Bicarbonate-Reversible Inhibition of Plastoquinone Reductase in Photosystem II

Abstract

Abstract In this paper the current status of the so-called bicarbonate effect is presented. Several chemicals (such as formate, azide, nitrite and nitric oxide) are known to inhibit the two-electron gate of photosystem II (PS II). A remerkable slowing down of QA- reoxidation and an increase in equilibrium [QA- ] have been observed after the second or the subsequent, but not the first, flash when thylakoid membranes are treated with formate, etc. And, significantly, these effects are totally and uniquely reversed upon bicarbonate addition. The current hypothesis is that bicarbonate functions as a proton shuttle that stabilizes the binding niche of QB- and stimulates platoquinol formation. This bicarbonate effect must involve both the D 1 and D 2 proteins since various herbicide-resistant D 1 mutants (e.g., D 1 -S264A , D 1 -L275F), as well as some D 2 mutants (e.g., D 2 -R251S, D 2 -R 233Q) have been found to be differentially sensitive to formate. The D 2-arginine (233, 251) effects are specific since D 2 -R 139H mutant and an­ other mutant in which an extra arginine was inserted, between F 223 and E 224 , behaves like the wild type. Data in the literature suggest that the bicarbonate binding must also involve Fe in the PS II QA-Fe -QB complex. In contrast, the QA-Fe -QB complex and the two-electron gate of both green and purple photosynthetic bacteria, including the M -E 234 G , Q and V mutants, are insensitive to bicarbonate-reversible inhibitors. We will also address the question of the nature of the active species involved and the possible role of bicarbonate in vivo.