Syngas fermentation to biofuels: Effects of ammonia impurity in raw syngas on hydrogenase activity

Hydrogenase activity plays an important role in the fermentation of biomass-generated syngas (containing CO, CO2, and H2) to obtain ethanol and other biofuels. One process efficiency issue for producing biofuels from syngas fermentation is the ability of key cellular enzymes to produce reducing equivalents from syngas. For microbes using the Wood–Ljungdahl pathway, inhibition of the hydrogenase enzyme would decrease reducing equivalent production from H2, thus potentially reducing carbon conversion efficiency and biofuel production. During biomass gasification to produce syngas, several impurities are generated. These impurities, such as ammonia (NH3), can potentially impact the fermentation process. In this work, it was shown that NH3 rapidly converts to ammonium ion (NH4+) following exposure of fermentation media to NH3. The accumulated NH4+ also inhibited hydrogenase activity and cell growth. A kinetic model for hydrogenase activity that included inhibition effects from NH4+ was developed. Model parameters included KH2KH2 (Michaelis–Menten constant) and KNH4+ (the inhibition constant for NH4+). Experimental results showed that NH4+ is a non-competitive inhibitor for hydrogenase activity with KNH4+ of (649 ± 35) mol m−3. As part of the work to distinguish the unique aspect of NH4+ inhibition, additional work showed that potassium and phosphate ions had no effect on hydrogenase activity. Since NH4+ can easily be accumulated in fermentation media and transport across the cell membrane, it is necessary to remove NH3 impurity from raw syngas to minimize the reduction in hydrogenase activity.

► Ammonium ion (NH4+) can accumulate significantly in media as a result of ammonia impurity in syngas.
► Ammonium ion non-competitively inhibits hydrogenase with an inhibition constant (Ki) of (649 ± 35) mol m−3.
► Ammonium ion (NH4+) accumulated in media can inhibit cell growth.
► Ammonia impurity should be removed from syngas to maximize hydrogenase activity and product formation.