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, CO(2), H(2), and N(2)) was generated in a fluidized-bed gasifier and 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.