The photoinitiated reaction pathway of full-length cyanobacteriochrome Tlr0924 monitored over twelve orders of magnitude. [Molecular Biophysics]

May 9th, 2014 by Hauck, A. F. E., Hardman, S. J. O., Kutta, R. J., Greetham, G. M., Heyes, D. J., Scrutton, N. S.

The coupling of photochemistry to protein chemical and structural change is crucial to biological light-activated signaling mechanisms. This is typified by cyanobacteriochromes (CBCRs), members of the phytochrome superfamily of photoreceptors that exhibit a high degree of spectral diversity, collectively spanning the entire visible spectrum. CBCRs utilize a basic E/Z isomerization of the bilin chromophore as the primary step in their photocycle, which consists of reversible photoconversion between two photostates. Despite intense interest in these photoreceptors as signal transduction modules, a complete description of light-activated chemical and structural changes has not been reported. The CBCR Tlr0924 contains both phycocyanobilin and phycoviolobilin chromophores, and these two species photoisomerize in parallel via spectrally and kinetically equivalent intermediates before the second step of the photoreaction where the reaction pathways diverge, the loss of a thioether linkage to a conserved cysteine residue occurs and the phycocyanobilin reaction terminates in a red-absorbing state, while the phycoviolobilin reaction proceeds more rapidly to a final green-absorbing state. Here time resolved visible transient absorption spectroscopy (femtosecond to second) has been used, in conjunction with time resolved IR spectroscopy (femtosecond to nanosecond) and cryo-trapping techniques, to follow the entire photoconversion of the blue-absorbing states to the green- and red-absorbing states of the full-length form of Tlr0924 CBCR. Our analysis shows that Tlr0924 undergoes an unprecedented long photoreaction that spans from picoseconds to seconds. We show that the thermally driven, long time scale changes are less complex than those reported for the red/far-red photocycles of the related phytochrome photoreceptors.
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