Orange carotenoid protein (OCP) is a water-soluble protein which plays a role in photoprotection in diverse cyanobacteria. It is the only photoactive protein known to use a carotenoid as the photoresponsive chromophore. The protein consists of two domains, with a single keto-carotenoid molecule non-covalently bound between the two domains. It is a very efficient quencher of excitation energy absorbed by the primary light-harvesting antenna complexes of cyanobacteria, the phycobilisomes. The quenching is induced by blue-green light. It is also capable of preventing oxidative damage by directly scavenging singlet oxygen (1O2).
History OCP was first described in 1981 by Holt and Krogmann who isolated it from the unicellular cyanobacterium Arthrospira maxima, although its function would remain obscure until 2006. The crystal structure of the OCP was reported in 2003. At the same time the protein was shown to be an effective quencher of singlet oxygen and was suggested to be involved in photoprotection, or carotenoid transport. In 2000, it was demonstrated that cyanobacteria could perform photoprotective fluorescence quenching independent of lipid phase transitions, differential transmembrane pH, and inhibitors. The action spectrum for this quenching process suggested the involvement of carotenoids, and the specific involvement of the OCP was later demonstrated by Kirilovsky and coworkers in 2006. In 2008, OCP was shown to require photoactivation by strong blue-green light for its photoprotective quenching function. Photoactivation is accompanied by a pronounced color change, from orange to red, which had been previously observed by Kerfeld et al in the initial structural studies. In 2015 a combination of biophysical methods by researchers in Berkeley showed that the visible color change is the consequence of a 12Å translocation of the carotenoid.
Physiological significance For a long time, cyanobacteria were considered incapable of performing non-photochemical quenching (NPQ) as a photoprotective mechanism, relying instead on a mechanism of energy redistribution between the two photosynthetic reaction centers, PSII and PSI, known as "state transitions". OCP is found in a majority of cyanobacterial genomes, with remarkable conservation of its amino acid sequence, implying evolutionary constraints to preserve an important function. Mutant cells engineered to lack OCP photobleach under high light and become photoinhibited more rapidly under fluctuating light. Under nutrient stress conditions, which are expected to be norm in marine environments, photoprotective mechanisms such as OCP become important even at lower irradiances. This protein is not found in chloroplasts, and appears to be specific to cyanobacteria.
Function
Photoactivity
Upon illumination with blue-green light, OCP switches from an orange form (OCPO) to a red form (OCPR). The reversion of OCPR to OCPO is light independent and occurs slowly in darkness. OCPO is considered the dark, stable form of the protein, and does not contribute to phycobilisome quenching. OCPR is considered to be essential for induction of the photoprotection mechanism. The photoconversion from the orange to red form has a poor light efficiency (very low quantum yield), which helps to ensure the protein's photoprotective role only functions during high light conditions; otherwise, the dissipative NPQ process could unproductively divert light energy away from photosynthesis under light-limiting conditions.
Energy quenching As evidenced by a decreased fluorescence, OCP in its red form is capable of dissipating absorbed light energy from the phycobilisome antenna complex. According to Rakhimberdieva and coworkers, about 30-40% of the energy absorbed by phycobilisomes does not reach the reaction centers when the carotenoid-induced NPQ is active. The exact mechanism and quenching site in both the carotenoid as well as the phycobilisome still remain uncertain. The linker polypeptide ApcE in the allophycocyanin (APC) core of the phycobilisomes is known to be important, but is not the site of quenching. Several lines of evidence suggest that it is the 660 nm fluorescence emission band of the APC core which is quenched by OCPR. The temperature dependence of the rate of fluorescence quenching is similar to that of soluble protein folding, supporting the hypothesis that OCPO slightly unfolds when it converts to OCPR.
Singlet oxygen quenching As first shown in 2003, the auxiliary function of carotenoids as quenchers of singlet oxygen contributes to the photoprotective role of OCP has also been demonstrated under strong orange-red light, which are conditions where OCP cannot be photoactivated for its energy-quenching role. This is significant because all oxygenic phototrophs have a particular risk of oxidative damage initiated by singlet oxygen (1O2), which is produced when their own light-harvesting pigments act as photosensitizers.
Structure
3D structure The three-dimensional protein structure of OCP (in the OCPO form) was solved in 2003, before its photoprotective role had been defined. The 35 kDa protein contains two structural domains: an all-α-helical N-terminal domain (NTD) consisting of two interleaved 4-helix bundles, and a mixed α/β C-terminal domain (CTD). The two domains are connected by an extended linker. In OCPO, the carotenoid spans both domains, which are tightly associated in this form of protein. In 2013 Kerfeld and co-workers showed that the NTD is the effector (quencher) domain of the protein while the CTD plays a regulatory role.
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