The ability of all cells to regulate levels of reactive oxygen species (ROS) is vital for controlling many aspects of proliferation and survival and we have discovered that pyruvate kinase M2 (PKM2) is important for cancer cell biology. PKM2 is directly oxidized on Cys358 to inhibit its catalytic activity, which allows for diversion of glucose-6-phosphate into the pentose phosphate pathway. This, in turn, allows the synthesis of NADPH, which is critical for generating reduced glutathione, necessary for ROS detoxification. In a cellular context, our PKM2 activator, ML285 protects the enzyme from oxidation by ROS and results in sensitization to oxidative stress and increased apoptosis.