Triacylglycerol Cycle in Lipid Metabolism

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Questions and Answers

Where is most of the glycerol 3-phosphate synthesized in animals?

Muscles

Adipocytes

Seeds and nuts

Liver and kidney (correct)

What is the primary source of glycerol 3-phosphate in animals?

TAGs degradation

Glycogen breakdown

Dihydroxyacetone phosphate from glycolysis (correct)

Glycerol kinase

What is the role of glycerol kinase in animals?

To convert glycerol to glycerol 3-phosphate (correct)

To synthesize glycerol 3-phosphate from dihydroxyacetone phosphate

To synthesize phospholipids

To break down TAGs

In which cells is glycerol kinase absent?

Adipocytes Signup and view all the answers

What is the backbone of triacylglycerides and phospholipids?

Glycerol Signup and view all the answers

What is the typical fat storage in a 70-kg human?

15 kg Signup and view all the answers

How long can a human survive on stored fat?

12 weeks Signup and view all the answers

What is the comparison of fat storage to glycogen storage in the human body?

Fat storage is 12 times greater than glycogen storage Signup and view all the answers

What is the effect of high levels of free fatty acids in blood on glucose utilization in muscle?

It interferes with glucose utilization Signup and view all the answers

What is the role of thiazolidinediones in type 2 diabetes treatment?

Increasing glyceroneogenesis Signup and view all the answers

What is the effect of thiazolidinediones on PEP carboxykinase synthesis?

It promotes synthesis Signup and view all the answers

What is the effect of thiazolidinediones on TAGs resynthesis?

It increases resynthesis Signup and view all the answers

What is the first step in the biosynthesis of phospholipids?

Synthesis of the backbone molecule (glycerol or sphingosine) Signup and view all the answers

What is the location of phospholipid biosynthesis in the cell?

Primarily on the surface of the smooth ER and the inner mitochondrial membrane Signup and view all the answers

What is the final step in the biosynthesis of phospholipids?

Alteration or exchange of the head group Signup and view all the answers

What is the fate of phospholipids after their biosynthesis?

They are transported to other cellular locations (mechanism not fully understood) Signup and view all the answers

What percentage of free fatty acids released by lipolysis are reesterified to form TAGs?

75% Signup and view all the answers

What is the primary role of glyceroneogenesis in the TAG cycle?

To generate glycerol 3-phosphate Signup and view all the answers

During lipolysis, what is the fate of glycolysis in adipocytes?

It is inhibited Signup and view all the answers

What is the primary source of glycerol 3-phosphate in adipose tissue?

Glyceroneogenesis Signup and view all the answers

What is the role of glycerol kinase in adipose tissue?

It is not present in adipose tissue Signup and view all the answers

What is the precursor for glycerol 3-phosphate in glyceroneogenesis?

All of the above Signup and view all the answers

What is the relationship between glyceroneogenesis and gluconeogenesis?

Glyceroneogenesis contains some of the same steps as gluconeogenesis Signup and view all the answers

Where does glyceroneogenesis occur?

In both the liver and adipose tissue Signup and view all the answers

Study Notes

The Triacylglycerol Cycle

75% of free fatty acids (FFAs) released by lipolysis are reesterified to form TAGs, rather than being used for fuel under all metabolic conditions, including starvation.
Some FFAs from adipose cells are transported to the liver, remade into TAG, and redeposited in adipose cells.

Glyceroneogenesis

Glyceroneogenesis makes DHAP for glycerol 3-phosphate generation during the TAG cycle.
During lipolysis, glycolysis in adipocytes is inhibited, making DHAP unavailable for glycerol 3-phosphate production.
Adipose cells lack glycerol kinase to make glycerol 3-phosphate on site.
Glyceroneogenesis converts pyruvate, alanine, glutamine, or other substances from the Citric acid cycle into DHAP.

Treatment of Type 2 Diabetes

High levels of free fatty acids in the blood interfere with glucose utilization in muscle and promote insulin resistance, leading to type 2 diabetes.
Thiazolidinediones promote the synthesis of PEP carboxykinase, increasing the rate of TAG resynthesis.

Biosynthesis of Membrane Phospholipids

Phospholipid biosynthesis involves the synthesis of the backbone molecule (glycerol or sphingosine), attachment of fatty acid(s) to the backbone, and addition of a hydrophilic head group.
The final phospholipid product may undergo alteration or exchange of the head group.

Phospholipid Biosynthesis - Place of Action

Phospholipid biosynthesis occurs in every cell, primarily on the surface of the smooth ER and the inner mitochondrial membrane.
The transport of phospholipids to other cellular locations is not fully understood.

Synthesis of Fat (Triacylglycerides) and Phospholipids

Animals and plants store fat for fuel, with a typical 70-kg human having ~15 kg fat, enough to last 12 weeks.
Both TAGs and phospholipids contain glycerol backbone and 2 or 3 fatty acids.

Synthesis of Backbone of TAGs and Phospholipids

Most glycerol 3-phosphate comes from siphoning off dihydroxyacetone phosphate (DHAP) from glycolysis.
Some glycerol 3-phosphate is made from glycerol via glycerol kinase, a minor pathway in the liver and kidney.

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Description

Understand the process of reesterification of free fatty acids into triacylglycerols (TAGs) in adipose tissue and liver, and how it affects energy storage and usage.