page_title: Glyceroneogenesis
section_title: Brown adipose tissue
hierarchical_section_title: Glyceroneogenesis / Location / Brown adipose tissue
context_page_description: Glyceroneogenesis is a metabolic pathway which synthesizes glycerol 3-phosphate or triglyceride from precursors other than glucose. Usually glycerol 3-phosphate is generated from glucose by glycolysis, but when glucose concentration drops in the cytosol, it is generated by another pathway called glyceroneogenesis. Glyceroneogenesis uses pyruvate, alanine, glutamine or any substances from the TCA cycle as precursors for glycerol 3-phosphate. Phosphoenolpyruvate carboxykinase, which is an enzyme that catalyzes the decarboxylation of oxaloacetate to phosphoenolpyruvate is the main regulator for this pathway. Glyceroneogenesis can be observed in adipose tissue and also liver. It is a significant biochemical pathway which regulates cytosolic lipid levels. Intense suppression of glyceroneogenesis may lead to metabolic disorder such as type 2 diabetes.
context_section_description: Brown adipose tissue is another type of adipose tissue which stores free fatty acids. Brown adipose tissue is especially abundant in new born mammals and hibernating mammals. The differences between brown and white adipose tissue are brown adipose tissues have higher activity in glyceroneogenesis than white adipose tissues and glyceroneogenesis in brown adipose tissue is related to thermogenesis. The activity of glyceroneogenesis in brown adipose tissue is greater than that of white adipose tissue because it contains more enzymes involved in the glyceroneogenesis. Compared to white adipose tissue, brown adipose tissue has considerably higher activity of PEPC-K and glycerol kinase. The activity of PEPC-K in brown adipose tissue is almost 10 times the activity of that in white adipose tissue.  PEPC-K which is involved in the conversion of oxaloacetate to phosphoenolpyruvate is the key enzyme that regulates glyceroneogenesis. Increase in activity of the enzyme will increase the activity of the pathway. Furthermore, not only PEPC-K but brown adipose tissue is also rich in activity of glycerol kinase. Glycerol kinase is the enzyme which phosphorylates glycerol in order to generate the backbone of triglycerides, glycerol 3-phosphate. Increase in the activity of glycerol kinase will result in an increase in production of glycerol 3- phosphate. As a result, brown adipose tissue will have greater activity in glyceroneogenesis, because it contains more enzymes involved in the pathway.
  In addition, glyceroneogenesis in brown adipose tissue is related to thermogenesis in the organism. In mammals, heat is generated by delivering free fatty acids to the mitochondria. When glyceroneogenesis is proceeding regularly, the concentration of free fatty acid is low in intercellular environment because glyceroneogenesis re-esterifies fatty acids to triglycerides. In other words, thermogenesis by free fatty acids is less likely to occur when glyceroneogenesis is proceeding. However, when exposed to cold, a neurotransmitter hormone called norepinephrine will suppress the activity of PEPC-K. When the activity of PEPC-K is suppressed, glyceroneogenesis will be unable to re-esterify the free fatty acids. Eventually, the free fatty acid concentration within the cell will increase leading to excessive free fatty acids in cytosol, which will consequently be delivered to the mitochondria for thermogenesis. Therefore, when a mammal is exposed to cold, heat is generated in the brown adipose tissue by decreasing the activity of glyceroneogenesis.
caption: (Figure 7) Chemical structure of Norepinephrine