Minor Structural Differences Tune Functional Specialization of Antenna Subunits in the PSII Energy Transfer Network
Photosystem II (PSII) contains a diverse array of pigment-protein subunits with specialized roles in the antenna network. We investigate the functional significance of this subunit diversity in two structurally similar yet functionally distinct PSII subunits: minor light-harvesting complex CP29 and major light-harvesting complex LHCII. We combine two-dimensional electronic-vibrational spectroscopy with lifetime density analysis and structure-based kinetic modeling to assign spectral features to exciton states, providing molecular detail to probed spectral features. We show how the distinct pigment compositions of CP29 and Lhcb1, a monomeric subunit of the major LHCII trimer, reshape excitonic connectivity and energy transfer pathways in the complexes. Compared to Lhcb1, energy transfer pathways in CP29 are more spatially connected and reversible, suggesting CP29 evolved to distribute excitation between the core and periphery of the PSII antenna. We find that small structural modifications from a common framework tune CP29 and Lhcb1 to complementary roles in the PSII supercomplex.