Abstract:

Reactive oxygen species such as superoxide, hydrogen peroxide, and the hydroxyl radical can have a direct effect on cells by modifying proteins and DNA, lipid peroxidation, signaling a heat shock response, and inhibiting metabolic pathways. However, further investigations into downstream effects and the way cells responds to oxidative stress hold clues as to how the cells are able to cope with these stresses. In this dissertation, the study of oxidative stress is modeled in the Saccharomyces cerevisiae strain lacking copper zinc superoxide dismutase (sod1 ?). The resulting superoxide increase has effects on metabolism and energy which are manifested as severely inhibited growth under certain conditions.

This dissertation begins by exploring particular medium conditions that affect the growth of sod1 ? yeast. In fermentable carbon sources growth impairment is observed in these sod1 ? yeast cells but in non-repressing carbon sources there is not a significant growth difference between the wildtype and mutant strains. Further investigation revealed an inability of sod1 ? cells to accomplish the diauxic shift, that is, to shift from fermentative to respiratory metabolism as glucose is exhausted. They do not store glycogen and appear to be unable to reuse the ethanol previously released into in the medium. These results suggest that a consistently higher level of energy is required to deal with the superoxide stress. Further evidence for this conclusion comes from chapter three which shows that mutant yeast do better in cases where they do not have to expend energy to acidify their medium and that the sod1 ? mutants contain approximately two-fold more mitochondrial mass than their wildtype counterparts. This discovery is particularly striking because mitochondria are the main source of superoxide production but their increased presence strongly suggests that they are needed to keep up with an increased energy demand on the cell. This highlights the detrimental role among other things that reactive oxygen species may have in decreasing total cellular energy efficiency.