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.