The structure at 2Â Ã resolution of Phycocyanin from Gracilaria chilensis and the energy transfer network in a PC-PC complex

Phycocyanin is a phycobiliprotein involved in light harvesting and conduction of light to the reaction centers in cyanobacteria and red algae. The structure of C-phycocyanin from Gracilaria chilensis was solved by X-ray crystallography at 2.0 Å resolution in space group P21. An interaction model between two PC heterohexamers was built, followed by molecular dynamic refinement. The best model showed an inter-hexamer rotation of 23°. The coordinates of a PC heterohexamer (αβ)6 and of the PC-PC complex were used to perform energy transfer calculations between chromophores pairs using the fluorescence resonance energy transfer approach (FRET). Two main intra PC (Iβ382 → Iα184 → Iα584 → Iβ682 and Iβ3153 → Iβ5153) and two main inter PC (Iβ682 → IIβ382 and Iβ5153 → IIβ3153) pathways were proposed based on the values of the energy transfer constants calculated for all the chromophore pairs in the hexamer and in the complex.