Orange Carotenoid Protein Quenches Excess Energy and Singlet Oxygen

Harnessing light for energy gives photosynthetic organisms free energy—great, right? Well, like so many free things, too much of a good thing can turn into a bad thing, like a drought-ending rain that turns into a levy-breaching flood. The photosynthetic apparatus, beautifully adapted to capture light, can incur serious damage when too much light energy floods in. Given the variability of light conditions in the environment, photosynthetic organisms have, by necessity, adapted to manage excess light. For example, in cyanobacteria, blue light causes a conformational switch of the Orange Carotenoid Protein (OCP) from its dark-stable, orange form (OCP^O) to its light-activated, red form (OCP^R) via absorption by OCP’s associated carotenoid, 3′-hydroxyechinenone (reviewed in Kirilovsky and Kerfeld, 2013). OCP^R binds to the light-harvesting phycobilisome complex, where it quenches excess energy, diverting energy away from the photosystems and dispersing it as heat. The unstable OCP^R readily reverts to OCP^O; therefore, OCP^R only acts as a floodgate for excess energy under high-light conditions.

Blue-green light activates the energy quenching activity of OCP, but red-orange light does not. However, Sedoud et al. (pages 1781–1791) showed that OCP has an additional activity in photoprotection, as it protects Synechocystis cells from photodamage by red light (see figure): Cells lacking OCP show stronger inhibition of photosystem activity than the wild type. Also, cells overexpressing OCP show the least inhibition, even in the ΔpsbA2 sensitized background, which lacks one of the genes encoding the photosystem II protein D1, a key target of photoinhibition. Based on the known activities of carotenoids in other systems, the authors hypothesized that this effect might involve singlet oxygen, ¹O₂. In addition to inundating the photosynthetic apparatus with excess energy, excess light energy arriving at the reaction centers can induce the accumulation of singlet oxygen ¹O₂ via charge recombination reactions and chlorophyll triplet formation (reviewed in Fischer et al., 2013). Indeed, the authors used histidine-mediated chemical trapping to show that OCP decreased ¹O₂ production in intact Synechocystis cells (figure). Moreover, the authors found that OCP isolated from different Synechocystis strains could quench ¹O₂ produced in response to the photosynthesizers methylene blue and Rose Bengal. The different strains used to isolate OCP differed in carotenoid composition, indicating that the OCP protein structure affects ¹O₂ quenching more than the associated carotenoid. OCP quenches ¹O₂ by physical means (whereby the carotenoid gains energy from ¹O₂ and then returns to the ground state) and by chemical means (whereby ¹O₂ oxidizes the carotenoid). However, not all carotenoid binding proteins can quench ¹O₂; the authors found that Red Carotenoid Protein from Anabaena did not quench ¹O₂, despite its similarity to the N-terminal domain of OCP.

OCP protects Synechocystis cells from photodamage caused by strong red light. Activity of photosystem II (PSII) in Synechocystis cells illuminated with strong red light for the indicated times. Synechocystis genotypes: wild type (WT), cells lacking OCP (ΔOCP), photodamage-sensitized cells (ΔpsbA2), and cells overexpressing OCP in the ΔpsbA2 background (OE-OCP). (Reprinted from Sedoud et al. [2014], Figure 1.)

The presence of singlet oxygen indicates a serious situation for the cell, the first trickles of impending disaster, and singlet oxygen also functions as a key signaling molecule to trigger high-light responses, including transcriptional and translational changes that result in acclimation or programmed cell death. However, the pathway by which such a short-lived molecule, with a microsecond half-life, functions in long-distance signaling remains unclear. This identification of OCP’s function in quenching singlet oxygen provides new information on the fate of ¹O₂ in the cell and reveals another mechanism by which cells protect themselves from the flood of light.