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Protonation-Dependent Structural Heterogeneity in the Chromophore Binding Site of Cyanobacterial Phytochrome Cph1
Citation key VelazquezEscobar2017
Author Velazquez Escobar, Francisco and Lang, Christina and Takiden, Aref and Schneider, Constantin and Balke, Jens and Hughes, Jon and Alexiev, Ulrike and Hildebrandt, Peter and Mroginski, Maria Andrea
Pages 47-57
Year 2017
DOI 10.1021/acs.jpcb.6b09600
Journal The Journal of Physical Chemistry B
Volume 121
Number 1
Note PMID: 27966353
Abstract Phytochromes are biological red/far-red light sensors found in many organisms. Photoisomerization of the linear methine-bridged tetrapyrrole triggers transient proton translocation events in the chromophore binding pocket (CBP) leading to major conformational changes of the protein matrix that are in turn associated with signaling. By combining pH-dependent resonance Raman and UV–visible absorption spectroscopy, we analyzed protonation-dependent equilibria in the CBP of Cph1 involving the proposed Pr-I and Pr-II substates that prevail below and above pH 7.5, respectively. The protonation pattern and vibrational properties of these states were further characterized by means of hybrid quantum mechanics/molecular mechanics calculations. From this combined experimental–theoretical study, we were able to identify His260 as the key residue controlling pH-dependent equilibria. This residue is not only responsible for the conformational heterogeneity of CBP in the Pr state of prokaryotic phytochromes, discussed extensively in the past, but it constitutes the sink and source of protons in the proton release/uptake mechanism involving the tetrapyrrole chromophore which finally leads to the formation of the Pfr state. Thus, this work provides valuable information that may guide further experiments toward the understanding of the specific role of protons in controlling structure and function of phytochromes in general.
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