Question

Enzymes of the "Kennedy pathway" for the triacylglycerol biosynthesis in plants:
A. Glycerol-3-P dehydrogenase
B. Glycerol-3-phosphate acyltransferase
C. Lysophosphatidic acid acyltransferase
D. Phosphatidic acid phosphatase
E. Diacylglycerol acyltransferase
Choose the correct sequence of the action of aforementioned enzymes of the "Kennedy pathway" in the triacylglycerol biosynthesis.

• A. A, B, C, D, E
• B. B, A, D, C, E
• C. B, A, C, D, E
• D. B, A, C, E, D

Hint:

To remember the Kennedy pathway, focus on the acyltransferases: first is GPAT (B) to make Lysophosphatidic acid, then LPAAT (C) to make Phosphatidic acid, followed by a Phosphatase (D) to make Diacylglycerol, and finally DGAT (E) to make Triacylglycerol.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The Kennedy pathway (or glycerol-3-phosphate pathway) is the primary metabolic pathway responsible for the de novo synthesis of triacylglycerol (TAG) in plants and animals.
It involves sequential acylation and dephosphorylation of a glycerol backbone.

Step 2: Detailed Explanation:
Let us examine the step-by-step enzymatic sequence of the Kennedy pathway:
1. Glycerol-3-P dehydrogenase (A): This enzyme reduces dihydroxyacetone phosphate (DHAP) to produce Glycerol-3-phosphate, providing the active glycerol backbone for lipid assembly.
2. Glycerol-3-phosphate acyltransferase (GPAT - B): This enzyme catalyzes the transfer of a fatty acid from acyl-CoA to the sn-1 position of glycerol-3-phosphate, producing Lysophosphatidic acid (LPA).
3. Lysophosphatidic acid acyltransferase (LPAAT - C): This enzyme transfers a second fatty acid to the sn-2 position of LPA, producing Phosphatidic acid (PA).
4. Phosphatidic acid phosphatase (PAP - D): This enzyme removes the phosphate group from the sn-3 position of PA, yielding Diacylglycerol (DAG).
5. Diacylglycerol acyltransferase (DGAT - E): This final, rate-limiting enzyme transfers a third fatty acid to the sn-3 position of DAG, producing the fully assembled neutral lipid, Triacylglycerol (TAG).
Thus, the correct chronological sequence of enzymes is: A \(\to\) B \(\to\) C \(\to\) D \(\to\) E.

Step 3: Final Answer:
The correct sequential order of the Kennedy pathway is A, B, C, D, E, which corresponds to option (A).