|MadSci Network: Biochemistry|
No, Manganese dioxide is not an enzyme. An enzyme is defined as a complex polymer of biological origin (usually a protein, but sometimes not) that acts as a catalyst in one or more chemical reactions. A catalyst, on the other hand, is any substance that increases the rate of a chemical reaction without itself being changed by the reaction. In your case, MnO2 (Manganese dioxide) is a catalyst for the degradation of hydrogen peroxide (H2O2) into water (H2O) and oxygen (O2).
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