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Issue 6, 2004
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Tryptophan fluorescence monitors structural changes accompanying signalling state formation in the photocycle of photoactive yellow protein

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Abstract

Photoactive yellow protein, a small, water-soluble blue-light absorbing photoreceptor protein from Ectothiorhodospira (Halorhodospirahalophila has a structure with two hydrophobic cores, of which the main one houses its light-sensitive chromophore (p-coumaric acid), separated by a central β-sheet. This photoreceptor protein contains a single tryptophan residue (W119) that is situated at the interface between the central β-sheet and its N-terminal cap. The fluorescence properties of W119 in the dark state pG (λmax = 328 nm; Φfl = 0.01; nearly pH-independent) are typical for a buried tryptophan in a hydrophobic environment with significant quenching by nearby amino acid residues. Signalling state formation leads to pH-dependent fluorescence changes: At pH values <6.5 the fluorescence emission increases, with a minor blue shift of the emission maximum. Above this pH, the emission maximum of the tryptophan shifts considerably to the red, whereas its total intensity decreases. These results further support the contention that signalling state formation in PYP leads to significant changes in the structure of this protein, even at sites that are at a considerable distance from the chromophore. The nature of these changes in pB, however, depend upon the pH imposed upon the protein: At slightly alkaline pH, which presumably is closest to the pH to which this protein is exposed in vivo, these changes lead to an exposure of the part of the central β-sheet harbouring W119. At slightly acidic pH the polarity of the environment of W119 is hardly affected by the formation of the signalling state but the quenching of its fluorescence emission, possibly by nearby amino acids, is reduced. On the other hand, its accessibility for quenching by small molecules in the solution is enhanced at acidic and alkaline pH in the signalling state (pB) compared to the dark state (pG). This latter observation points towards a more flexible structure of the N-terminal cap, having a looser interaction with the central β-sheet in pB.

Graphical abstract: Tryptophan fluorescence monitors structural changes accompanying signalling state formation in the photocycle of photoactive yellow protein

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Publication details

The article was received on 03 Feb 2004, accepted on 19 Apr 2004 and first published on 06 May 2004


Article type: Paper
DOI: 10.1039/B401600A
Citation: Photochem. Photobiol. Sci., 2004,3, 531-536
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    Tryptophan fluorescence monitors structural changes accompanying signalling state formation in the photocycle of photoactive yellow protein

    T. Gensch, J. Hendriks and K. J. Hellingwerf, Photochem. Photobiol. Sci., 2004, 3, 531
    DOI: 10.1039/B401600A

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