Question

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.

Hint:

PEPCase is {phosphorylated when it needs to fix CO2: day for C4, night for CAM. “Phosphorylation = protection from malate inhibition; dephosphorylation = off.”

Answer 1

The Correct Option is B, C

Step 1: Biochemical control logic. PEPCase catalyzes \(\mathrm{PEP + HCO_3^- ⇒ OAA + P_i}\). Its activity is tuned by reversible phosphorylation on a conserved serine:
\(\bullet\) Phosphorylated PEPCase \(⇒\) more active, less sensitive to feedback inhibition by malate, and more responsive to activation by glucose-6-phosphate.
\(\bullet\) Dephosphorylated PEPCase \(⇒\) less active (greater malate inhibition).
The kinase responsible is PEPCase kinase (PPCK); a specific phosphatase removes the phosphate. PPCK expression/activity is under circadian and light control, but the phase differs in C4 vs CAM.
Step 2: C4 plants (daytime CO2 fixation).
In C4 species (e.g., maize), initial CO2 fixation in mesophyll occurs during the day. To support this, PEPCase must be active by day: PPCK is expressed/active in the light, phosphorylating PEPCase \(⇒\) high catalytic rate and malate resistance. At night, PPCK activity falls and phosphatase predominates \(⇒\) PEPCase is dephosphorylated and inactivated. Therefore, (C) is correct and (A) is incorrect.
Step 3: CAM plants (night-time CO2 fixation).
CAM species (e.g., Kalanchoë) open stomata at night to minimize water loss; they fix CO2 into malate at night and decarboxylate by day. Consequently, PPCK is under circadian control with a night peak: at night PEPCase becomes phosphorylated (active); during the day it is dephosphorylated (inactive) to suppress futile cycling while malate is being decarboxylated. Hence, (B) is correct and (D) is incorrect.
Step 4: Concept map linking statements to regulation.
C4: Day (PPCK\(\uparrow\) \(⇒\) PEPCase–P\(\uparrow\) active), Night (dephosphorylation \(⇒\) inactive) \(⇒\) (C) true.
CAM: Night (PPCK\(\uparrow\) \(⇒\) PEPCase–P\(\uparrow\) active), Day (dephosphorylation \(⇒\) inactive) \(⇒\) (B) true.