Water Bridges Conduct Sequential Proton Transfer in Photosynthetic Oxygen Evolution.

Area of Science:

• Biochemistry
• Photosynthesis research
• Enzyme kinetics

Background:

• Proton transfer is crucial in biological systems, including photosystem II (PSII) where it drives ATP synthesis.
• Acetate inhibits the S2 to S3 transition in the water-oxidizing cycle of PSII.
• Understanding early proton transfer steps is key to elucidating PSII mechanism.

Purpose of the Study:

• To investigate the effect of acetate on the S1 to S2 transition in spinach photosystem II.
• To elucidate the role of acetate in PSII proton transfer mechanisms.
• To characterize the interaction of acetate with the catalytic site using spectroscopy.

Main Methods:

• Reaction-induced infrared spectroscopy was used to study spinach PSII.
• 532 nm laser flashes induced the S1 to S2 transition.
• Isotope-editing of acetate and density functional studies were employed.

Main Results:

• Acetate directly contributes to the S2-minus-S1 infrared spectrum, indicating protonation.
• A decrease in a band assigned to internal hydronium ions (Wn+) was observed with acetate.
• Density functional calculations predicted proton transfer from hydronium to acetate.

Conclusions:

• Acetate facilitates proton transfer by accepting a proton from an internal hydronium ion.
• The S1 to S2 transition in PSII involves proton hopping through a water network.
• This finding provides insights into the mechanism of proton translocation in PSII.