Electron flow and energy conservation in hydrogenotrophic methanogenesis

Abstract

Methanogenesis is a globally important process responsible for the generation of >90% of the CH4 present on Earth. Despite this importance, key biochemical details concerning methanogenesis have eluded characterization. Presented here is a global analysis of the bioenergetics and substrate utilization in hydrogenotrophic methanogenesis: the reduction of CO2 to CH4. It was long thought that hydrogenotrophic methanogenesis proceeded linearly; however, this model fails to take into account how energy conservation occurs. The first step of the pathway is endergonic, but the source of energy to power this reaction was unknown. Results presented here show that the first and last steps are physically and energetically coupled. Hence the methanogenesis pathway is cyclic rather than linear. This presents another problem in that all metabolic cycles must replenish intermediates to guard against decaying flux. This replenishing reaction is facilitated by a membrane-bound hydrogenase activity. Finally, methanogens are thought to require hydrogen for growth. By generating a model for methanogenesis, I show this is not the case: hydrogenotrophic methanogens are capable H2- independent growth in the presence of alternative substrates such as formate. Taken together, these data provide for a model for the biological generation of CH4 through the hydrogenotrophic pathway. Although these data generate a molecular a model for methanogenesis, key details about how the process is regulated remain a mystery. Later chapters of this work describe the early stages of an experimental system to identify elements essential for the regulation and expression of methanogenesis genes.