Reversible and irreversible effects of alkaline pH on Photosystem II electron-transfer reactions

Incubation of highly active, O₂-evolving PS II preparations at alkaline pH inhibits donor side electron-transfer reactions in two distinct fashions, one reversible the other irreversible. In both cases, O₂ evolution is inhibited, with concomitant loss of the light-induced multiline and g = 4.1 EPR signals and an increased steady-state level of EPR Signal II induced by continuous illumination. However, the inhibition that is observed between pH 7.0 and 8.0 is readily reversible by resuspension at low pH, while above pH 8.0 the effect is irreversible. In addition, under repetitive flash conditions the ms decay kinetics remains largely unchanged at pH ≤ 8.0 but shows about a 2-fold increase in amplitude and is slowed at pH above 8.0. The irreversible component of inhibition most likely can be attributed to the loss of Mn and the 16, 24 and 33 kDa proteins. The reversible component may be mediated by displacement of Cl^- from an anion-binding site by OH^- or by titration of ionizable groups on the protein(s) associated with water-splitting. We propose that the reversible inhibition blocks electron transfer between the O₂-evolving complex and an intermediate which serves as the direct donor to Signal II, while the irreversible inhibition blocks the reduction of Signal II by this intermediate donor species.