Light and Rubisco Activase Stimulate CO2 Fixation

The Calvin cycle enzymes that catalyze CO2 fixation are rapidly inactivated in the dark, thereby conserving ATP that is generated in the dark for other synthetic reactions, such as lipid and amino acid biosynthesis. One mechanism that contributes to this control is the pH dependence of several

Calvin cycle enzymes. Because protons are transported from the stroma into the thylakoid lumen during photoelectron transport (see Figure 8-37), the pH of the stroma increases from «7 in the dark to «8 in the light. The increased activity of several Calvin cycle enzymes at the higher pH promotes CO 2 fixation in the light.

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:

PSI 2e-

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