Role of hydrogen bond alternation and charge transfer states in photoactivation of the Orange Carotenoid Protein

Igor A Yaroshevich,Alexey V Stepanov,Thomas Friedrich,Eugene G Maksimov,A B Rubin ,Vladimir V Poddubnyy,Ivan Gushchin,Ф Е Гостев ,Alina Remeeva,Timofey S Gostev,Kirill Kovalev,В А Надточенко ,Ivan V Shelaev ,Dmitry Khakhulin,Yury B Slonimskiy,Vladimir Z Paschenko,Tomáš Polı́vka ,Nikolai N Sluchanko,Dmitry A Cherepanov,Valentin Gordeliy ,Д В Зленко ,Ekaterina A Slutskaya,Miroslav Kloz,Viacheslav S Botnarevskii,Mikhaǐl P Kirpichnikov

doi:10.1038/s42003-021-02022-3

Abstract

Here, we propose a possible photoactivation mechanism of a 35-kDa blue light-triggered photoreceptor, the Orange Carotenoid Protein (OCP), suggesting that the reaction involves the transient formation of a protonated ketocarotenoid (oxocarbenium cation) state. Taking advantage of engineering an OCP variant carrying the Y201W mutation, which shows superior spectroscopic and structural properties, it is shown that the presence of Trp201 augments the impact of one critical H-bond between the ketocarotenoid and the protein. This confers an unprecedented homogeneity of the dark-adapted OCP state and substantially increases the yield of the excited photoproduct S*, which is important for the productive photocycle to proceed. A 1.37 Å crystal structure of OCP Y201W combined with femtosecond time-resolved absorption spectroscopy, kinetic analysis, and deconvolution of the spectral intermediates, as well as extensive quantum chemical calculations incorporating the effect of the local electric field, highlighted the role of charge-transfer states during OCP photoconversion.

Highlights

We propose a possible photoactivation mechanism of a 35-kDa blue light-triggered photoreceptor, the Orange Carotenoid Protein (OCP), suggesting that the reaction involves the transient formation of a protonated ketocarotenoid state
We suggested that this effect might be due to the reduction of the number of hydrogen bond donors in the C-terminal domain (CTD)
Our work shows that the presence of two hydrogen bonds in WT OCP causes spectral heterogeneity

Summary

Introduction

We propose a possible photoactivation mechanism of a 35-kDa blue light-triggered photoreceptor, the Orange Carotenoid Protein (OCP), suggesting that the reaction involves the transient formation of a protonated ketocarotenoid (oxocarbenium cation) state. During the conversion into OCPR, the structure of OCP undergoes global rearrangement: the domains separate from each other, after the carotenoid moves 12 Å deeper into the NTD13–15 These events lead to the exposure of sites for protein-protein interactions with the light-harvesting antenna complex, the phycobilisome (PBS), enabling quenching of its excitation[16,17,18,19]. ICT signatures are valuable indicators for the contribution of the carbonyl group(s) to the conjugated system, and might inform on the configuration of the carotenoid end rings[42,43] These effects are of particular interest for the chromophores embedded in OCP since the only specific carotenoid-protein interactions in the OCPO state are two short and strong hydrogen bonds between the 4-keto oxygen of the carotenoid and two hydrogens in the CTD, one belonging to the Tyr-201 hydroxyl group and another to the N-H group of

Methods

Results

Conclusion