5. What are the missing pieces in the lumen thiol-disulfide regulation mechanism?
So far, we have discussed the redox regulation from the stroma to the transmembrane region to the lumen and its photoprotective roles. The molecular players in the redox regulation and relevant photoprotective forms are summarized in Figure 5. In brief, reducing equivalents from the stroma are transferred through CcdA and HCF164 to VDE and STN7. These proteins are inactive in their reduced forms, as indicated by the arrows pointing in the opposite direction (-). Another reducing mediator, SOQ1, transfers reducing power to LCNP, which is inactivated in its reduced form. The electron donors for KEA3-1, PsbO, and Deg1 are still unknown, as indicated by the dashed arrows. The oxidizing system appears to be mediated by LTO1. LTO pulls electrons from VDE, STN7, and PsbO. The oxidizing mediators for KEA3-1, Deg1, and LCNP are not known. All of the redox-regulated proteins in the lumen, including VDE (qE, qZ), KEA3 (pmf partitioning into ∆ψ and ∆pH and effects on qE), STN7 (qT), LNCP (qH), Deg1, and PsbO (qI), orchestrate photoprotective mechanisms.
However, as seen in Figure 5, many of the pathways in lumenal redox regulation are still hypothetical (so many dashed lines!). Certainly, we have just begun to scratch the surface of this complex lumenal redox regulation network. While there are many unanswered questions regarding this complex redox network, there are two significant yet unresolved questions we would like to focus on:
How do electrons get out of the lumen? Recall that LTO1 may be involved in numerous oxidizing events but the final acceptor of LTO1 has not been identified (See Figure 5).
What keeps fine-tuning redox potentials and kinetics to achieve the appropriate regulatory levels in the “acidic lumen?” As we discussed in section 3.2, due to the ∆pH component of pmf , the lumen has quite a unique redox environment, where the redox midpoint potentials of the regulatory thiols substantially increase upon illumination (See Figure 4).
One hypothetical model that could answer both of these questions is that reactive oxygen species (ROS) act as a strong oxidant. It has been shown that in the stroma, the 2CP-mediated oxidation system pulls electrons from target enzymes and transfers them to the final acceptor hydrogen peroxide (H2O2), which is then reduced to water. In the hypothetical model for lumenal redox regulation, the Trx-domain of LTO1 would pull electrons from target enzymes, and these electrons would then be transferred to the VKOR domain of LTO1 in the thylakoid membrane. From there, the electrons could be transferred to an unknown oxidizing mediator (marked as ”?” in the figure), which could further reduce H2O2. Another possibility is that there is another oxidoreductase in the thylakoid membrane that uses H2O2 as a final electron acceptor. This hypothesis is supported by the results of state transition experiments in the lto1 mutant, which showed that state transitions were not completely abolished in this mutant (Wu et al 2021). This result strongly suggests that other factors are also involved in keeping STN7 oxidized and active. The other possible strong oxidants could be plastoquinone (PQ) pool. More research will be needed to determine the exact mechanisms involved.