Week 3: Lipid digestion & absorption

Question 1

Most abundant dietary lipid

Answer 1

Triacylglycerols

Question 2

Triacylglycerols chemical properties

Answer 2

very hydrophobic

2 carbon backbone

Question 3

How to digest Triacylglycerols?

Answer 3

Lipases

Question 4

Types of lipases

Answer 4

Salivary lipase (lingual lipase)

Gastric lipase

Pancreatic lipase

Question 5

Primary infantile lipase type

Answer 5

Gastric lipase (only 10% of total lipase activity in adults) this is because newborns do not have a fully formed pancreas yet

Question 6

Primary adult lipase

Answer 6

Pancreatic lipase

Question 7

Products of a lipase reaction

Answer 7

2 x FA structures

1 x 2-monoglyceride (1 FA attached)

Question 8

lipase chemical equation of Triacylglycerol digestion

Answer 8

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

Question 9

How does lipase not damage other lipid structures?

Answer 9

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

Question 10

Cholesterol chemical properties

Answer 10

very hydrophobic

no processing need to absorb

Question 11

How is cholesterol absorbed?

Answer 11

• Cholesterol binds to NPC-1-L1 (Niemann Pick-C1- like-1 protein (cholesterol transporter) for transport

Question 12

How are cholesterol esters absorbed?

Answer 12

Cholesterol backbone and FA are hydrolized apart by cholesterol esterase

Question 13

Cholesterol ester structure

Answer 13

• cholesterol backbone
• FA tail ((ester portion) with ester linkage)

Question 14

Phospholipid structure

Answer 14

Question 15

How are phospholipids digested

Answer 15

Pancreatic phospholipase A2

Question 16

Phospholipid digestion equation

Answer 16

Phospholipid -pancreatic phospholipase A2-> 1 x FA

1 x lysophospholipid (glycerol backbone)

1 x lysolecithin (choline head group)

Question 17

Lysophospholipid and Lysolecithin function

Answer 17

They are amphipathic from FA (hydrophobic) and head group (hydrophilic) so they help with emulsification of lipids

Question 18

long to very long chain FA # of Cs

Answer 18

C16-C22

Question 19

of Carbons of Short-chain FAs

Answer 19

C5-C12

Question 20

What prevents FAs from just crossing the membrane

Answer 20

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

Question 21

Long to very long FA transporter

Answer 21

FATP (Fatty Acid Transporter Protein)

Question 22

FATP transport mechanism

Answer 22

facilitated diffusion (depends on natural FA concentration gradient in the lumen vs enterocyte

Question 23

How is the reverse reaction of FATP prevented?

Answer 23

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

Question 24

Co-enzyme A AKA

Answer 24

Co-Ash

Question 25

How does the FA-CoA get into the blood from an enterocyte

Answer 25

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

Question 27

Chylomicron structure

Answer 27

outer would have polar groups such as cholesterol, PLs while cholesterol esters and ApoB48=TG complexes would be in the core of the cylomicron

Question 28

How do chylomicrons leave enterocytes?

Answer 28

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