In oxygenic photosynthesis, water is the substrate of the enzyme, photosystem II (PSII). PSII converts water to molecular oxygen through four successive, photo-driven oxidation events at a manganese-calcium cluster. This reaction is essential for the maintenance of aerobic life on earth. A tyrosyl radical, YZ, is essential in photosynthetic water oxidation and transfers oxidizing equivalents from an oxidized, primary chlorophyll donor, to the metal center. Based on X-ray crystal structures, the metal ions of the catalytic site and YZ are predicted to be extensively hydrogen bonded in an internal water network that involves amide carbonyl groups and amino acid side chains. This hydrogen-bonded network is predicted to link YZ with the lumenal surface of the thylakoid membrane; the pathway includes an extrinsic subunit known as PsbO. Such an extensive network may be essential in facilitating proton release from substrate water and may play other roles in catalysis. To test the role of the water network, reaction induced FT-IR and EPR spectroscopies were employed. PSII was prepared in selected S states by laser flash illumination; these S states correspond to steps along the catalytic cycle to produce oxygen from water. This technique shows that the network is dynamic. In addition, the data support the conclusion that the water network can be transiently protonated and deprotonated during the catalytic cycle. Taken together, these spectroscopic results support the presence of a catalytically functional, water network, extending from YZ to the lumenal surface of PSII.

Proton coupled electron transfer in photosynthetic oxygen evolution: Role for internal water and hydrogen bonded networks

253rd National Meeting (2017)

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