Investigation of nitrous oxide biosynthesis by a bacterial nitric oxide reductase (NOR) and an engineered NOR mimic using stable isotope ratio mass spectrometry

While carbon dioxide (CO2) is the most prevalent greenhouse gas, nitrous oxide (N2O) is far more potent, with a global warming potential ~265 times greater than that of carbon dioxide over a 100-year period.1 Additionally, N2O is capable of destroying ozone, making it doubly concerning as a greenhouse gas. Approximately half of the N2O produced yearly is from anthropogenic sources. The largest contributor to anthropogenic N2O is the over-fertilization of agricultural soils, which fuels a host of microbial nitrogen cycling processes that produce N2O. One of these processes is denitrification, and N2O is known to be an obligate intermediate in this process. In denitrification, N2O is synthesized by an enzyme known as nitric oxide reductase (NOR). A thorough understanding of the enzymatic mechanisms by which N2O is produced is essential to mitigating anthropogenic N2O emissions. To this end, this thesis contains the examination of N2O produced by a bacterial cytochrome c NOR (cNOR) from Paracoccus dentrificans and a cNOR mimic, I107EFeBMb, using stable isotope ratio mass spectrometry (IRMS). The first chapter provides the reader with an introduction to the nitrogen cycle, the known NORs, and the basics of isotope theory. The studies on the native cNOR and I107EFeBMb are contained in the second and third chapters, respectively. Conclusions and future directions are presented in the final chapter.