In photosynthetic organisms, electrons abstracted during the water splitting cycle are transferred along a series of cofactors in the photosystem II (PSII) protein. The final electron transfer step happens between two identical plastoquinone (PQ) molecules bound near the stromal side of the thylakoid membrane. While the first bound quinone, QA, serves as a one-electron acceptor, QB is able to accept two electrons and two protons to form plastoquinol (H2PQ) and subsequently diffuse out to the membrane. We use classical molecular dynamics and quantum electronic structure calculations to show how the charge distribution due to the protein environment creates the driving force for electron transfer by shifting the redox potential of each PQ cofactor in different ways. For example, these calculations show that the variable protonation of His252, hydrogen bonded to QB via Ser264, controls the redox potential of QB significantly more than the redistribution of charges that results from a hydrogen bond interaction with Ser264 alone. The PQ at the QA site lacks a titratable residue in such close proximity. As a result, the QA redox potential is more subtly affected by the surrounding charge distribution. With the methods presented, we can predict the effect of stromal side mutations on the final electron transfer step of PSII.

Effect of protein charge distributions on redox cofactors in photosystem II

246th National Meeting (2013)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.