Energetics of tryptophan residues in electron transfer and photoprotection of type-II photosynthetic reaction centers

Tryptophan is the strongest UV chromophore in proteins, and its biosynthesis is the most energy-consuming among all amino acids. In the transmembrane region of purple bacterial photosynthetic reaction centers (PbRC), tryptophan residues are densely concentrated near the inactive electron-transfer branch in subunit M, forming part of the carotenoid binding site. We investigated the redox potentials (Em) of tryptophan residues in PbRC and O2-evolving photosystem II (PSII) by solving the linear Poisson-Boltzmann equation, considering equilibrium with all titratable sites in the entire protein. The tryptophan mediating superexchange electron transfer between the active (bacterio)pheophytin and primary quinone exhibits the highest Em value in both PbRC and PSII. In contrast, in PSII, D1-Trp14, oxidized under strong light to trigger the degradation of photodamaged D1 protein, has the lowest Em value. In PbRC, a chain of tryptophan residues near the inactive branch forms an Em cascade. Quantum mechanical/molecular mechanical calculations suggest that this chain enables electron hole hopping toward the carotenoid, effectively dissipating harmful UV energy. This mechanism likely reflects the photoprotective strategy of PbRC, focusing on UV tolerance rather than oxidative stress.