Catalase-free photosystem II: The O₂-evolving complex does not dismutate hydrogen peroxide

A photosystem II (PSII) membrane-associated heme catalase has been identified as a major source of the dark H₂O₂-dismutation reaction in PSII membrane samples [Sheptovitsky, Y. G., and Brudvig, G. W. (1996) Biochemistry 35, 16255-16263]. Based on this finding, a catalase-free PSII membrane sample was prepared by using mild heat treatment to deplete most of the PSII membrane-associated heme catalase followed by inhibition of the residual catalase with 50 mM 3-amino-1,2,4-triazole, a specific heme catalase inhibitor that binds covalently to compound I. After these treatments, the PSII membrane sample exhibited only 0.02% of the original H₂O₂-dismutation activity when assayed in the presence of 20 mM 3-amino-1,2,4-triazole. This small residual H₂O₂-dismutation activity is attributed to adventitious metal ions or the non-heme iron in PSII because the activity was still present in a Mn-depleted PSII sample but was completely suppressed by adding 5 mM ferricyanide to the assay buffer; the effect of ferricyanide is attributed to oxidation of H₂O₂-dismutating cations. Although the H₂O₂- dismutation activity was completely eliminated by these treatments, the light-induced O₂-evolution activity was retained. A single saturating flash given to catalase-free PSII membranes did not induce any H₂O₂-dismutation activity. These results demonstrate that the S₁/S_-1 and S₂/S₀ cycles of the O₂-evolving complex of PSII do not occur in the presence of H₂O₂, as proposed by Velthuys, B., and Kok, B. [(1978) Biochim. Biophys. Acta 502, 211-221]. The light-induced O₂-evolution activity in catalase-free PSH was found to be irreversibly impaired by micromolar concentrations of H₂O₂. Thus, it is possible that the PSII membrane-associated heme catalase plays an important role in protection of the O₂-evolving complex from damage by H₂O₂.