Oxidative stress is caused by an imbalance between production and accumulation of oxygen reactive species (ROS) in cells and cells . Manganese superoxide dismutase (MnSOD) is an antioxidant enzyme that protects the mitochondria from oxidative damage by using a mechanism involving electron and proton transfers to lower ROS levels in mitochondria. More than a quarter of known enzymes also rely on electron and proton transfers to facilitate cellular activities that are essential for human health. However, most of their mechanisms are unclear.