Nitrogenase Bioelectrocatalysis: ATP-Independent Ammonia Production Using a Redox Polymer/MoFe Protein System

Nitrogenase is the only biological catalyst that is known to be able to convert nitrogen gas to ammonia. In microorganisms, the MoFe catalytic protein of nitrogenase is reduced by a transient Fe protein binding and separate hydrolysis of ATP. However, the requirement of 16 ATP molecules by the Fe protein for the 8 electron transfer is an energy-intense caveat to the enzymatic synthesis of NH3 and is challenging from an electrochemical perspective. Thus, we report the redox polymer-based ATP-free mediated electron-transfer system of MoFe nitrogenase using cobaltocene-functionalized poly(allylamine) Cc-PAA), which is able to reduce the MoFe nitrogenase directly with a low redox potential of -0.58 V vs SHE. An efficient immobilization of MoFe nitrogenase via Cc-PAA allowed for the bioelectrocatalytic reduction of N-3(-), NO2-, and N-2 to NH3. Bulk bioelectrosynthetic experiments produced 7 +/- 2 and 30 +/- 5 nmol of NH3 from NO2- and N-3(-) reduction for 30 min, respectively. In addition, biosynthetic N-2 reduction to NH3 was confirmed by N-15(2) labeling experiments with NMR analysis. This mediated electrontransfer approach of the immobilized nitrogenase using the Cc-PAA redox polymer provides a valuable technological basis for scaleup and industrial uses in the future of bioelectrosynthesis.