Fig. 5.
Structural scheme showing the working model for O2 reduction by in PSII. (A) When photosynthesis is limited by low CO2, illumination results in the reduction of the PQ pool, QB and QA. (B) The presence of long-lived 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 . (D) The superoxide formed is released from the Fe2+ and diffuses away.