Scientists shed light on the mechanism of photoactivation of orange carotenoid protein

Biochemists FITs biotechnology RAS as part of an international team of scientists deciphered the mechanism underlying the activation of orange carotenoid protein – photoreceptor cyanobacteria, protecting them from excessive exposure to light. The creation of a unique mutant variant and the use of the latest spectroscopic approaches made it possible to describe a photochemical reaction not previously known for carotenoids. The results will open up future prospects for the development of new light-controlled systems and biomaterials. The paper can be found in the pages of the journal Communications Biology.

Exposure to light is necessary for photosynthetic organisms to convert inorganic compounds into organic ones. But there is a limit to this: too much sunlight energy can be devastating to cells. Thanks to the pigments carotenoids, which can be part of special photoprotective proteins, the excess excitation is converted into heat – the so-called non-photochemical quenching occurs. The authors of this article studied one of such protectors – orange carotenoid protein (OCP). It was first isolated in 1981 from representatives of an ancient group of photosynthetic prokaryotes, the cyanobacteria. OCP has two protein-domain parts that form a cavity, inside which a single carotenoid molecule is contained.

“When a carotenoid molecule absorbs light, the OCP protein is able to change from an inactive orange to an active red form. This process is multistep and obeys a complex hierarchy of events. Previously, we showed asynchrony of these changes, but the mechanism of the very first stage of photoactivation of OSR, related to breaking hydrogen bonds between carotenoid and protein, remained unsolved,” – says Evgeny Maximov, PhD in biology, a senior researcher FITs biotechnology RAS.

Scientists conducted a comprehensive study involving methods of structural biology, spectroscopy, biochemistry and quantum chemistry. Employees of FIC Biotechnology RAS created “super orange” variant of OSR with unique spectral and structural properties and determined its crystal structure with the highest spatial resolution among all known OSR-like proteins.

Analysis of the data obtained showed that as a result of photon absorption in the OSR a charge separation reaction can take place along the hydrogen bond between the carotenoid molecule and one of the amino acid residues of the protein. This hydrogen bond in the dark stabilizes the orange state of the OSR, but in the light it breaks extremely quickly due to the redistribution of electron density in the carotenoid molecule. As a result, negative and positive poles are formed in the protein, which leads to a change in its entire structure. Such a photochemical reaction was first described for carotenoids.

“Our discovery will allow us to control the process of OSR activation and its spectral properties. As a result, this can lead to the creation of new light-controlled systems and “smart” biocompatible materials based on photoactive proteins for optogenetics and functional imaging”, – says Nikolay Sluchanko, Doctor of Biology, leading researcher of the RAS Biotechnology Research Center.

The work was carried out jointly with colleagues from the Biology Department of Lomonosov Moscow State University, MIPT, the N.N. Semenov Institute of Chemical Physics, the M.M. Shemyakin Institute of Bioorganic Chemistry and Yu.

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