Bicarbonate-controlled reduction of oxygen by the Q _A semiquinone in Photosystem II in membranes

Abstract

Significance In Photosystem II (PSII), O 2 reduction by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mtext>Q</mml:mtext> <mml:mtext>A</mml:mtext> <mml:mrow> <mml:mo>•</mml:mo> <mml:mo>−</mml:mo> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> is often discussed but has not been demonstrated. Here, we show in PSII membranes that <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mtext>Q</mml:mtext> <mml:mtext>A</mml:mtext> <mml:mrow> <mml:mo>•</mml:mo> <mml:mo>−</mml:mo> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> can reduce O 2 to superoxide, but only when bicarbonate is absent from its binding site on the nonheme Fe 2+ . Bicarbonate’s role in PSII was recently shown to involve a regulatory/protective redox-tuning mechanism linking PSII function to CO 2 concentration. A key aspect is the presence of stable <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mtext>Q</mml:mtext> <mml:mtext>A</mml:mtext> <mml:mrow> <mml:mo>•</mml:mo> <mml:mo>−</mml:mo> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> causing release of bicarbonate from its site on Fe 2+ . Here, we show that under these conditions, O 2 binds to the empty site on the Fe 2+ and is reduced by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mtext>Q</mml:mtext> <mml:mtext>A</mml:mtext> <mml:mrow> <mml:mo>•</mml:mo> <mml:mo>−</mml:mo> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> . This unexpected reaction may be a further indication of cross-talk between the regulation of PSII and CO 2 fixation.

Topics & Concepts