Characterization of the interaction between manganese and tyrosine Z in acetate-inhibited photosystem II

When acetate-inhibited photosystem II (PSII) membranes are illuminated at temperatures above 250 K and quickly cooled to 77 K, a 240 G-wide electron paramagnetic resonance (EPR) signal is observed at 10 K. This EPR signal arises from a reciprocal interaction between the spin 1/2 ground state of the S-2 stale of the Mns cluster, for which a multiline EPR signal with shifted Mn-55 hyperfine peaks is observed, and the oxidized tyrosine residue, Y-Z(.), for which a broadened Y-Z(.) EPR spectrum is observed. The S2YZ. EPR signal in acetate-inhibited PSII is the first in which characteristic spectral features from both paramagnets can be observed. The observation of distinct EPR signals from each of the paramagnets together with the lack of a half-field EPR transition indicates that the exchange and dipolar couplings are weak. Below 20 K, the S2YZ. EPR signal in acetate-inhibited PSII is in the static limit. Above 20 K, the line width narrows dramatically as the broad low-temperature S2YZ. EPR signal is converted to a narrow Y-Z(.) EPR signal at room temperature. The line width narrowing is interpreted to be due to averaging of the exchange and dipolar interactions between Y-Z(.) and the S-2 state of the Mn-4 cluster by rapid spin-lattice relaxation of the Mn-4 cluster as the temperature is increased. Decay of the S2YZ. intermediate at 200 K shows that the g = 4.1 form of the S-2 slate is formed and that a noninteracting S-2-state multiline EPR signal is not observed as an intermediate in the decay. This result shows that a change in die redox state of Y-Z induces a spin state change in the Mn-4 cluster in acetate-inhibited PSII. The interconversion between spin slates of the Mn-4 cluster in acetate-inhibited PSII supports the idea that Y-Z oxidation or Y-Z(.) reduction is communicated to the Mn-4 cluster through a direct hydrogen-bonding pathway, possibly involving a ligand bound to the Mn-4 cluster.