Molecular genetics of superoxide dismutases in yeasts and related fungi

This chapter discusses the molecular genetics of superoxide dismutases in yeasts and related fungi. The chapter provides information on various enzymes that are responsible for the establishing themselves as first component of defense mechanism, such as the superoxide dismutases. These enzymes catalyze the disproportionation of O₂^-, to H₂O₂ and O₂. As discussed in the chapter, eukaryotes contain at least two superoxide dismutases that are catalytically equivalent but evolutionarily, genetically, and structurally distinct. Superoxide can be produced in a wide variety of cellular redox processes. The simple type of reaction that can generate O₂^- is auto-oxidation. In Saccharomyces cerevisiae, the dominant source of O₂^- appears to be leakage from the mitochondrial electron transport chain. These enzymes are also functionally distinct because these are found in different cell compartments. The regulation of expression of these enzymes is also discussed in the chapter along with the discussion of their physiologic functions in light of the phenotypes of strains of yeast and fungi that lack either or both activities. In addition to these enzymes, an extracellular Cu,ZnSOD is characterized in eukaryotes. The human enzyme, designated ECSOD1, is a secreted glycoprotein containing Cu and Zn; the gene for this SOD has been cloned. Although ECSOD and the intracellular SODl differ in primary sequence, they share a number of structural homologies with respect to the metal centers. Some evidences for this type of dismutase in S. cerevisiae and N. crassa are also presented in the chapter.