Is the Proton Release Due to Water Oxidation Directly Coupled to Events in the Manganese-Centre?

Abstract

By input of four light quanta photosystem II (PS II) oxidizes 2 H₂O molecules in a four stepped cycle to yield 1 O₂ and 4 H⁺. This is formally described by cycling through 4 transitions between 5 states (S₀→S→S₂→S₃→S₄, S₀) with O₂ release during the transition S₃→S₄, S₀. S₁ is the most stable state in darkness. In thylakoids 3 of the 4 transitions between these states are coupled to proton release with the given stoichiometry (1) and half-rise times (2): S₀→S₁: 1 H⁺ (250 μs), S₁→S₂: 0 H⁺, S₂→S₃:1 H⁺ (200 |μs), S₃→S_4’0: 2 H⁺ (1.2 ms). Artificial electron donors to the catalytic manganese-centre as NH₂OH shift the dark state of the water oxidizing complex 2 steps backwards (3,4). Cl⁻ and Ca²⁺ are essential cofactors for water oxidation and the lack of Cl⁻ inhibits the S₂→S₃ transition (5).