Question

Question: Describe how newly synthesized fatty acids are incorporated into triacylglycerols.

Short answer

The newly synthesized fatty acids are incorporated into triacylglycerols through the following reactions:

$$\begin{gathered} Dihydroxyacetonephosphate+NADH+H^{+}\underset{Gycerol-3-phosphatedehydrogenase}{\to }Gycerol-3-phosphate+NA{D}^{+} \\ Dihydroxyacetonephosphate+fattyacyl-CoAester\underset{}{\to }Acyl-dihydroxyacetonephosphate+HSCoA \\ Gycerol-3-phosphate+NADH+H^{+}\underset{glycerol-3-phosphateacyltransferase}{\to }Lysophosphatidicacid+HSCoA \\ Acyl-dihydroxyacetonephosphate+NADH+H^{+}\underset{Acyl-dihydroxyacetonephosphatereductase}{\to }Lysophosphatidicacid+NA{D}^{+} \end{gathered}$$

$$\begin{gathered} Lipophosphatidicacid+fatty-acylCoAester\underset{1acylglycerol-3phosphateacyltransferase}{\to }Phosphatidicacid+HSCoA \\ Phosphatidicacid\underset{Phosphatidicacidphosphatase}{\to }Diacylglycerol+P_i \\ 2-Monoacylglycerol+fatty-acylCoAester\underset{2-Monoacylglycerolacyltransferase}{\to }Diacylglycerol+HSCoA \\ Diacylglycerolacid+fatty-acylCoAester\underset{Diacylglycerolacyltransferase}{\to }Tricylglycerol+HSCoA \end{gathered}$$

Step-by-step solution

 Step 1: Triacylglycerols

The glycerol-3-phosphate or dihydroxyacetone phosphate and fatty acyl-CoA esters produce the triacylglycerols.The process involves the use of three fatty acids.

Biosynthesis of triacylglycerols

Dihydroxyacetone phosphate is first converted top either glycerol-3-phosphate by glycerol-3-phosphate dehydrogenase or acyl-dihydroxyacetone phosphate by

dihydroxyacetone phosphate acyltransferase. Both of these products are then converted to lysophosphatidic acid by glycerol-3-phosphate acyltransferase and acyl-dihydroxyacetone phosphate reductase, respectively.

$$\begin{gathered} Dihydroxyacetonephosphate+NADH+H^{+}\underset{Gycerol-3-phosphatedehydrogenase}{\to }Gycerol-3-phosphate+NA{D}^{+} \\ Dihydroxyacetonephosphate+fattyacyl-CoAester\underset{}{\to }Acyl-dihydroxyacetonephosphate+HSCoA \\ Gycerol-3-phosphate+NADH+H^{+}\underset{glycerol-3-phosphateacyltransferase}{\to }Lysophosphatidicacid+HSCoA \\ Acyl-dihydroxyacetonephosphate+NADH+H^{+}\underset{Acyl-dihydroxyacetonephosphatereductase}{\to }Lysophosphatidicacid+NA{D}^{+} \end{gathered}$$

Lysophosphatidic acid is converted to phosphatidic acid by 1-acylglycerol-3-phosphate acyltransferase, which is then converted to diacylglycerol by phosphatidic acid phosphatase. The diacylglycerol can also be produced from a reaction between 2-monoacylglycerol and a fatty acyl-CoA ester by 2-monoacylglycerol acyltransferase.

$$\begin{gathered} Lipophosphatidicacid+fatty-acylCoAester\underset{1acylglycerol-3phosphateacyltransferase}{\to }Phosphatidicacid+HSCoA \\ Phosphatidicacid\underset{Phosphatidicacidphosphatase}{\to }Diacylglycerol+P_i \\ 2-Monoacylglycerol+fatty-acylCoAester\underset{2-Monoacylglycerolacyltransferase}{\to }Diacylglycerol+HSCoA \\ Diacylglycerolacid+fatty-acylCoAester\underset{Diacylglycerolacyltransferase}{\to }Tricylglycerol+HSCoA \end{gathered}$$