Thioredoxin is a small protein that carries two reversibly oxidizable sulfhydryl (-SH) groups that participates in a wide variety of reduction/oxidation reactions. In photosynthesis regulation, reduction of thioredoxin is promoted by oxidation of the reduced form of ferredoxin. In strongly irradiated chloroplasts, in which  NADP⁺ stores are depleted, reduced ferredoxin accumulates. High levels of reduced ferredoxin yield reduced thioredoxin, which thereby lead to activation of the Calvin cycle enzymes (Figure 17.23), stimulating the Calvin cycle reactions when the light reactions are very active. The same compound, reduced thioredoxin, also stimulates the CF0-CF1 complexes, ensuring a high rate of ATP generation when illumination is intense.

Thioredoxin is also a potential carrier of electrons to ribonucleotide reductase for reduction of ribose to deoxyribose. A proposed mechanism of action of ribonucleotide reductase is shown in Figure 22.15.

Thioredoxin is also involved in reducing sulfate in sulfur fixation (see here)

Some of the interesting biological activities of thioredoxin are listed in Table 22.1.