Oxygen and ROS in Photosynthesis

Abstract

Oxygen is a natural acceptor of electrons in the respiratory pathway of aerobic organisms and in many other biochemical reactions. Aerobic metabolism is always associated with the formation of reactive oxygen species (ROS). ROS may damage biomolecules but are also involved in regulatory functions of photosynthetic organisms. This review presents the main properties of ROS, the formation of ROS in the photosynthetic electron transport chain and in the stroma of chloroplasts, and ROS scavenging systems of thylakoid membrane and stroma. Effects of ROS on the photosynthetic apparatus and their roles in redox signaling are discussed.

Keywords: chloroplasts; photodamage; photosynthetic electron transport chain; reactive oxygen species; redox signaling.

Figure 1

Singlet oxygen (¹O₂) generation in the antenna complex (A) and in the reaction center of PSII via charge recombination reactions (B). Proposed mechanism for the formation of ¹O₂ on the donor side of PSII via formation of organic peroxyl radicals (C). (A) In the antenna complex, the absorption of a photon by a molecule of Chl leads to the formation of a singlet excited state, ¹Chl*, that can transform to the triplet state ³Chl via intersystem crossing. The formation of ¹O₂ via intersystem crossing has been demonstrated to occur in isolated LHCII [164,165]. (B) In the reaction centre of PSII, [P680⁺Pheo⁻] is originally formed via electron transfer from ¹P680* to Pheo in a virtual singlet state ¹[P680⁺Pheo⁻] that recombines to ¹P680*. Charge recombination of P680⁺PheoQ_A⁻] causes the formation of ¹[P680⁺Pheo⁻]. The long lifetime of the state [P680⁺PheoQ_A⁻] destroys spin correlation, and therefore the recombination [P680⁺PheoQ_A⁻] to ¹[P680⁺Pheo⁻Q_A] often produces a “virtual triplet state” of the primary radical pair, i.e., a radical pair with such a spin configuration that, in its recombination to an excited state of the primary donor, produces a triplet, ³P680 [172,173,174]. (C) On the donor side of PSII, carbon centered radicals (R^•) can be formed via the oxidation of lipids and proteins by P680⁺ if electron donation from Mn-cluster to P680⁺ does not function. R^• are able to react with O₂ to form peroxyl radical (ROO^•). Two peroxyl radicals react with each other to form linear tetraoxide (ROOOOR) that decomposes to ¹O₂, carbonyl (R=O) and alcohols (R–OH) via the Russell mechanism [200,201,202].

Figure 2

Formation of reactive oxygen species (ROS) in PSII. (A) Formation of superoxide (O₂^•−) can occur with the interaction of O₂ with a semiquinone anion radicals at the Q_A and Q_B sites, when the electron flow from Q_B to the PQ pool is limited. The low potential form of cyt b559 can reduce O₂ to O₂^•− inside the thylakoid membrane [234,235,236]. (B) Formation of H₂O₂ and HO^•. Cyt b559 can catalyze the formation of H₂O₂ inside the thylakoid membrane by a O₂^•− dismutation mechanism [72,160]. O₂^•− can reduce cyt b559 (Fe³⁺) to cyt b559 (Fe²⁺). O₂^•− + cyt b559 (Fe³⁺) ∆ O₂ + cyt b559 (Fe²⁺). The following interaction of HO₂^• with Cyt b559 (Fe²⁺) leads to the formation of a ferric–hydroperoxo intermediate of cyt b559 (Fe³⁺–OOH) which can spontaneously decompose to cyt b559 (Fe³⁺) and H₂O₂. HO₂^• + cyt b559 (Fe²⁺) → cyt b559 (Fe³⁺–OOH) + H⁺→ cyt b559 (Fe³⁺) + H₂O_2. The formation of H₂O₂ in a cyt b559-dependent way requires the protonation of O₂^•− to form HO₂^•. The interaction of O₂^•− with Fe²⁺ on the acceptor side of PSII can result in the formation of a ferric–peroxo intermediate [Fe³⁺–OO⁻] that can be protonated to a ferric–hydroperoxo intermediate [Fe³⁺–OOH]. O₂^•− + [Fe²⁺] → [Fe³⁺–OO⁻] + H⁺ → [Fe³⁺–OOH]. [Fe³⁺–OOH] can be reduced by an electron from Q_A⁻, which causes its decomposition to a ferric–oxo intermediate [Fe³⁺–O⁻] and HO^•. Q_A⁻ + [Fe³⁺–OOH] → Q_A + [Fe³⁺–O⁻] + HO^•. (C) Formation of organic hydroperoxides on the donor side of PSII. Charge separation when the OEC is inactive leads to the formation of P680^•+ and TyrZ^• which have a long lifetime and are therefore able to interact with surrounding molecules such as Chls, carotenoids and amino acids. The interaction of P680^•+ or TyrZ ^• with an organic molecule (RH) can proceed via a radical chain mechanism [200,201,202].

Figure 3

(A) Forward electron transfer chain, lifetimes and midpoint redox potentials of the cofactors of PSI; (B) charge recombination reactions and recombination lifetimes of the cofactors of PSI; the values were taken from [145,268,269,270,271]; (C) possible means of ROS formation in PSI [152,266,272,273,274,275]. PC is plastocyanin; P700 is a dimer of Chl a molecules, the primary electron donor; A_0A and A_0B are Chl a molecules located in branches A and B, respectively, both act as primary electron acceptors; A_1A and A_1B are phylloquinone molecules located in branch A and B, respectively, both acting as electron acceptors; F_X, a 4Fe-4S cluster, a secondary electron acceptor; F_A and F_B, 4Fe-4S clusters, terminal electron acceptors.

Figure 4

Autocatalytic oxidation of reduced plastoquinone (PQH₂) by O₂ in the thylakoid membrane. 1—formation of a plastosemiquinone radical (PQH^•) by a dismutation reaction of PQH₂ with PQ; 2—reduction in O₂ by a plastosemiquinone anion radical (PQ^•−) with formation of superoxide anion radical (O₂^•−); 3—oxidation of PQH₂ by O₂^•− with formation of hydrogen peroxide (H₂O₂) and PQ; 4—diffusion of O₂^•− from thylakoid membrane to stroma and to lumen. In the autocatalytic oxidation of PQH₂, the reaction of O₂^•− with PQH₂ provides excess PQ^•− that can be involved in the formation of O₂^•− and, in turn, accelerates the oxidation of PQH₂ [128].

Figure 5

The proposed generation of O₂^•− in Cyt b6f. (A) In the Q_O site of Cyt b6f, PQH^• is generated (Table 2) by the 2Fe-2S cluster of the high-potential Rieske iron−sulfur protein. PQ^•− that has a long residence time within the Q_O pocket, and cytochrome b_L can also serve as a reductant for the generation of O₂^•− in Cyt b6f. In addition, HO₂^•, can be reduced by cytochrome b_L to form H₂O₂ [317]. (B) The proposed generation of O₂^•− in Cyt b6f in the presence of DNP-INT, an inhibitor of PQH₂ oxidation by Cyt b6f. The oxidation of PQH₂ does not occur in Q_O site, formation of PQH^• and its deprotonation can occur in the Qi site. PQH^• can be oxidized in subsequent reactions with O₂ or with hemes b_H or b_L. H₂O₂ can be formed via the reaction of O₂^•− with PQH₂ or via the reaction of HO₂^• with cytochrome b_H or cytochrome b_L.