Week 3: Lipid digestion & absorption Flashcards
Most abundant dietary lipid
Triacylglycerols
Triacylglycerols chemical properties
very hydrophobic
2 carbon backbone
How to digest Triacylglycerols?
Lipases
Types of lipases
Salivary lipase (lingual lipase)
Gastric lipase
Pancreatic lipase
Primary infantile lipase type
Gastric lipase (only 10% of total lipase activity in adults) this is because newborns do not have a fully formed pancreas yet
Primary adult lipase
Pancreatic lipase
Products of a lipase reaction
2 x FA structures
1 x 2-monoglyceride (1 FA attached)
lipase chemical equation of Triacylglycerol digestion
1 Triacylglycerol + 2H2O -lipase-> 2 x FA structures
1 x 2-monoglyceride (1 FA attached)
Co-lipase is an accessory protein to combine with lipase and be able to interact with hydrophobic lipid droplets
How does lipase not damage other lipid structures?
co-lipase has to be activated from Pro-co-lipase by trypsin so it only occurs in the intestinal lumen because trypsinogen is activated by enterokinase which is on enterocyte luminal membranes
Cholesterol chemical properties
very hydrophobic
no processing need to absorb
How is cholesterol absorbed?
- Cholesterol binds to NPC-1-L1 (Niemann Pick-C1- like-1 protein (cholesterol transporter) for transport
How are cholesterol esters absorbed?
Cholesterol backbone and FA are hydrolized apart by cholesterol esterase
Cholesterol ester structure
- cholesterol backbone
- FA tail ((ester portion) with ester linkage)
Phospholipid structure
How are phospholipids digested
Pancreatic phospholipase A2
Phospholipid digestion equation
Phospholipid -pancreatic phospholipase A2-> 1 x FA
1 x lysophospholipid (glycerol backbone)
1 x lysolecithin (choline head group)
Lysophospholipid and Lysolecithin function
They are amphipathic from FA (hydrophobic) and head group (hydrophilic) so they help with emulsification of lipids
long to very long chain FA # of Cs
C16-C22
of Carbons of Short-chain FAs
C5-C12
What prevents FAs from just crossing the membrane
Long to very long fatty acids carboxylic acid from crossing the membrane by pure diffusion (need a transporter)
Short chain FAs: C5-C12 are the perfect balance of hydrophobicity and hydrophilicity and can pass through by simple diffusion
Long to very long FA transporter
FATP (Fatty Acid Transporter Protein)
FATP transport mechanism
facilitated diffusion (depends on natural FA concentration gradient in the lumen vs enterocyte
How is the reverse reaction of FATP prevented?
Fatty acyl CoA Synthetase bound to FATP adds a Co-enzyme A to FAs that are transported into enterocytes by FATP
The FA-CoA structure is trapped in the cell and cannot go back into the lumen from FATP
Co-enzyme A AKA
Co-Ash
How does the FA-CoA get into the blood from an enterocyte
FA-CoA binds to FABP (Fatty acid-binding protein) which creates FA-CoA-FABP complex which is soluble so it can be transported
FA-CoA-FABP is transported to Smooth ER
in the smooth ER take 2 FA-CoA + 2-monglyceride -> TG + 2CoA
TGs bind to ApoB48 forming structure including cholesterol and cholesterol esters and dietary phospholipids to form a chylomicron
Chylomicron structure
outer would have polar groups such as cholesterol, PLs while cholesterol esters and ApoB48=TG complexes would be in the core of the cylomicron
How do chylomicrons leave enterocytes?
secreted out of the basal surface and are too large to enter the bloodstream so they enter the lymphatics (lymphatic endothelial cells are leaky so they have large junctions to allow large structures in)
Then the chylomicrons enter the systemic circulation through the vena cava
