Question

Consider the following compounds:
1. Phosphoenolpyruvate
2. Malic acid
3. Pyruvic acid
4. Oxaloacetic acid
Which is the correct sequence of these intermediate compounds as they form in $C_4$ dicarboxylic acid pathway flowing the $PEP-$ carboxylation reaction?

• A. $1,2,3,4$
• B. $2,4,3,1$
• C. $4,1,2,3$
• D. $4,2,3,1$

Answer 1

Answer: (D)

↵ The C​​​​​_{​​​​​​4} pathway is also known as Hatch and Slack pathway, occurs in the plant that are adapted to dry tropical regions, e.g., Sugarcane, Maize. It is a cyclic process in which the primary CO​​​_{​​​​​2} acceptor is three-carbon molecule phosphoenolpyruvate which is present in mesophylls cells. PEP carboxylase or PEPcase is the enzyme that catalyzes the CO​​​_{​​​​​2} fixation. In the C​​​​​_{​​​​​​4} plants, the mesophyll cells lack the enzyme RuBisCO.

The 4-Carbon oxaloacetic acid is formed in the mesophyll cell, which is the first product of this pathway. In the mesophyll cells, the oxaloacetic acid is then converted to other 4-carbon compounds like malic acid or aspartic acid and then it is transported to the bundle sheath cells. To release CO​​​_{​​​​​2} and a 3-carbon molecule in the bundle sheath cells, these C​​​₄ acids are broken down, and the CO​​​_{​​​​​2} released in the bundle sheath cells enter the Calvin pathway. The bundle sheath cells lack PEPcase enzyme but have RuBisCO enzyme. The three-carbon molecule is transported back to the mesophyll cell where with the help of a cold sensitive enzyme called PEP synthase, it is converted to PEP again and thus completing the cycle.