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

Abstract

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 N3 –, NO2 –, and N2 to NH3. Bulk bioelectrosynthetic experiments produced 7 ± 2 and 30 ± 5 nmol of NH3 from NO2 – and N3 – reduction for 30 min, respectively. In addition, biosynthetic N2 reduction to NH3 was confirmed by 15N2 labeling experiments with NMR analysis. This mediated electron-transfer approach of the immobilized nitrogenase using the Cc-PAA redox polymer provides a valuable technological basis for scale-up and industrial uses in the future of bioelectrosynthesis.

Insights into Nitrogenase Bioelectrocatalysis for Green Ammonia Production

“…However, when a CdS nanorod replaced the Fe‐protein, lower turnover frequencies were reported (75 min −1 ) . In other reports, using only the MoFe protein, the turnover frequencies are significantly slower 0.1–0.6 min −1 . Therefore, further work is required to improve the rate of catalysis using only the MoFe‐protein.…”

“…Recently there have been reports of two separate redox mediators, cobaltocene and neutral red being tethered to a polymer for which mediated bioelectrolysis of N 2 reduction to NH 3 was demonstrated.…”

Recent Progress in Applications of Enzymatic Bioelectrocatalysis

“…N 2 ase reduces not only N 2 but also nitrite (NO 2 − ) and azide (N 3 − ) to NH 3 on electrodes, and the MoFe protein disassociated with the Fe protein shows the DET-type bioelectrocatalytic activity without ATP [154]. Furthermore, ATP-independent NH 3 bioelectrosynthesis was improved in the MET-type system using N 2 ase and a cobaltocene-functionalized polymeric mediator [51]. On the other hand, Milton et al reported a NH 3 -producing H 2 /N 2 biofuel cell using MET-type reactions of N 2 ase and H 2 ase [148].…”

“…In addition, once both enzymes and mediators are stably immobilized on electrodes, the measurement systems work as pseudo-DET-type systems [10,11,36]. Particularly, redox polymers anchoring osmium complexes [3,[37][38][39][40][41][42][43][44][45], ferricyanide [46,47], metallocenes [48][49][50][51], and viologen units [52][53][54][55] are constructed as polymeric mediators immobilized on electrodes. In summary, a DET-type system is often more ideal than a MET-type system, whereas it seems to be practical to utilize an MET-type system for several objectives.…”

Gemischte Photosystem‐I‐Monoschichten ermöglichen einen verbesserten anisotropen Elektronenfluss in Biophotovoltaik‐Systemen durch Unterdrückung elektrischer Kurzschlüsse

“…Die Redoxpolymere wurden ausgewählt, da sie in der Lage sind, Enzyme, die interessante Reduktionsreaktionen katalysieren, [22] elektrisch an Elektroden zu koppeln. Dies wurde kürzlich für P‐vio in Kombination mit Hydrogenasen [12, 23] sowie für verschiedene Cobaltocen‐modifizierte Polymere in Kombination mit Hydrogenasen, [7, 24] Formiatdehydrogenase, [25] Diaphorase für die NADH‐Regeneration [26] und Nitrogenase gezeigt [27] . Die Photostromantwort wurde unter Verwendung von Umgebungs‐O 2 als terminalem Elektronenakzeptor gemessen, wobei die polymergebundenen Redoxzentren einen effizienten Elektronentransfer mit dem terminalen Fe‐S‐Cluster am PSI ermöglichten.…”