The curious life of human mitochondrial SOD2.

Human manganese superoxide dismutase (MnSOD2) is a critical mitochondrial antioxidant that catalyzes the conversion of highly reactive superoxide radicals into molecular oxygen and hydrogen peroxide. The peroxide molecules are subsequently neutralized by other antioxidant systems, positioning MnSOD2 as the primary defense against mitochondrial oxidative stress and diseases associated with disrupted in vivo redox balance. MnSOD2 has been studied since its discovery in the early 1960s, particularly in the context of cellular pathology and as a therapeutic target. Recent studies combining neutron protein crystallography (NPC), X-ray absorption spectroscopy (XAS), and quantum mechanical (QM) computations have uncovered previously uncharacterized protonation states and atypically short and strong hydrogen bonds within the active site of MnSOD2. Together, these drive the enzyme's exceptionally rapid turnover. This focused review summarizes emerging insights to generate an updated landscape of MnSOD2's structure-function relationship and to highlight remaining challenges. The primary bottleneck to a complete understanding of the structural mechanism of MnSOD2 catalysis is the lack of a superoxide-bound MnSOD2 structure that resolves all proton positions, defines the redox state of the catalytic metal, the metal ligands, and the position of superoxide. Additionally, another largely unexplored area is how Fe substitution converts MnSOD2 into a peroxidase, and how this metal promiscuity affects mitochondrial redox homeostasis. This review synthesizes current evidence and states an informed hypothesis for the catalytic mechanism of Fe-substituted SOD2 (FeSOD2). Clarifying these gaps will advance our understanding of the structural basis of SOD2 catalysis and how it shapes mitochondrial redox biology in health and disease.