Ferredoxin-thioredoxin reductase EC 1.8.7.2, systematic name ferredoxin:thioredoxin disulfide oxidoreductase, is a [4Fe-4S] protein that plays an important role in the ferredoxin/thioredoxin regulatory chain. It catalyzes the following reaction:

Ferredoxin-Thioredoxin reductase (FTR) converts an electron signal (photoreduced ferredoxin) to a thiol signal (reduced thioredoxin), regulating enzymes by reduction of specific disulfide groups. It catalyses the light-dependent activation of several photosynthesis enzymes and constitutes the first historical example of a thiol/disulfide exchange cascade for enzyme regulation. It is a heterodimer of subunit alpha and subunit beta. Subunit alpha is the variable subunit, and beta is the catalytic chain. The structure of the beta subunit has been determined and found to fold around the FeS cluster.

Major groups of oxygen-producing, photosynthetic organisms such as cyanobacteria, algae, C4, C3, and crassulacean acid metabolism (CAM) plants use Ferredoxin-thioredoxin reductase for carbon fixation regulation. FTR, as part of a greater Ferredoxin-Thioredoxin system, allows plants to change their metabolism based on light intensity. Specifically, the Ferredoxin-Thioredoxin system controls enzymes in the Calvin Cycle and Pentose phosphate pathway - allowing plants to balance carbohydrate synthesis and degradation based on the availability of light. In the light, photosynthesis harnesses light energy and reduces Ferredoxin. Using FTR, reduced Ferredoxin then reduces Thioredoxin. Thioredoxin, through thiol/disulfide exchange, then activates carbohydrate synthesis enzymes such as chloroplast fructose-1,6-bisphosphatase, Sedoheptulose-bisphosphatase, and phosphoribulokinase. As a result, light uses FTR to activate carbohydrate biosynthesis. In the dark, Ferredoxin remains oxidized. This leaves Thioredoxin inactive and allows carbohydrate breakdown to dominate metabolism.

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