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

Oxygen consumption in Mn-depleted photosystem if (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 mu M, corresponding to 2-4 Mn per RC) of Mn2+, while at higher concentrations (> 100 mu M) Mn2+ 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 Mn2+) 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. (C) 2010 Elsevier B.V. All rights reserved.