Q4:

RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in $C_4$ plants?

Initial Theoretical Framework: The Duality of RuBisCO

Ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO) is the most abundant enzyme on Earth. Its catalytic activity is governed by the relative concentrations of $CO_2$ and $O_2$ at the active site, as well as the temperature. The dual nature is defined by the following reactions:

• Carboxylation: $RuBP + CO_2 \rightarrow 2(3\text{-PGA})$ [Catalyzed by RuBisCO in the Calvin Cycle]
• Oxygenation: $RuBP + O_2 \rightarrow 1(3\text{-PGA}) + 1(\text{Phosphoglycolate})$ [Photorespiration]

The oxygenation process is metabolically wasteful, as it releases fixed carbon and consumes ATP. $C_4$ plants have evolved a specialized mechanism to bypass this inefficiency.

Step 1: Anatomical Specialization (Kranz Anatomy)

Unlike $C_3$ plants, $C_4$ plants possess Kranz Anatomy, which spatially separates the primary $CO_2$ fixation from the Calvin Cycle. This separation is the prerequisite for increasing the $CO_2:O_2$ ratio at the site of RuBisCO.

Step 2: The $CO_2$ Concentrating Mechanism

In $C_4$ plants, the initial fixation of $CO_2$ occurs in the mesophyll cells, while the Calvin Cycle (involving RuBisCO) is restricted to the bundle sheath cells.

1. Atmospheric $CO_2$ is converted to $HCO_3^-$ and fixed by PEP carboxylase to form oxaloacetate ($OAA$). [PEP carboxylase has a high affinity for $CO_2$ and no oxygenase activity].
2. $OAA$ is converted into malate or aspartate (4-carbon compounds) and transported into the bundle sheath cells.
3. Inside the bundle sheath cells, these 4-carbon compounds undergo decarboxylation, releasing a high concentration of $CO_2$.

Visual Representation: Spatial Separation in $C_4$ Plants

Step 3: Creating a Low-Oxygen Environment

The bundle sheath cells in $C_4$ plants are characterized by:

• Reduced or absent Photosystem II (PSII) activity, which prevents the photolysis of water and the subsequent production of $O_2$. [Per the Hill Reaction].
• Thick, suberized cell walls that create a gas-tight barrier, preventing $O_2$ from diffusing into the bundle sheath cells.

Step 4: The Kinetic Outcome

By concentrating $CO_2$ in the bundle sheath and simultaneously minimizing the presence of $O_2$, the $C_4$ cycle ensures that the probability of RuBisCO binding $O_2$ is virtually zero. Consequently, the enzyme is "forced" to function as a carboxylase. This adaptation prevents the energy-intensive process of photorespiration, thereby significantly increasing the photosynthetic efficiency of the plant under high-light and high-temperature conditions.

Final Solution: RuBisCO carries out more carboxylation in $C_4$ plants because they utilize a $CO_2$ concentrating mechanism that elevates the local concentration of $CO_2$ around the enzyme while excluding $O_2$ within the bundle sheath cells, thereby suppressing the oxygenase activity of RuBisCO.