Question

Photorespiration reaction catalyzed by RuBisCO is shown below :
\[ \text{RuBP} + \text{O}_2 \rightarrow \text{3-Phosphoglycerate} + \text{X} \] Identify "X" from the given options :

• A. Malate
• B. Phosphoenolpyruvate
• C. 2-Phosphoglycolate
• D. Oxaloacetate

Hint:

Photorespiration is often termed the $C_2$ cycle precisely because the very first stable organic product formed outside the main path contains a chain of 2 carbon atoms (2-Phosphoglycolate). Remembering this carbon-counting trick will help you solve problems involving alternative photosynthetic cycles instantly!

Answer 1

The Correct Option is C

Concept: RuBisCO (Ribulose-1,5-bisphosphate carboxylase-oxygenase) is a dual-functional enzyme. It has active binding sites for both carbon dioxide ($\text{CO}_2$) and oxygen ($\text{O}_2$).

• Under normal conditions where $\text{CO}_2$ concentrations are high, RuBisCO acts as a carboxylase, leading to the productive Calvin cycle.

• Under conditions of high temperature, high light intensity, and high oxygen concentration relative to $\text{CO}_2$, RuBisCO binds to oxygen instead. This initiates a wasteful metabolic pathway known as

photorespiration (also called the $C_2$ cycle or photosynthetic carbon oxidation cycle).

Step 1: Analyze the stoichiometry of the reaction
Let us observe the substrate involved in the reaction:

• RuBP (Ribulose-1,5-bisphosphate): This is a

5-carbon sugar molecule containing two phosphate groups.

• Oxygen ($\text{O}_2$): Adds no carbon atoms to the system.
When RuBisCO exhibits its oxygenase activity, it breaks down the 5-carbon RuBP molecule via oxygenation. The total carbon pool entering the reaction equals 5 carbons.

Step 2: Tracking the products of oxygenation
The reaction splits the 5-carbon substrate asymmetricaly into two specific fragments:

• 3-Phosphoglycerate (PGA): A

3-carbon molecule that can enter the Calvin cycle.

• The unknown molecule "X": Since the total number of reactant carbons is 5, and 3 carbons are contained in PGA, the remaining molecule must contain exactly: \[ 5 \text{ carbons} - 3 \text{ carbons} = 2 \text{ carbons} \]
The standard 2-carbon phosphorylated compound produced during this step of photorespiration is

2-Phosphoglycolate. Let us write out the balanced biochemical equation explicitly: \[ \text{RuBP (5C)} + \text{O}_2 \xrightarrow{\text{RuBisCO (Oxygenase activity)}} \text{3-Phosphoglycerate (3C)} + \text{2-Phosphoglycolate (2C)} \] Comparing this balanced equation with the equation given in the question statement, we find that the structural entity represented by

"X" is precisely 2-Phosphoglycolate.

Step 3: Verification of other options

• Option (1) Malate: A 4-carbon organic acid primarily active in $C_4$ and CAM pathways.

• Option (2) Phosphoenolpyruvate (PEP): A 3-carbon primary $\text{CO}_2$ acceptor molecule in $C_4$ plants.

• Option (4) Oxaloacetate (OAA): A 4-carbon dicarboxylic acid that acts as the first stable product in $C_4$ photosynthesis.
None of these options match the 2-carbon product of the RuBP oxygenase step, confirming that option (3) is correct.