(2e) Hydrogenase in Acetogenic Production of Fuel-Grade Ethanol

Volatile oil prices, insufficient domestic resources, and global depletion of fossil fuels have led to a need for alternative energy sources. Ethanol is one of the alternatives, with many advantages. It is renewable and carbon neutral, it can be produced domestically, and it has great potential for low-cost production.

Despite the skepticism of ethanol produced from corn, research in this field has led to other methods of production. One of these methods is cellulosic ethanol production. The cellulose and hemi cellulose in plants can be converted to ethanol through various processes, such as saccharification followed by enzymatic conversion, or gasification followed either by metal catalysis or fermentation. This study investigates gasification followed by fermentation, a process that uses a combination of carbon dioxide, carbon monoxide, and hydrogen, also known as syngas. Syngas is obtained by gasifying biomass such as wood chips or prairie grass. These sources can be grown on marginal land and offer potential for an energy input to output ratio of 1:6 or more.

Anaerobic bacteria called acetogens carry out this process by following the Wood-Ljungdahl metabolic pathway, which includes two anabolic processes: acetogenesis and solventogenesis. Acetogenesis produces acetic acid, while solventogenesis produces ethanol. The two processes do not happen simultaneously, but rather acetogenesis preceeds solventogenesis. It is therefore necessary to determine the factors that cause the switch from acetogenesis to solventogenesis. Reduction of carbon to an alcohol requires four more electrons than reduction to a carboxylic acid, so some source of these electrons is necessary.

One of the key enzymes in acetogens which can provides electrons is hydrogenase. Hydrogenase catalyzes the oxidation of hydrogen gas to protons and electrons. These electrons can then be used to reduce Acetyl CoA to ethanol. Tthe activity of hydrogenase was measured over time using benzyl viologen (BV) as an artificial electron acceptor. The ethanol and acetic acid concentrations and the pH in connection with hydrogenase activity was also measured. It was found that as media pH decreased, hydrogenase became more active, more ethanol was produced, and acetic acid production stopped. Additionally, the pH gradient across the cell membrane was measured. This information is valuable for optimizing conditions that will enhance ethanol production.