Food Chemistry and Fish in Nutrition

4.3.4.Synthesis of triglycerides

4.3.4.Synthesis of triglyceridess

.  (ii) Synthesis of triglycerides

Triglycerides are synthesized from glycerol and fatty acids. Glycerol is a product of the metabolism of adipose tissue, and only tissues that possess the activating enzyme, glycerol kinase, can utilize it. This enzyme, which requires ATP, is found in liver and kidney, among other tissues.

1. Synthesis of glycerol 3-phosphate

Glycerol kinase catalyzes the conversion of glycerol to glycerol 3-phosphate.

Glycerol kinase

Glycerol + ATP  → Glycerol 3-phosphate + ADP

This pathway connects with the stages of the glycolysis pathway triose phosphate, because glycerol 3-phosphate by NAD ⁺ in the presence of glycerol-3-phosphate dehydrogenase.

Liver and kidney are able to convert glycerol to blood glucose by making use of the above enzymes, some enzymes of glycolysis, and the specific enzymes of the gluconeogenic pathway, fructose-1,6-bisphosphatase and glucose 6- phosphatase.

2. Activation of fatty acid to acyl CoA

 Fatty acids are activated to acyl-CoA by the enzyme acyl-CoA synthetase, utilizing ATP and CoA.

Acyl-CoA synthetase

Fatty acids +ATP + CoA. → Acyl-CoA + ADP

3. Formation of phosphadidate

Two molecule of acyl-CoA combine with glycerol 3-phosphate to form phosphatidate (1,2- diglyceride phosphate). This takes place in two stages via lysophosphatidate, catalyzed first by glycerol-3-phosphate acyltransferase and then by 1-acylglycerol-3-phosphate acyltransferase.

Glycerol-3-phosphate acyltransferase

2 Acyl-CoA + Glycerol 3-phosphate →1-Acylglycerol-3-phosphate

1-Acylglycerol-3-phosphate acyltransferase

1-Aacylglycerol-3-phosphate→Phosphatidate (1,2- Diglyceride phosphate)

4. Formation of diglyceride

Phosphatidate (1, 2- diglyceride phosphate) is converted by phosphatidate phosphohydrolase to a 1,2 diglyceride.

Phosphatidate phosphohydrolase
Phosphatidate (1,2- diglyceride phosphate) → 1,2 diglyceride.

5. Fomation of triglyceride

A further molecule of acyl-Co.A is esterified with the diglyceride to form a triglyceride, catalyzed by diglyceride acyltransferase.

Diglyceride acyltransferase
Diglyceride + acyl-Co.A→ Triglyceride + CoA

In intestinal mucosa, a monoacyl-glycerol pathway exists whereby monoglyceride is converted to 1,2- diglyceride as a result of the presence of monoglyceride acyltransferase.

Most of the activity of these enzymes resides in the endoplasmic reticulum of the cell, but some is found in mitochondria, e.g, glycerol-3-phosphate acyltransferase