Energetic insights into two electron transfer pathways in light-driven energy-converting enzymes

Abstract

We report redox potentials (Em) for one-electron reduction for all chlorophylls in the two electron-transfer branches of water-oxidizing enzyme photosystem II (PSII), photosystem I (PSI), and purple bacterial photosynthetic reaction centers (PbRC). In PSI, Em values for the accessory chlorophylls were similar in both electron-transfer branches. In PbRC, the corresponding Em value was 170 mV less negative in the active L-branch (BL) than in the inactive M-branch (BM), favoring BL- formation. This contrasted with the corresponding chlorophylls, ChlD1 and ChlD2, in PSII, where Em(ChlD1) was 120 mV more negative than Em(ChlD2), implying that to rationalize electron transfer in the D1-branch, ChlD1 would need to serve as the primary electron donor. Residues that contributed to Em(ChlD1)