CHARACTERIZING THE ROLE OF C. ALBICANS SOD4: AN FE REGULATED CU-ONLY SOD ENZYME

Candida albicans is one of the most prevalent human fungal pathogens. Interestingly, it encodes three Cu-only superoxide dismutase (SOD) enzymes (SOD4, SOD5, SOD6). These Cu-only SODs represent a new class of extracellular SODs that are found throughout the fungal kingdom. C. albicans SOD5 was the first member in this class to be characterized structurally via X-ray crystallography and remains the only member of this class for which a structure exists. Biochemical studies on SOD5 revealed it to be a highly active SOD enzyme. SOD5 is important for both fungal defense against reactive oxygen species and morphogenesis signaling in C. albicans. In addition, SOD5 has been shown to be a virulence factor in rodent models of Candida infection. SOD4 shares a 72% sequence identity with SOD5, but little is known about its unique role in C. albicans biology. In this thesis, I characterized the SOD4 protein and studied its role in C. albicans and related Candida spp. First, I found SOD4 has different surface charges compared to SOD5, but that they are otherwise very similar at the biochemical level (Chapter 2). I found that SOD4, like SOD5 is a highly efficient monomeric SOD enzyme requiring only Cu as a cofactor and has similar Cu binding properties (Chapter 2). C. albicans transcriptionally upregulates SOD4 during iron deprivation, a response that is also demonstrated by other Candida species and may be conserved in the CTG fungal clade (Chapter 2). C. tropicalis, like C. albicans, also produces a burst of ROS during hyphal morphogenesis that coincides with induction of a Cu-only SOD enzyme (Chapter 2). I also determined that in the emerging fungal pathogen C. auris, SOD4 is an active SOD enzyme despite its divergence from the SOD4 and SOD5 sequences of C. albicans (Chapter 2). Currently, there are no published structures of SOD4 proteins. In Chapter 3 of this thesis, I studied SOD4 and SOD5 by NMR to gain insight into their structures in solution and determine the feasibility of future NMR-based assays. Previous in vivo studies indicated that sod5∆/∆ mutants of C. albicans had decreased virulence in the lateral tail vein model of disseminated candidiasis in mice and were incapable of forming biofilms in a central venous rat catheter model. Surprisingly, we found that sod4∆/∆ yeast did not have decreased ability to cause invasive infection in mice and were still capable of producing robust biofilms on rat venous catheters in vivo (Chapter 4). We found there may be some increased immune cell infiltration in these models, and perhaps the loss of SOD4 alters immune cell signaling. This thesis research adds structural and biochemical information to the knowledge of this unique class of enzymes and their role in fungal pathogenesis