Novel Microbial Based Enzymatic CO 2 Fixation Mechanisms

Abstract

The broad, long term goal of the research is to provide insight into the mechanism of novel carboxylation reactions catalyzed by enzymes involved in bacterial alkene, ketone, and epoxide metabolism. Novel convergent pathways in the model microorganism Xanthobacter autotrophicus have been described in which epoxides produced by alkene oxidation and ketones produced by the oxidation of alcohols are further carboxylated and utilized in energy yielding pathways. Both alkene/epoxide and alcohol/ketone pathways involve distinct carboxylases with unique molecular properties and cofactor requirements that represent alternative mechanisms of CO 2 fixation. The main focus and aims of this project is to provide structural basis for the mechanistic understanding of the reactions catalyzed by 2-ketopropyl coenzyme M oxidoreductase / carboxylase and acetone carboxylase, the terminal steps in the convergent pathways of alkene/epoxide and alcohol ketone metabolism in X. autotrophicus strain Py2. Interestingly, coenzyme M (CoM), the simplest known organic coenzyme, has been shown to have a central role in the pathway of alkene/epoxide carboxylation. Prior to this discovery CoM was thought only to be involved in methanogenesis in archaea. In addition to the carboxylation mechanisms, we are examining the enzymology of the biosynthesis of CoM in bacteria and we have already determined that it is biochemically distinct from that of CoM synthesized by methanogens. The proposed studies will reveal new and unique insights into CO 2 fixation pathways and carboxylation chemistry. We are utilizing a multidisciplinary approach involving kinetic studies, site-specific amino acid substitution studies, and the determination of high-resolution structures of enzymes in the presence of substrate analogs and mechanism base inhibitors to ascertain information concerning the biochemical mechanisms of enzyme catalyzed reactions. Within the context of the mission of the Department of Energy and the core activities of the Energy Biosciences, the results obtained in the study will reveal new insights into these novel carboxylation reactions and will provide the basis for the comparison of the mechanism of these interesting enzymes to other well-characterized biological mechanisms for CO 2 fixation and carboxylation. Knowledge concerning the mechanism of these enzymes provides the basis for an increased fundamental understanding of carboxylation chemistry that could contribute to future mechanisms for CO 2 capture and fixation and, in turn, to mitigating the effects of increasing concentrations of CO 2 on global climate change.