37 - Degradation of Cellulose through Hydrogen Metabolism and Electron Bifurcation

Abstract

Cellulose, one of the most abundant polymers on earth, is difficult to break down. Select anaerobic bacteria are capable of metabolizing cellulose into glucose, then splitting the components apart further through glycolysis to produce ATP. During glycolysis, NAD+ is reduced to NADH and cannot continue acting as an electron carrier until NADH returns to an oxidized state. Though a hydrogen producing pathway is the preferred method of electron carrier recycling, an overabundance of hydrogen can impede the cycle. Anerobic organisms in nature have developed efficient pathways to maximize metabolic energy yields when exposed to little hydrogen. Bacteria present in cellulose degrading ecosystems like compost piles, leaf litter, pine straw, and pond sediment break down cellulose through a process called electron bifurcation. In electron bifurcation, enzymes are characterized by having two electron donors and one receptor or one donor and two accepters, which allows for exergonic and endergonic reactions to take place and limit free energy loss. When ATP production is inhibited by the accumulation of high hydrogen levels, the bifurcating enzymes switch to fermentative pathways that will allow them to continue a metabolic pathway at the expense of the products changing. In this study, we will examine the metabolic pathways taken by the bifurcating enzymes and seek to determine ways in which the metabolic pathways taken are influenced by low and high hydrogen concentrations.