Fig. 5.

Structural scheme showing the working model for O₂ reduction by ${\text{Q}}_{\text{A}}^{•-}$ in PSII. (A) When photosynthesis is limited by low CO₂, illumination results in the reduction of the PQ pool, Q_B and Q_A. (B) The presence of long-lived ${\text{Q}}_{\text{A}}^{•-}$ changes the dissociation constant of the bicarbonate ligand on the nonheme Fe²⁺, leading to its release and allowing O₂ to enter the channel and to bind to the Fe²⁺. (C) When the O₂ is bound to the Fe²⁺, it is rapidly reduced by the electron coming from ${\text{Q}}_{\text{A}}^{•-}$. (D) The superoxide formed is released from the Fe²⁺ and diffuses away.