Glycerol produced from hydrolysis of triglycerides in adipose tissue is a waste product, since it has to be in glycerol-3-P form and the enzyme for phosphorylation, glycerol kinase is absent in the adipose tissue. Glycerol -3-P can still be produced in the adipose tissue through interconversion of one of the glycolytic intermediates. Which of the following glycolytic intermediates can be used for the synthesis of glycerol-3-P?
A. Phosphoenol pyruvate
B. 1, 3 Bisphosphoglycerate
C. Dihydroxyacetone phosphate
D. Fructose 1, 6 bisphosphate
The correct answer is C- Dihydroxy acetone phosphate.
In adipose tissue glycerol-3-p is obtained through glycolysis from dihydroxy acetone phosphate for esterification to form triglyceride. In fact adipocytes require a basal level of glycolysis in order to provide them with DHAP as an intermediate in the synthesis of triacylglycerols.
The hydrolysis of triacylglycerols in fat cells yields glycerol and fatty acids. Glycerol may enter either the gluconeogenic or the glycolytic pathway at Dihydroxyacetone phosphate;however, the carbons of the fatty acids cannot be utilized for net synthesis of glucose.In the fasting state glycerol released from lipolysis of adipose tissue triacylglycerol is used solely as a substrate for gluconeogenesis in the liver and kidneys.Glycerol kinase is absent in adipose tissue, that is the reason glycerol released by hydrolysis of triglycerides cannot be reutilized there. It is a waste product. It is transported through blood to liver where it is first phosphorylated to form Glycerol-3-P, by glycerol kinase and then converted to Dihydroxy acetone phosphate by the action of glycerol-3-P dehydrogenase (Figure-1).
The G3PDH reaction is the same as that used in the transport of cytosolic reducing equivalents into the mitochondrion for use in oxidative phosphorylation. This transport pathway is called the glycerol-phosphate shuttle.
Figure-1-The conversion requires phosphorylation to glycerol-3-phosphate by glycerol kinase and dehydrogenation to Dihydroxyacetone phosphate (DHAP) by glyceraldehyde-3-phosphate dehydrogenase (G3PDH).
As regards other options, Phosphoenol pyruvate is a high energy compound, it is converted to pyruvate and by the process of substrate level phosphorylation ATP is produced (Figure-2).
Similarly 1, 3 Bisphosphoglycerate is another high energy compound and is converted to 3, phosphoglycerate with the production of ATP by substrate level phosphorylation. (Figure-2)
Fructose 1. 6 bisphosphate and 2, phosphoglycerate are also not used for glycerol production.
Figure-2- steps of glycolysis and gluconeogenesis
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