Question

Catalase testing is one of several tests used to detect presence of an electron transport chain in a cell. Catalase is made by a cell to detoxify H₂O₂. How does presence of H₂O₂ relate to the presence of an electron transport chain?

Answer 1

Catalse is an enzyme present in aerobic microorganisms and other eukaryotes. It catalyzes the decomposition of hydrogen peroxide into water and oxygen in order to protect the cells from oxidative damage by H₂O₂.

Reactive oxygen species are generated during electron transport chain (ETC), when electrons from complex I and III leaks and partially reduced dioxygen molecule to superoxide(^.O₂^-) that undergoes dismutation to form H₂O₂. This dismutation reaction is catalyzed by two superoxide dismutases (1 &2) and the resulting H₂O₂ is completely reduced to water by catalase.

Thus the presence of a functional ETC will always lead to passage of 0.1-2% of electrons prematurely to oxygen forming reactive oxygen species and ultimately hydrogen peroxide that in turn is detoxified by catalase. Hence, these reactive oxygen species are a natural byproduct of oxidative phosphorylation and presence of these ROS confirms the presence of a functional ETC in the organism.