Tsa1 is the dominant peroxide scavenger and a source of H₂O₂-dependent GSSG production in yeast

Hydrogen peroxide (H 2 O 2 ) is an important biological molecule, functioning both as a second messenger in cell signaling and, especially at higher concentrations, as a cause of cell damage. Cells harbor multiple enzymes that have peroxide reducing activity in vitro . However, the contribution of each of these enzymes towards peroxide scavenging in vivo is less clear. Therefore, to directly investigate in vivo peroxide scavenging, we used the genetically encoded peroxide sensors, roGFP2-Tsa2ΔC R and HyPer7, to systematically screen the peroxide scavenging capacity of yeast thiol and heme peroxidase mutants. We show that the 2-Cys peroxiredoxin Tsa1 alone is responsible for almost all exogenous H 2 O 2 and tert -butyl hydroperoxide scavenging. The two catalases and cytochrome c peroxidase only produce observable scavenging defects at higher H 2 O 2 concentrations when these three heme peroxidases are deleted in combination. We also analyzed the reduction of Tsa1 in vitro , revealing that the enzyme is efficiently reduced by thioredoxin 1 with a rate constant of 2.8×10 6 M −1 s −1 . When thioredoxins are oxidized, Tsa1 can become an important source of H 2 O 2 -dependent cytosolic glutathione disulfide production in yeast. Our findings clarify the importance of the various thiol and heme peroxidases for peroxide removal and suggest that most thiol peroxidases have alternative or specialized functions in specific subcellular compartments.