Solution-State ~(15)N NMR Spectroscopic Study of α-C-Phycocyanin: Implications for the Structure of the Chromophore-Binding Pocket of the Cyanobacterial Phytochrome Cph1

摘要

The detailed structure of the chromophore-binding pocket in phytochrome proteins and the structural changes associated with its photocycle are still matters of debate. Insight into the structure and dynamics of the binding pocket has been gained through the comparison of a ~(15)N NMR spectrum of α-C-phycocyanin, which is often used as a model system for the study of phyto-chromes, with the previously described ~(15)N NMR spectrum of the cyanobacterial phytochrome Cph1. The former spectrum supports the hypothesis that all four nitrogen atoms of the α-C-phycocya-nin chromophore are protonated, in analogy with the proposed protonation state for the P_r and P_(fr) forms of Cph1. The spectra show that the chromophores in both proteins exhibit a distinct dynamic behavior, as also indicated by a NOESY spectrum of Cph1. Finally, stereochemical arguments and a Cph1 homology model support the hypothesis that the chromophore in Cph1 is most likely in the ZZZssa conformation in the P_r form of the protein.