Electron Transfer Route between Quinones in Type-II Reaction Centers

Area of Science:

• Biochemistry
• Photosynthesis Research
• Electron Transfer Mechanisms

Background:

• Photosynthetic reaction centers (PbRCs and PSII) utilize electron transfer between quinones Q_A and Q_B to terminate photoinduced charge separation.
• Understanding the factors governing the rate of this electron transfer is crucial for comprehending photosynthetic efficiency.

Purpose of the Study:

• To investigate and compare the electron transfer pathways and superexchange coupling (H_QA-QB) between Q_A and Q_B in PbRCs and PSII.
• To elucidate the structural and environmental factors responsible for the observed differences in electron transfer rates.

Main Methods:

• Calculation of superexchange coupling (H_QA-QB) for electron transfer from Q_A to Q_B within the protein environment.
• Analysis of electron tunneling pathways involving the nonheme Fe complex, considering both unoccupied and occupied molecular orbitals.

Main Results:

• Superexchange coupling (H_QA-QB) is significantly larger in PbRCs compared to PSII.
• Electron transfer via unoccupied orbitals (Q_A → Fe → Q_B) dominates in PbRCs, while transfer via occupied orbitals (Fe → Q_B followed by Q_A → Fe) is pronounced in PSII.
• A water molecule donating a hydrogen bond to Glu-M234 in PbRCs, absent in PSII due to D1-Tyr246, significantly increases H_QA-QB. Ser-L223···Q_B H-bond formation further enhances charge delocalization.

Conclusions:

• The presence of a specific water molecule and an extended hydrogen bond network in PbRCs facilitates charge delocalization and enhances Q_A-to-Q_B electron transfer compared to PSII.
• These findings explain the discrepancy in electron transfer rates between PbRCs and PSII, despite their structural similarities.