The mechanism of amine inhibition of the photosynthetic oxygen evolving complex Amines displace functional chloride from a ligand site on manganese

Amines inhibit photosynthetic oxygen evolution by binding to manganese in the oxygen evolving complex. This inhibition is more effective if chloride is absent. Steady-state kinetic analyses show that ammonia and chloride compete for the same site in the oxygen evolving complex; inhibition of oxygen evolution by either chloride depletion or by ammonia addition induces Signal IIr, which saturates at high microwave power. These data indicate that chloride is bound at the manganese site in the oxygen evolving complex. We propose that the anion functions as a bridging ligand mediating electron transfer between manganese atoms and perhaps also between manganese and Z, the primary donor to P680.