Characterization of the Interaction between Manganese and Tyrosine Z in Acetate-Inhibited Photosystem II

Abstract

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 S2 state of the Mn4 cluster, for which a multiline EPR signal with shifted 55Mn hyperfine peaks is observed, and the oxidized tyrosine residue, YZ*, for which a broadened YZ* 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 YZ* EPR signal at room temperature. The line width narrowing is interpreted to be due to averaging of the exchange and dipolar interactions between YZ* and the S2 state of the Mn4 cluster by rapid spin-lattice relaxation of the Mn4 cluster as the temperature is increased. Decay of the S2YZ* intermediate at 200 K shows that the g = 4.1 form of the S2 state is formed and that a noninteracting S2-state multiline EPR signal is not observed as an intermediate in the decay. This result shows that a change in the redox state of YZ induces a spin-state change in the Mn4 cluster in acetate-inhibited PSII. The interconversion between spin states of the Mn4 cluster in acetate-inhibited PSII supports the idea that YZ oxidation or YZ* reduction is communicated to the Mn4 cluster through a direct hydrogen-bonding pathway, possibly involving a ligand bound to the Mn4 cluster.

The Amyloid-β Peptide of Alzheimer’s Disease Binds CuI in a Linear Bis-His Coordination Environment: Insight into a Possible Neuroprotective Mechanism for the Amyloid-β Peptide

“…Tyrosyl radical, in particular, has been proposed as an intermediate in ROS generation by the AβCu complex 40,42,43. Under our conditions, AβCu II with ascorbate does not produce a detectable amount of tyrosine radical, a species that is easily identifiable by low-temperature EPR spectroscopy 44. This result indicates that this radical does not reach a steady-state concentration above our detection limit.…”

Structure-based mechanism of photosynthetic water oxidation

“…40 The different temperature-dependences of the S-state transitions also offer clues about proton transfer. 44 The S 2 -S 3 transition is also shown by EPR to be inhibited in samples depleted of calcium or chloride 22,45 or inhibited by acetate; 46 the Kok cycle stalls under illumination at the S 2 Y Z d state. 44 The S 2 -S 3 transition is also shown by EPR to be inhibited in samples depleted of calcium or chloride 22,45 or inhibited by acetate; 46 the Kok cycle stalls under illumination at the S 2 Y Z d state.…”

STRUCTURE, DYNAMICS, AND ENERGETICS OF THE PRIMARY PHOTOCHEMISTRY OF PHOTOSYSTEM II OF OXYGENIC PHOTOSYNTHESIS

“…There has been considerable debate over this issue, as it has repercussions for the hydrogen atom abstraction model for water oxidation that implicates Y Z directly in this mechanism (13,120). This model is a consequence of recent demonstrations of the close proximity between Y Z and the Mn cluster (113,116), the lowered activation energy that results from having a strong base prepositioned to accept a proton from bound water as the Mn cluster is being oxidized (67,145), and the difference in bond dissociation energies that favor hydrogen atom transfer from the Mn cluster to Y Z…”