68 Which one of the following statements regarding PEPCase is INCORRECT?
(1) During the day, C4 PEPCase is inactive whereas CAM PEPCase is active.
(2) PEPCase is inactivated by dephosphorylation.
(3) PEPCase kinase phosphorylates PEPCase.
(4) The synthesis of PEPCase kinase is modulated by circadian rhythm in CAM leaves.
Introduction
Phosphoenolpyruvate carboxylase (PEPCase) is a key enzyme in C4 and CAM photosynthesis, responsible for initial CO2 fixation. Its activity is tightly controlled by phosphorylation and circadian rhythms to match carbon fixation with stomatal behavior and environmental changes.
Explanation of Each Statement
(1) During the day, C4 PEPCase is inactive whereas CAM PEPCase is active.
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Incorrect. In C4 plants, PEPCase is active during the day, capturing CO2 when stomata are open and light is available, supporting the high photosynthetic rate. In CAM plants, PEPCase is active at night for nocturnal CO2 fixation and typically less active during the day due to decarboxylation processes and enzyme regulation. This statement is therefore factually incorrect.
(2) PEPCase is inactivated by dephosphorylation.
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Correct. PEPCase activity is reduced when the enzyme is dephosphorylated; phosphorylation enhances its activity and reduces sensitivity to feedback inhibition.
(3) PEPCase kinase phosphorylates PEPCase.
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Correct. The regulatory enzyme PEPCase kinase phosphorylates and thereby activates PEPCase, playing a crucial role in diurnal regulation of C4 and CAM metabolism.
(4) The synthesis of PEPCase kinase is modulated by circadian rhythm in CAM leaves.
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Correct. In CAM plants, PEPCase kinase expression and activity follow a circadian pattern, peaking at night to match the nocturnal carbon fixation phase.
Why Option (1) Is Incorrect
The central misconception is the proposed inactivity of C4 PEPCase during the day. In both C4 and CAM pathways, PEPCase activity aligns with environmental CO2 uptake—daytime for C4 and nighttime for CAM. This difference underlies an important adaptive mechanism in plant photosynthesis.
