based on nodular CO₂ production.