Structure and function of chromophores in R-phycoerythrin at 1.9 Å resolution

The crystal structure of R-Phycoerythrin (R-PE) from Polysiphonia urceolata has been refined to a resolution of 1.9 Angstrom based on the atomic coordinates of R-PE determined at 2.8 Angstrom resolution, through the use of difference Fourier method and steorochemistry parameters restrained refinement with model adjustment according to the electron density map, Crystallographic R-factor of the refined model is 0.195 (Rfree = 0.282) from 8-1.9 Angstrom. High resolution structure of R-PE showed precise interactions between the chromophores and protein residues, which explained the spectrum characteristic and function of chromophores. Four chiral atoms of phycourobilin (PUB) were identified as C(4)-S, C(16)-S, C(21)-S, and C(20)-R. In addition to the coupling distances of 19 Angstrom to 45 Angstrom between the chromophores which were observed and involved in the energy transfer pathway high resolution structure of R-PE suggested other pathways of energy transfer, such as the ultrashort distance between alpha 140a and beta 155.It has been proposed that aromatic residues in linker proteins not only influence the conformation of chromophore, but may also bridge chromophores to improve the energy transfer efficiency (C) 1999 Wiley-Liss, Inc.