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Conversion of Small Carbon Compounds by Nitrogenase Proteins

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Abstract

Nitrogenases are complex metalloenzymes capable of catalyzing two of the most challenging reactions in Nature: the reduction of atmospheric dinitrogen to ammonia and the reduction of carbon monoxide (CO) to hydrocarbons. The Azotobacter vinelandii molybdenum (Mo) and vanadium (V)-nitrogenases are homologous systems consisting of two components: the reductase component (Fe protein) and the catalytic component (MoFe or VFe protein, respectively). The reductase component contains a [Fe4S4]-cluster, whereas the catalytic component contains two unique metal clusters, the P-cluster and the M or V cluster for Mo- and V-nitrogenase, respectively.

This dissertation focuses on the conversion of small carbon compounds such as the toxic exhaust CO and the greenhouse gas carbon dioxide (CO2) by nitrogenase proteins. The CO2 reducing capability of Mo- and V-nitrogenase was investigated under physiologically relevant assay conditions. Indeed, both nitrogenases reduce CO2 to CO, but only V nitrogenase reduces CO2 to hydrocarbons. These studies demonstrated that V-nitrogenase directly couples CO2 or CO2-derived species to hydrocarbon chains.

Building on this, the efficiency of this reaction was improved by using the strong reductant europium(II) diethylenetriaminepentaacetate, rendering the ATP-dependent electron transfer obsolete and increasing the hydrocarbon chain length as well as the yield.

Excitingly, the reductase component of V- and Mo-nitrogenase alone can reduce CO2 to CO. Strikingly, the reverse reaction, the oxidation of CO to CO2, is catalyzed under oxidizing conditions by Fe protein. The interconversion of CO and CO2 establishes the reductase component housing a [Fe4S4]-cluster as simple Fe/S-based mimic of CO dehydrogenase. The physiological relevance of this reduction is suggested by the observation that A. vinelandii strains expressing only the Fe protein component form CO2-derived CO in vivo.

These discoveries led to the question of whether V-nitrogenase expressing A. vinelandii strains could also in vivo form hydrocarbons from CO or CO2. Indeed, this strain can convert CO to hydrocarbons in vivo. Furthermore, this process is a secondary metabolism since the carbon from CO is not incorporated into the cell mass, but released. Coupling the two in vivo processes could result in a biofuel process that catalytically converts CO2 into combustible hydrocarbon fuels.

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