The Conversion of Carbon Monoxide and Carbon Dioxide by Nitrogenases

Nitrogenases are the only known family of enzymes that catalyze the reduction of molecular nitrogen (N2 ) to ammonia (NH3 ). The N2 reduction drives biological nitrogen fixation and the global nitrogen cycle. Besides the conversion of N2 , nitrogenases catalyze a whole range of other reductions, including the reduction of the small gaseous substrates carbon monoxide (CO) and carbon dioxide (CO2 ) to hydrocarbons. However, it remains an open question whether these 'side reactivities' play a role under environmental conditions. Nonetheless, these reactivities and particularly the formation of hydrocarbons have spurred the interest in nitrogenases for biotechnological applications. There are three different isozymes of nitrogenase: the molybdenum and the alternative vanadium and iron-only nitrogenase. The isozymes differ in their metal content, structure, and substrate-dependent activity, despite their homology. This minireview focuses on the conversion of CO and CO2 to methane and higher hydrocarbons and aims to specify the differences in activity between the three nitrogenase isozymes.