title: Structural basis for the photochemistry of alpha-phycoerythrocyanin.
abstract: Phycobiliproteins and phytochromes are light-harvesting and light-sensing proteins containing linear tetrapyrroles, so-called bile chromophores. The chromophores in certain biliproteins, including the phytochromes, isomerize reversibly from a stable Z-configuration to a stable E-configuration when irradiated with light of the appropriate wavelength. Here, we report the crystal structure of alpha-phycoerythrocyanin with its chromophore in the E-configuration, compare it with the Z-configuration found in trimeric phycoerythrocyanin, and reveal the structural bases of the isomerization. The geometric changes of the chromophore account for the large spectral shift, which characterizes the overall transition. Interactions of the chromophore A and D pyrrole rings with flexible protein moieties are required for the formation and stabilization of the isomers. We predict that the results will hold for all photoactive biliproteins.
authors.affiliation: Physikdepartment E17, Technische Universität München, James Franck Strasse, 85747 Garching, Germany. marius.schmidt@ph.tum.de
authors.forename: Marius
authors.initials: M
authors.lastname: Schmidt
authors.forename: Anamika
authors.initials: A
authors.lastname: Patel
authors.forename: Yi
authors.initials: Y
authors.lastname: Zhao
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Reuter
mesh_terms: D018360:Crystallography, X-Ray; D000458:Cyanobacteria / Q000737:chemistry; D008958:Models, Molecular; D046912:Multiprotein Complexes; D010777:Photochemistry; D052980:Phycobilins / Q000737:chemistry*; D010798:Phycocyanin / Q000737:chemistry*; D011487:Protein Conformation; D020836:Protein Structure, Quaternary; D013053:Spectrophotometry; D055672:Static Electricity; D013237:Stereoisomerism
chemical_list: D046912:Multiprotein Complexes; D052980:Phycobilins; C038397:phycoerythrocyanin; D010798:Phycocyanin