Photoconsumption of molecular oxygen on both donor and acceptor sides of photosystem II in Mn-depleted subchloroplast membrane fragments

Abstract

Oxygen consumption in Mn-depleted photosystem II (PSII) preparations under continuous and pulsed illumination is investigated. It is shown that removal of manganese from the water-oxidizing complex (WOC) by high pH treatment leads to a 6-fold increase in the rate of O 2 photoconsumption. The use of exogenous electron acceptors and donors to PSII shows that in Mn-depleted PSII preparations along with the well-known effect of O 2 photoreduction on the acceptor side of PSII, there is light-induced O 2 consumption on the donor side of PSII (nearly 30% and 70%, respectively). It is suggested that the light-induced O 2 uptake on the donor side of PSII is related to interaction of O 2 with radicals produced by photooxidation of organic molecules. The study of flash-induced O 2 uptake finds that removal of Mn from the WOC leads to O 2 photoconsumption with maximum in the first flash, and its yield is comparable with the yield of O 2 evolution on the third flash measured in the PSII samples before Mn removal. The flash-induced O 2 uptake is drastically (by a factor of 1.8) activated by catalytic concentration (5–10 μM, corresponding to 2–4 Mn per RC) of Mn 2+, while at higher concentrations (> 100 μM) Mn 2+ inhibits the O 2 photoconsumption (like other electron donors: ferrocyanide and diphenylcarbazide). Inhibitory pre-illumination of the Mn-depleted PSII preparations (resulting in the loss of electron donation from Mn 2+) leads to both suppression of flash-induced O 2 uptake and disappearance of the Mn-induced activation of the O 2 photoconsumption. We assume that the light-induced O 2 uptake in Mn-depleted PSII preparations may reflect not only the negative processes leading to photoinhibition but also possible participation of O 2 or its reactive forms in the formation of the inorganic core of the WOC.