Fermentation of biomass-generated synthesis gas: Effects of nitric oxide

The production of renewable fuels, such as ethanol, has been steadily increasing owing to the need for a reduced dependency on fossil fuels. It was demonstrated previously that biomass-generated synthesis gas (biomass-syngas) can be converted to ethanol and acetic acid using a microbial catalyst. The biomass-syngas (primarily CO, CO2, H-2, and N-2) was generated in a fluidizedbed gasifier and T used as a substrate for Clostridium carboxidivorans P7(T). Results showed that the cells stopped consuming H-2 when exposed to biomass-syngas, thus indicating that there was an inhibition of the hydrogenase enzyme due to some biomass-syngas contaminant. It was hypothesized that nitric oxide (NO) detected in the biomass-syngas could be the possible cause of this inhibition. The specific activity of hydrogenase was monitored with time under varying concentrations of H-2 and NO. Results indicated that NO (at gas concentrations above 40 ppm) was a non-competitive inhibitor of hydrogenase activity, although the loss of hydrogenase activity was reversible. In addition, NO also affected the cell growth and increased the amount of ethanol produced. A kinetic model of hydrogenase activity with inhibition by NO was demonstrated with results suggesting there are multiple binding sites of NO on the hydrogenase enzyme. Since other syngas-fermenting organisms utilize the same metabolic pathways, this study estimates that NO < 40 ppm can be tolerated by cells in a syngas-fermentation system without compromising the hydrogenase activity, cell growth, and product distribution.