Absorption and transport of fats Flashcards
What are bile salts?
Bile salts/acids solubilise fat in the GI tract. Synthesised from cholesterol in liver and stored in gall bladder as bile. Secreted into small intestine in response to cholecystokinin
Form micelles with triacylglycerol to increase SA for digestion
What are pancreatic lipases?
Pancreatic secreted lipase can hydrolyse parts of the triacylglycerol structure, requires a co-lipase to enhance its activity
Can hydrolyse ester linkage and remove R1 and R3 fatty acid, cannot hydrolyse ester linkage from R2
Produces two free fatty acids and one glycerol molecule with a fatty acid still attached (2-monoacylglycerol)
Products turned into micelles which are absorbed across the intestinal cell membrane
How do bile salts solubilise triacylglycerol (TAG) and assist with hydrolysis?
One side is hydrophobic and the other is hydrophilic hence the molecule can behave as a detergent molecule in a hydrophilic/hydrophobic intrasalt environment (hydrophilic side faces outside into hydrophilic environment and hydrophobic faces inside). Facilitates the activity pancreatic lipase as it can interact with micelles and enable the hydrolysis of triacylglycerol
What is fat malabsorption?
Excess fat and fat soluble vitamins in feces that cant be absorbed can be caused by conditions that interfere with bile or pancreatic lipase secretion
Xenical forms a covalent bond with the lipase and can inhibit the active site hence pancreatic lipase cannot be secreted
What are lipoproteins?
Evolved to help solubilise lipid molecules for transport in blood
Provide a delivery system for transporting lipids in and out of cells
Some involve ‘apoproteins’ which are accessory proteins used for structural assembly (apoB), ligands for cell surface receptors (apoE and B) and enzyme cofactors (apoCII)
Lipoprotein contents
Exist in an aqueous environment hence phospholipids with polar hydrophilic heads facing outwards and non polar hydrophobic tails facing inwards allowing hydrophobic molecules to be contained within structure and transported around
What are the four main lipoprotein classes?
Chylomicrons, very low density (VLDL), low density (LDL) and high density (HDL)
Need CoA to be activated and to facilitate triacylglycerol synthesis
Density of lipoprotein class increases as TAG decreases and protein amount increases
What is the exogenous lipid transport pathway?
TAG manipulated into chylomicron which facilitates getting them into blood for circulation. Lipase interacts with chylomicron in capillaries and hydrolyse free amino acids which can be transferred to adipose tissue and muscle. The remnants (lipid depleted chylomicron lipoprotein particles) are taken up by the liver and reprocess into VLDL/LDL/HDL depending on the composition of the lipoprotein
What is endogenous lipid transport pathway?
Lipid handling in the liver, mobilisation of adipose tissue storage or fall out from dietary lipid from exogenous pathway and various circulation and uptake to various peripheral tissue around the body
What happens if we have defects in apoCII or lipoprotein lipase?
Lipoprotein lipase (lipoprotein found on capillary surfaces) is activated by apoCII, if we have defects in apoCII or lipoprotein lipase it can result in elevated levels of chylomicrons and plasma TAG causing various secondary presentation issues
What is Familial hypercholesterolemia?
Leads to premature atherosclerosis (harding and thickening of arteries) caused by defect in LDL receptor gene, dominant disorder. LDL levels are 2-3x higher than normal, can be treated with statin to lower LDL and increase HDL
