Question

Some Calvin Cycle enzymes contain disulfide bonds that must be reduced through a mechanism involving thioredoxin in order for the enzyme to be active in sunlight. What is the name of the key regulatory protein that inactivates these same Calvin Cycle enzymes by oxidation when the sun goes down? Choose the ONE best answer.

Thioredoxin requires sunlight for proper folding into its functional three dimensional structure, so in the absence of light, it is unfolded and cannot reduce these enzymes.
The protein is called ferredoxin-thioredoxin reductase.
There is no regulatory protein that oxidizes these Calvin cycle enzymes; oxidation is spontaneous Sunlight is a reducing energy and moonlight is an oxidizing energy, so when the moon comes out at night, the Calvin Cycle enzymes are oxidized spontaneously.
None of these answers are correct
Thioredoxin has multiple light-sensing properties and when the sun goes down it turns into a strong oxidant.

Answer 1

There is no regulatory protein that oxidizes these Calvin cycle enzymes; oxidation is spontaneous.

Step-by-step explanation:

Four enzymes of the Calvin cycle are regulated by sunlight. These enzymes are namely Ribulose 5-phosphate kinase, fructose 1,6-bisphosphatase, sedoheptulose 1,7-bisphosphatase, and glyceraldehyde 3-phosphate dehydrogenase.

The inactive form of these enzymes has disulfide bonds between two Cys residues. In the presence of sunlight, the reduced thioredoxin that obtains electrons from PS-I via ferredoxin reduces the disulfide bonds between the Cys residues of these enzymes.

The reduction of the bonds brings about the confirmation changes to make the enzyme active.

At nightfall, these bonds are spontaneously re-oxidized and there is no reduced thioredoxin available to reduce them again. The spontaneous oxidation of the disulfide bonds of these enzymes at nightfall makes them inactive again.