Cryptophyte algae differ from cyanobacteria and red algae in the architecture of their photosynthetic light harvesting systems, even though all three are evolutionarily related. Central to cryptophyte light harvesting is the soluble antenna protein phycoerythrin 545 (PE545). The ultrahigh resolution crystal structure of PE545, isolated from a unicellular cryptophyte Rhodomonas CS24, is reported at both 1.1 Å and 0.97 Å resolution, revealing details of the conformation and environments of the chromophores. Absorption, emission and polarized steady state spectroscopy (298 K, 77 K), as well as ultrafast (20 fs time resolution) measurements of population dynamics are reported. Coupled with complementary quantum chemical calculations of electronic transitions of the bilins, these enable assignment of spectral absorption characteristics to each chromophore in the structure. Spectral differences between the tetrapyrrole pigments due to chemical differences between bilins, as well as their binding and interaction with the local protein environment are described. Based on these assignments, and considering customized optical properties such as strong coupling, a model for light harvesting by PE545 is developed which explains the fast, directional harvesting of excitation energy. The excitation energy is funnelled from four peripheral pigments (β158,β82) into a central chromophore dimer (β50/β61) in ∼1 ps. Those chromophores, in turn, transfer the excitation energy to the red absorbing molecules located at the periphery of the complex in ∼4 ps. A final resonance energy transfer step sensitizes just one of the α19 bilins on a time scale of 22 ps. Furthermore, it is concluded that binding of PE545 to the thylakoid membrane is not essential for efficient energy transfer to the integral membrane chlorophyll a-containing complexes associated with PS-II.
All Science Journal Classification (ASJC) codes
- Structural Biology
- Molecular Biology
- energy transfer
- ultrahigh resolution X-ray crystallography