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Highly conserved residues Asp-197 and His-250 in Agp1 phytochrome control the proton affinity of the chromophore and Pfr formation
Citation key ISI:000243451300062
Author von Stetten, David and Seibeck, Sven and Michael, Norbert and Scheerer, Patrick and Mroginski, Maria Andrea and Murgida, Daniel H. and Krauss, Norbert and Heyn, Maarten P. and Hildebrandt, Peter and Borucki, Berthold and Lamparter, Tilman
Pages 2116-2123
Year 2007
ISSN 0021-9258
DOI 10.1074/jbc.M608878200
Address 9650 ROCKVILLE PIKE, BETHESDA, MD 20814-3996 USA
Journal J. Biol. Chem.
Volume 282
Number 3
Month JAN 19
Abstract The mutants H250A and D197A of Agp1 phytochrome from Agrobacterium tumefaciens were prepared and investigated by different spectroscopic and biochemical methods. Asp-197 and His-250 are highly conserved amino acids and are part of the hydrogen-bonding network that involves the chromophore. Both substitutions cause a destabilization of the protonated chromophore in the Pr state as revealed by resonance Raman and UV-visible absorption spectroscopy. Titration experiments demonstrate a lowering of the pK(a) from 11.1 ( wild type) to 8.8 in H250A and 7.2 in D197A. Photoconversion of the mutants does not lead to the Pfr state. H250A is arrested in a meta-Rc-like state in which the chromophore is deprotonated. For H250A and the wild-type protein, deprotonation of the chromophore in meta-Rc is coupled to the release of a proton to the external medium, whereas the subsequent proton re-uptake, linked to the formation of the Pfr state in the wild- type protein, is not observed for H250A. No transient proton exchange with the external medium occurs in D197A, suggesting that Asp-197 may be the proton release group. Both mutants do not undergo the photoinduced protein structural changes that in the wild- type protein are detectable by size exclusion chromatography. These conformational changes are, therefore, attributed to the meta-Rc -> Pfr transition and most likely coupled to the transient proton re- uptake. The present results demonstrate that Asp-197 and His-250 are essential for stabilizing the protonated chromophore structure in the parent Pr state, which is required for the primary photochemical process, and for the complete photo-induced conversion to the Pfr state.
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