Orange Carotenoid Protein Picosecond Dynamics Changes with Photo and Chemical Activation

Abstract

Orange carotenoid protein (OCP) is the only photosensory protein described to date having a carotenoid as the active chromophore. Upon strong green-blue (or white) illumination, OCP goes from the orange inactive state (OCPO) to the red active state (OCPR) [1]. OCPR interacts with the phycobilisome (PB), assisting in thermal energy dissipation. This dissipation diminishes the formation of reactive oxygen which damages the photosynthetic apparatus and leads to cell death [2]. OCP has a large conformational change as it goes from OCPO to OCPR, and this structural change is thought to be essential to the energy quenching interaction with the PB. However, the conversion from inactive to active state has been recently shown to also occur with high chaotrope concentrations. Specifically high concentration NaSCN produces a long lived red state in the absence of photoexcitation [3]. We suggest that both the susceptibility of OCP to large conformational change and its interaction with PB are associated with changes in the long range picosecond structural flexibility. We measure OCP protein flexibility changes with photoactivation and chemical activation using terahertz absorption spectroscopy. Temperature dependent terahertz time domain spectroscopy is measured in the 100 - 290 K range on OCP solutions as a function of illumination and NaSCN concentration. A rapid increase in the THz absorbance is observed in the 180-220 K range. We quantify the picosecond structural flexibility by both the net THz absorption and the dynamical transition temperature, which scales with structural stability. R.E.B acknowledges DOE award DE-FG02-07ER15902 and A.G.M acknowleges NSF awards DBI 1556359 and MCB 1616529, and DOE award DE-SC0016317 for support of the work.1. Wilson, A. et al. Proc. Natl. Acad. Sci. U. S. A. 105:12075-12080(2008).2. Wilson, A. et al. Plant Cell, 18:992-1007(2006).3. King, J.D. et al. FEBS Lett, 588:4561-4565(2014).