The Signaling State of Orange Carotenoid
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Eugene G. and Shirshin, Evgeny A. and Sluchanko, Nikolai N. and
Zlenko, Dmitry V. and Parshina, Evgenia Y. and Tsoraev, Georgy V. and
Klementiev, Konstantin E. and Budylin, Gleb S. and Schmitt,
Franz-Josef and Friedrich, Thomas and Fadeev, Victor V. and Paschenko,
Vladimir Z. and Rubin, Andrew B.
||Department of Biophysics, Faculty of Biology, M.V. Lomonosov
Moscow State University, Moscow, Russia.
carotenoid protein (OCP) is the photoactive protein that is
responsible for high light tolerance in cyanobacteria. We studied the
kinetics of the OCP photocycle by monitoring changes in its absorption
spectrum, intrinsic fluorescence, and fluorescence of the Nile red dye
bound to OCP. It was demonstrated that all of these three methods
provide the same kinetic parameters of the photocycle, namely, the
kinetics of OCP relaxation in darkness was biexponential with a ratio
of two components equal to 2:1 independently of temperature. Whereas
the changes of the absorption spectrum of OCP characterize the
geometry and environment of its chromophore, the intrinsic
fluorescence of OCP reveals changes in its tertiary structure, and the
fluorescence properties of Nile red indicate the exposure of
hydrophobic surface areas of OCP to the solvent following the
photocycle. The results of molecular-dynamics studies indicated the
presence of two metastable conformations of 3'-hydroxyechinenone,
which is consistent with characteristic changes in the Raman spectra.
We conclude that rotation of the ?-ionylidene ring in the C-terminal
domain of OCP could be one of the first conformational rearrangements
that occur during photoactivation. The obtained results suggest that
the photoactivated form of OCP represents a molten globule-like state
that is characterized by increased mobility of tertiary structure
elements and solvent accessibility.
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