Structure and function of chromophores in R-phycoerythrin at 1.9 angstrom 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 coord inates of R-PE determined at 2.8 Angstrom resolution, through the use of di fference Fourier method and steorochemistry parameters restrained refinemen t with model adjustment according to the electron density map, Crystallogra phic R-factor of the refined model is 0.195 (Rfree = 0.282) from 8-1.9 Angs trom. High resolution structure of R-PE showed precise interactions between the chromophores and protein residues, which explained the spectrum charac teristic 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 chromop hores which were observed and involved in the energy transfer pathway high resolution structure of R-PE suggested other pathways of energy transfer, s uch as the ultrashort distance between alpha 140a and beta 155.It has been proposed that aromatic residues in linker proteins not only influence the c onformation of chromophore, but may also bridge chromophores to improve the energy transfer efficiency (C) 1999 Wiley-Liss, Inc.