Question

Which of the following metabolites is not an intermediate in the triglyceride biosynthesis by Kennedy pathway?

• A. Lysophosphatidic acid
• B. Phosphatidic acid
• C. Diacylglycerol
• D. Glyceraldehyde-3-Phosphate

Hint:

The Kennedy pathway sequentially builds triglycerides on a glycerol-3-phosphate backbone:
Glycerol-3-P $\rightarrow$ Lysophosphatidic acid $\rightarrow$ Phosphatidic acid $\rightarrow$ Diacylglycerol $\rightarrow$ Triacylglycerol.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The Kennedy pathway (also known as the glycerol-3-phosphate pathway) is the primary metabolic route for the biosynthesis of triacylglycerols (triglycerides) and structural glycerophospholipids in cells.

Step 2: Detailed Explanation:
Let us trace the enzymatic intermediates of the Kennedy pathway:
1. The pathway initiates with the acylation of Glycerol-3-phosphate by glycerol-3-phosphate acyltransferase (GPAT) to yield Lysophosphatidic acid (1-acyl-glycerol-3-phosphate).
2. Lysophosphatidic acid is then acylated by lysophosphatidic acid acyltransferase (LPAAT) at the sn-2 position to produce Phosphatidic acid (1,2-diacylglycerol-3-phosphate).
3. The phosphate group is subsequently cleaved from phosphatidic acid by phosphatidic acid phosphatase (PAP) to yield Diacylglycerol (DAG).
4. Finally, diacylglycerol is acylated by diacylglycerol acyltransferase (DGAT) to produce Triacylglycerol (triglyceride).
Let us analyze Glyceraldehyde-3-phosphate:
Glyceraldehyde-3-phosphate is a three-carbon sugar phosphate intermediate of glycolysis and the Calvin cycle.
Although it can be converted into dihydroxyacetone phosphate (DHAP) and then reduced to glycerol-3-phosphate, it is not a direct metabolic intermediate within the sequential steps of the Kennedy pathway itself.
Therefore, Glyceraldehyde-3-phosphate is the correct answer.

Step 3: Final Answer:
Glyceraldehyde-3-Phosphate is not an intermediate of the Kennedy pathway.