Question

1. The Calvin cycle is sometimes referred to as the dark reactions, even though light is required for flux to be maintained. Describe the two primary mechanisms that regulate Calvin cycle enzymes so that they are most active in the light.

Answer 1

Primary mechanisms that regulate enzymes of Calvin cycle:

A) Thioredoxin/ferridoxin system:

In this, reduced ferridoxin formed in photosystem II, reduces thioredoxin which then reduces the disulfide bonds of certain enzymes. As primary requirement is of reduced ferriodoxin, which is formed due to light reactions, this regulation is by light reactions.

Enzymes involved are glyceraldehyde-3-phosphate dehydrogenase, seduheptulose-1,7-bisphosphatase, fructose-1,6-bisphosphatase etc.

B) Rubisco activation:

Activation of RuBisCO is brought about by the enzyme rubisco activase which functions to deprotonate lysine and makes it favorable for carbamoylation. Rubisco activase requires high concentration ATP in stroma which is possible only if ATP is generated by light reactions.

Now, functioning of rubisco is also dependent on presence of Mg2+ ions. These are realesed from lumen of thylakoid only when the internal pH is low. pH will be low if proton concentration is high which is possible only if electron flow occurs in Z scheme. This is also based on light reactions.