Local Protein Flexibility As a Prerequisite for Reversible Chromophore Isomerization in Alpha-phycoerythrocyanin

Phycoerythrocyanin is the only cyanobacterial phycobiliprotein containing phycoviolobilin as a chromophore. The phycoviolobilin chromophore is photo-reactive; upon irradiation, the chromophore undergoes a Z/E-isomerization involving the rotation of pyrrole-ring D. We have determined the structure of trimeric phycoerythrocyanin at three different experimental settings: monochromatically at 110 K and 295 K as well as with the Laue method at 288 K. Based on their chemical structures, the restraints for the phycoviolobilin of the alpha-subunit and for the phycocyanobilin chromophores of the beta-subunit were newly generated, which allows a chemically meaningful refinement of both chromophores. All three phycoerythrocyanin structures are very similar; the subunits match within 0.5 A. The detailed comparison of the data obtained with the different measurements provided information about the protein properties around the phycoviolobilin chromophore. For the first time, crystals of a phycobilisome protein are used successfully with the Laue technique. This paves the way for time-resolved macromolecular crystallography, which is able to elucidate the exact mechanisms of the phycoviolobilin photoactivity including the protein involvement.