Calvin cycle thioredoxin

Thioredoxin in Regulation of Calvin Cycle Enzymes Motohashi and colleagues used thioredoxin as a hook to fish out from plant extracts the proteins that are activated by thioredoxin. To do this, they prepared a mutant thioredoxin in which one of the reactive Cys residues was replaced with a Ser. Explain why this modification was necessary for their experiments. [Motohashi, K., Kondoh, A., Stumpp, M.T., Hisabori, T. (2001) Comprehensive survey of proteins targeted by chloroplast thioredoxin. Proc. Natl. Acad. Sci. USA 98, 11,224-11,229.]... 

Thioredoxin Plays a Key Role in Regulating the Calvin Cycle... 

Figure 20.15. Thioredoxiu. The oxidized form of thioredoxin contains a disulfide bond. When thioredoxin is reduced by reduced ferredoxin, the disulfide bond is converted into two free sulfbydryl groups. Reduced thioredoxin can cleave disulfide bonds in enzymes, activahng certain Calvin cycle enzymes and inactivating some degradahve enzymes.

Figure 20.16. Enzyme Activation by Thioredoxin. Reduced thioredoxin activates certain Calvin cycle enzymes hy cleaving regulatory disulfide bonds.

Chloroplast ferredoxin is a small water soluble protein M W 000) containing an Fe-S center [245]. Its midpoint potential ( — 0.42 V [246]) is suitable for acting as an electron acceptor from the PSI Fe-S secondary acceptors (Centers A and B) and as a donor for a variety of functions on the thylakoid membrane surface and in the stroma. Due to its hydrophylicity and its abundance in the stromal space, ferredoxin is generally considered as a diffusable reductant not only for photosynthetic non-cyclic and cyclic electron flow, but also for such processes as nitrite and sulphite reduction, fatty acid desaturation, N2 assimilation and regulation of the Calvin cycle enzyme through the thioredoxin system [245]. Its possible role in cyclic electron flow around PSI has already been discussed. The mobility of ferredoxin along the membrane plane could be an essential feature of this electron transfer process the actual electron acceptor for this function and the pathway of electron to plastoquinone is, however, still undefined. 

Reduced thioredoxin formed by the light-driven transfer of electrons from ferredoxin activates enzymes of the (Calvin cycle by reducing disulfide bridges. The light-induced increase in pH and level of... 

A stromal protein called thioredoxin (Tx) also plays a role in controlling some Calvin cycle enzymes. In the dark, thioredoxin contains a disulfide bond in the light, electrons are transferred from PSI, via ferredoxin, to thioredoxin, reducing its disulfide bond ... 

Reduced thioredoxin then activates several Calvin cycle enzymes by reducing disulfide bonds In them. In the dark, when thioredoxin becomes reoxIdIzed, these enzymes are reoxidized and thus inactivated. 

FIGURE 20-19 Light activation of several enzymes of the Calvin cycle. The light activation is mediated by thioredoxin, a small, disulfide-containing protein. In the light, thioredoxin is reduced by electrons moving from photosystem I through ferredoxin (Fd) (blue arrows), then thioredoxin reduces critical disulfide bonds in each... 

---