Figure 5. Hypothetical model for redox regulation in the stroma and the lumen.
The redox-regulated stromal enzymes are shown in the upper panel (for details, see section 2) and the redox-regulated thylakoid membrane and lumenal proteins (section 4) are at the bottom of the panel. The arrows indicate the flow of reducing equivalents. Hypothetical electron flow is shown as dashed lines. The reductive pathway is indicated by the blue and cyan arrows for stromal, and lumenal respectively, and oxidative pathways are shown in red and pink arrows for stromal, and lumenal respectively. To clarify, the use of a plus (+) sign in reductive arrows denotes that the reduced state of the protein in the direction of the arrow is in an active or stable form. On the other hand, the presence of a minus (-) sign in the reductive arrow indicates that the reduced state of the protein in the direction of the arrow is in an inactive or unstable form. In summary, reducing equivalents from the stroma are transferred to the lumen via CcdA and HCF164 to VDE and STN7. These proteins are inactive when in their reduced forms, as indicated by the minus (-) sign in the arrows. Additionally, SOQ1 also serves as a reducing mediator, transferring reducing power to LCNP, which is deactivated when reduced. The oxidizing system is mediated by LTO1, which pulls electrons from VDE, STN7, and PsbO. However, the oxidizing mediators for KEA3, Deg1, and LCNP are still not known. LTO1 mediates the oxidizing system, extracting electrons from VDE, STN7, and PsbO. Nevertheless, the oxidizing mediators for KEA3, Deg1, and LCNP remain unknown. All of these redox-regulated proteins in the lumen, including VDE (qE, qZ), KEA3 (qE), STN7 (qT), LNCP (qH), Deg1, and PsbO (qI), are cooperating to induce photoprotective mechanisms. The hypothetical model for removing electrons from the lumen is shown that reactive oxygen species (ROS) act as a strong oxidant via unknown strong oxidant. 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. See more details in Table 1 and section 3 and 4.