Question

The enzyme required for carboxylation in the Calvin cycle is :

• A. Carboxypeptidase
• B. PEP carboxylase
• C. RuBP carboxylase–oxygenase
• D. Hexokinase

Hint:

RuBisCO has a "dual personality"—it can bind both $CO_2$ (Carboxylation) and $O_2$ (Oxygenation) depending on their relative concentrations.

Answer 1

The Correct Option is C

The Calvin cycle ($C_3$ cycle) is the primary pathway for carbon fixation in all photosynthetic plants. The most critical step is the carboxylation of Ribulose-1,5-bisphosphate (RuBP).
• RuBP carboxylase-oxygenase (RuBisCO): This is the enzyme that catalyzes the reaction between $CO_2$ and RuBP to form two molecules of 3-phosphoglyceric acid (3-PGA). It is considered the most abundant enzyme on Earth.
• PEP carboxylase: This enzyme is involved in the initial fixation of carbon in $C_4$ plants and CAM plants, but not in the $C_3$ Calvin cycle.
• Hexokinase: This enzyme is used in the first step of glycolysis (respiration), not photosynthesis.
• Carboxypeptidase: This is a digestive protease enzyme found in animals.