Superoxide dismutase (SOD) catalyzes the dismutation of superoxide (O2•−) to H2O2. Which metalloenzyme variant is located in the cytoplasm, and which is in the mitochondrial matrix?

• A Cu/Zn-SOD (SOD1) in cytoplasm; Mn-SOD (SOD2) in mitochondrial matrix ✓
• B Mn-SOD (SOD2) in cytoplasm; Cu/Zn-SOD (SOD1) in mitochondria
• C Fe-SOD in cytoplasm; Cu/Zn-SOD in mitochondrial matrix
• D Catalase in cytoplasm; SOD in peroxisome only

Explanation

Superoxide dismutase has distinct metalloenzyme isoforms compartmentalized by subcellular location: Cu/Zn-SOD (SOD1) is predominantly cytosolic (also in peroxisomes and intermembrane space); Mn-SOD (SOD2) is located in the mitochondrial matrix, where it scavenges superoxide generated by Complex I and Complex III of the electron transport chain. Extracellular Cu/Zn-SOD (SOD3/EC-SOD) is secreted. Mutations in SOD1 are associated with familial amyotrophic lateral sclerosis (ALS). Catalase (option D) is a peroxisomal enzyme that converts H2O2 to H2O + O2 and is not SOD.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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