Light activation of Orange Carotenoid Protein reveals bicycle-pedal single-bond isomerization

Abstract

Orange Carotenoid protein (OCP) is the only known photoreceptor which uses carotenoid for its activation. It is found exclusively in cyanobacteria, where it functions to control light-harvesting of the photosynthetic machinery. However, the photochemical reactions and structural dynamics of this unique photosensing process are not yet resolved. We present time-resolved crystal structures at second-to-minute delays under bright illumination, capturing the early photoproduct and structures of the subsequent reaction intermediates. The first stable photoproduct shows concerted isomerization of C9’-C8’ and C7’-C6’ single bonds in the bicycle-pedal (s-BP) manner and structural changes in the N-terminal domain with minute timescale kinetics. These are followed by a thermally-driven recovery of the s-BP isomer to the dark state carotenoid configuration. Structural changes propagate to the C-terminal domain, resulting, at later time, in the H-bond rupture of the carotenoid keto group with protein residues. Solution FTIR and UV/Vis spectroscopy support the single bond isomerization of the carotenoid in the s-BP manner and subsequent thermal structural reactions as the basis of OCP photoreception.

How orange carotenoid protein controls the excited state dynamics of canthaxanthin

“…Recent time-resolved X-ray crystallography experiments 62 revealed a concerted isomerization of two carotenoid single bonds located in the NTD, namely C 6′ –C 7′ and C 8′ –C 9′ ( Fig. 1 ), through the so-called bicycle-pedal mechanism.…”

“… 15 Furthermore, the experimental conditions employed in ref. 62 could have altered the photocycle of OCP due to the crystal packing. Finally, isomerization at the β 1 ring seems a more likely cause of photoactivation, as (i) the β 1 ring is conserved among different OCP-bound carotenoids, and (ii) it is the location of the H-bond disruption necessary for carotenoid translocation.…”

Orange Carotenoid Protein in Mesoporous Silica: A New System towards the Development of Colorimetric and Fluorescent Sensors for pH and Temperature

“…(1) They can help us better understand the mechanism of photo-activation, which is under intensive study and the object of lively debate ( [3,[35][36][37][38][39] and refs. therein); (2) They provide new information concerning the influence of the surrounding environment on the orange to red activation;…”

Photoactivation of the orange carotenoid protein requires two light-driven reactions mediated by a metastable monomeric intermediate

“…However, a recent study by Niziński et al 20 of pump–probe spectra with coverage of the near-IR region has suggested instead that the first intermediate P 1 in the ketocarotenoid translocation mechanism arises not from S* but from the S 1 state proper. Based on evidence from time-resolved X-ray crystal structures, Chukhutsina et al 21 raised the alternative proposal that a “bicycle pedal” configuration 22 of the ketocarotenoid's isoprenoid backbone is produced during the photoactivation reaction.…”