Q.52 Regulation of phosphoenolpyruvate carboxylase (PEPCase) governs CO2 fixation
in both C4 and CAM (crassulacean acid metabolism) plants. Which one or more
of the following statements with respect to PEPCase activity is/are CORRECT?
(A) PEPCase in C4 plants is inactivated by dephosphorylation during the day.
(B) PEPCase in CAM plants is inactivated by dephosphorylation during the day.
(C) PEPCase in C4 plants is inactivated by dephosphorylation at night.
(D) PEPCase in CAM plants is inactivated by dephosphorylation at night.

PEPCase in C4 and CAM plants is regulated by phosphorylation, which activates the enzyme, while dephosphorylation inactivates it to match CO2 fixation needs during light or dark periods.​

Option Analysis

(A) PEPCase in C4 plants is inactivated by dephosphorylation during the day.
Incorrect. In C4 plants, PEPCase is active during the day for CO2 fixation in mesophyll cells; light activates PEPCase kinase, causing phosphorylation for activation, not dephosphorylation.​

(B) PEPCase in CAM plants is inactivated by dephosphorylation during the day.
Correct. CAM plants fix CO2 at night; PEPCase is phosphorylated (active) at night but dephosphorylated during the day by PEPCase phosphatase to prevent futile cycling with decarboxylation.​

(C) PEPCase in C4 plants is inactivated by dephosphorylation at night.
Correct. At night in C4 plants, low light reduces PEPCase kinase activity, leading to dephosphorylation and inactivation to avoid unnecessary PEP use.​

(D) PEPCase in CAM plants is inactivated by dephosphorylation at night.
Incorrect. CAM plants require active PEPCase at night for CO2 uptake when stomata open; phosphorylation by circadian-controlled kinase activates it then.​

Correct answers: (B) and (C).​

PEPCase regulation in C4 and CAM plants optimizes CO2 fixation via reversible phosphorylation. This post-translational control ensures enzyme activity aligns with photosynthetic demands, crucial for competitive exams like CSIR NET.​

Phosphorylation Activates PEPCase

PEPCase (phosphoenolpyruvate carboxylase) catalyzes PEP + HCO3- to oxaloacetate, the first CO2 fixation step in C4 and CAM pathways.

• Phosphorylation at a serine residue decreases malate inhibition sensitivity and boosts activity.​

• PEPCase kinase (PPCK), light-activated in C4 or circadian-controlled in CAM, drives this.​

C4 Plants: Daytime Activation

C4 plants use spatial separation; PEPCase in mesophyll fixes CO2 daytime when stomata open.

• Day: Phosphorylation activates PEPCase for high activity.​

• Night: Dephosphorylation inactivates it, conserving resources.​

CAM Plants: Nighttime Activation

CAM plants temporally separate fixation to minimize water loss in arid conditions.

• Night: Stomata open; phosphorylation activates PEPCase, storing malate in vacuoles.​

• Day: Dephosphorylation inactivates PEPCase; malate decarboxylates for Rubisco.​