Adipose tissue: fed state and starved state

Fed state:

  • Lipogenesis is the synthesis of fatty acids from carbohydrate, thos occurs when glucose is available in excess of that which is required for aerobix metabolism, glycogenesis and other various biosynthetic reactions.
  • Glucose is metabolised by glycolysis to generate acetyl CoA, which serves as the initial substrate molecule for lipogenesis.
  • Acetyl coA carboxylase is the key regulatory enzyme of fat biosynthesis.
  • Fatty acids are hydrophobic and on their own are incapable of being transported in the blood. In order for these lipids to make their way to adipose tissue for storage they must be appropriately packaged for transport.
  • In the liver, fatty acids from lipogenesis are esterified in the ER to form TAGs. At the same time, a specialised lipid carrier in synthesises, termed a VLDL.
  • VLDLs are lipoproteins that encase TAGs and allow them to become water soluble so they can travel in the blood to adipose tissue where they are stored.
  • Dietary lipids get to adipose tissue in a similar fashion as TAGs from lipogenesis.
  • Dietary lipid is hydrolused in the cells of the small intestine into fatty acids and glycerol and absorbed into the intestinal epithelial cells.
  • Here fatty acids are re-esterified into TAGs and packages into chylomicrons.
  • Chylomicrons are the main lipoprotein for lipid transport in the lipid-fed state.

Starved state:

  • Hormones sensitive lipase is responsible for adipose tissue lipolysis.
  • It resides within fat cells and Is responsible for hydrolysing the stored TAGs generating fatty acids
  • HSL is regulated by cAMP-dependent phosphorylation, being active in the phosphorylated form.
  • Hormones such as glucagon and adrenaline increase the intracellular concentration of cAMP, allosterically activating PKA.
  • PKA catalyses the conversion of HSL to its active form.
  • Once released into the blood stream, the fatty acids generated by HSL are bound to the carrier protein, albumin and conveyed to those tissues capable of using fat as a feul.

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