The synthesis of lipid (fat)
Lipogenesis is the process of producing lipid or fat. In biology, lipogenesis is a biochemical process, e.g. acetyl-CoA is converted to triglyceride. It is to store biochemical energy for later metabolic use.
Cellular respiration is a cellular process wherein the biochemical energy is harvested from organic substance (e.g. glucose) and then stored in energy-carrying biomolecule (e.g. ATP). The major steps or processes of cellular respiration are (1) glycolysis, (2) Krebs cycle, and (3) oxidative phosphorylation. In glycolysis, for instance, glucose is converted to pyruvate. Before the next phase proceeds, the pyruvate is converted to acetyl-CoA. The acetyl-CoA enters the Krebs cycle to be oxidized through a cyclic series of enzymatic reactions. ATPs are produced as a source of energy for various metabolic processes.
In lipogenesis, the excess energy that will not be used immediately can be stored in the form of fats. The energy from acetyl-CoA is stored in fat biomolecules. The process wherein lipid is broken down to extract energy is called lipolysis. Lipolysis is activated when the circulating insulin level is low whereas the circulating epinephrine is high.
Insulin level rises when the pancreas releases insulin into the bloodstream. High glucose level (e.g. from dietary sources) promotes the release of insulin from the pancreas. In the presence of high insulin level, lipogenesis predominates over lipolysis. Insulin induces a cascade of reactions that lead to the reduction of cAMP level. While insulin slows down lipolysis, it induces lipogenesis. This ensures that lipolysis and lipogenesis do not occur at the same time.
When insulin is released by the pancreas to the bloodstream, the level of insulin increases. This results in the slowing down of lipolysis by inducing series of reactions that reduce cAMP level and lower sympathetic nervous outflow. Insulin also stimulates pyruvate dehydrogenase phosphatase that removes phosphate from pyruvate dehydronesase, thereby activating the latter to convert pyruvate to acetyl-CoA. The acetyl-CoA is carboxylated to form malonyl-CoA through the action of acetyl-CoA carboxylase. The malonyl-CoA plays a role in the chain elongation in fatty acid biosynthesis.
Lipogenesis includes (1) fatty acid synthesis and (2) triglyceride synthesis. Fatty acid synthesis occurs in the cytoplasm and characterized by the repeated addition of two-carbon units to acetyl-CoA. In triglyceride synthesis, three fatty acids are esterified to a glycerol in the endoplasmic reticulum. The cells that carry out lipogenesis are mostly adipocytes and liver cells. The liver cells, though, release triglycerides in the form of very-low-density lipoproteins (VLDL) into the bloodstream.
Word origin: Ancient Greek lípos (“animal fat”) + -genesis (origin)