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. Question 10 options: 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. 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. The protein is called ferredoxin-thioredoxin reductase. Thioredoxin has multiple light-sensing properties and when the sun goes down it turns into a strong oxidant. 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. 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. The protein is called ferredoxin-thioredoxin reductase. Thioredoxin has multiple light-sensing properties and when the sun goes down it turns into a strong oxidant. Solution Option 4 \" None of these answers are correct.\" \"Chloroplast thioredoxin systems are crucial components of redox network, mediating environmental signals to chloroplast proteins. One key protein in calvin cycle regulatory processes is thioredoxin. The reduced form of thioredoxin activates many biosynthetic enzymes by reducing disulfide bridges that control their activity and inhibits several degradative enzymes by the same means. In chloroplasts, oxidized thioredoxin is reduced by ferredoxin in a reaction catalyzed by ferredoxin-thioredoxin reductase. This enzyme contains a 4Fe-4S cluster that couples two one-electron oxidations of reduced ferredoxin to the two-electron reduction of thioredoxin. Thus, the activities of the light and dark reactions of photosynthesis are coordinated through electron transfer from reduced ferredoxin to thioredoxin and then to component enzymes containing regulatory disulfide bonds. Other means of control of calvin cycle also exist. For instance, phosphoribulose kinase and glyceraldehyde 3-phosphate dehydrogenase also are regulated by NADPH directly. Some plants synthesize a transition-state inhibitor, carboxyarabinitol-1-phosphate (CA1P), in the dark. In the dark, these enzymes associate with a small protein called CP12 to form a large complex in which the enzymes are inactivated. RuBP Carboxylase Activase facilitates release.

1. 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.
Question 10 options:
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.
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.
The protein is called ferredoxin-thioredoxin reductase.
Thioredoxin has multiple light-sensing properties and when the sun goes down it turns into a
strong oxidant.
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.
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.
The protein is called ferredoxin-thioredoxin reductase.
Thioredoxin has multiple light-sensing properties and when the sun goes down it turns into a
strong oxidant.
Solution
Option 4 " None of these answers are correct."
"Chloroplast thioredoxin systems are crucial components of redox network, mediating
environmental signals to chloroplast proteins. One key protein in calvin cycle regulatory
processes is thioredoxin. The reduced form of thioredoxin activates many biosynthetic enzymes
by reducing disulfide bridges that control their activity and inhibits several degradative enzymes
by the same means. In chloroplasts, oxidized thioredoxin is reduced by ferredoxin in a reaction
catalyzed by ferredoxin-thioredoxin reductase. This enzyme contains a 4Fe-4S cluster that

2. couples two one-electron oxidations of reduced ferredoxin to the two-electron reduction of
thioredoxin. Thus, the activities of the light and dark reactions of photosynthesis are coordinated
through electron transfer from reduced ferredoxin to thioredoxin and then to component enzymes
containing regulatory disulfide bonds. Other means of control of calvin cycle also exist. For
instance, phosphoribulose kinase and glyceraldehyde 3-phosphate dehydrogenase also are
regulated by NADPH directly. Some plants synthesize a transition-state inhibitor,
carboxyarabinitol-1-phosphate (CA1P), in the dark. In the dark, these enzymes associate with a
small protein called CP12 to form a large complex in which the enzymes are inactivated. RuBP
Carboxylase Activase facilitates release of CA1P from RuBP Carboxylase, when it is activated
under conditions of light by thioredoxin".