Protein and Lipid Digestion
how is cholic acid different from cholesterol structure?
2 hydroxyl groups and 1 carboxylic acid group
when a triglyceride is broken down by pancreatic lipase, what are the products of the reaction?
2-monoacylglycerols and fatty acids
during lipid digestion, what emulsifies lipids and what breaks them down?
bile salts emulsify lipids and pancreatic enzymes degrade lipids
what does Xenical or orlistat do?
binds to lipases which prevent the breakdown of ingested lipids (allow for the lipids to remain in the GI tract, resulting in secretion)
Ezetimibe (zetia) does what?
binds to the NPC1L1 transporter on enterocytes and prevents uptake of cholesterol into enterocytes (prevents dietary cholesterol from adding to blood cholesterol)
triacylglycerol is broken down to what via what enzyme, and what are some of the by-products?
broken down to 2-monoacylglycerol via pancreatic lipase; 2 fatty acids are by-products
what is cholesterol ester broken down to in the small intestine, what is it broken down by, and what are some of the by-products?
broken down to cholesterol by cholesteryl esterase; fatty acids are by-products
phospholipids are broken down to what via what enzyme in the small intestine, and what are some of the by-products?
broken down to glycerylphosphorylcholine via lipases; 2 fatty acids are by-products
once ingested proteins are broken down to polypeptides and amino acids, what are these molecules further broken down to and by what are they broken down by?
broken down to oligopeptides and amino acids via amino-peptidases and di/tri peptidases
what molecule is the precursor for bile acid synthesis?
cholesterol
what is the difference between cholic acid and chenodeoxycholic acid?
cholic acid has 3 OH groups, while chenodeoxycholic acid has only 2 OH groups
in bile acid synthesis, cholesterol is converted to what via what enzyme?
cholic acid or chenodeoxycholic acid (via cholesterol 7 alpha hydroxylase); addition of OH group
are bile salts conjugated or unconjugated?
conjugated (either glycine or taurine is added to COOH group)
what do you need to take along with xenical or orlistat?
daily multivitamin containing A, D, E, and K, and beta-carotene (don't take at same time as fat blocking agent)
what do bacteria convert cholic acid to in the small intestine?
deoxycholic acid (readily reabsorbed by the liver via the portal vein)
what is colipase?
enzyme produced by the pancreas which binds to lipase and restores activity to lipase in the presence of bile salts
what are the three possible pathways for cholesterol once it is taken up by enterocytes?
it can be exocytosed by the enterocyte (via chylomicron formation), it can be utilized by the enterocyte, or it can be pumped back out into the GI lumen
what do bacteria convert chenodeoxycholic acid to in the small intestine?
lithocholic acid (poorly reabsorbed by the liver via the portal vein)
3% of bile is what?
solids (80% is bile salts)
what do PCSK9 inhibitors do?
targets LDL cholesterol for destruction
chenodeoxycholic acid + taurine ->
taurochenodeoxycholic acid (bile salt)
chylomicrons (which form in the enterocyte) are mostly composed of what molecule?
triglycerides
are bile acids conjugated or unconjugated?
unconjugated (free COOH group)
how is cholesterol taken up by enterocytes?
via a transporter known as NPC1L1
97% of bile is what?
water
are dietary lipids essential?
yes (humans cannot synthesize fatty acids containing cis double bonds beyond position 9 - polyunsaturated acids like lineoleic and linolenic acids)
what are the primary products of lipid emulsification and degradation (in the gut)?
free fatty acids, 2-monoacylglycerol, cholesterol (products taken up by enterocytes)
cholic acid + glycine ->
glycocholic acid (bile salt)
amino acids (after being broken down by enzymes) can be absorbed where?
in the small intestine (then sent to the liver)
what does cholic acid do when it is in excess?
inhibits the enzyme, cholesterol 7 alpha hydroxylase
is absorption of carbohydrates and proteins affected with xenical or orlistat?
no - just fats
when dietary proteins are ingested, what enzyme breaks down ingested protein?
pepsin (breaks it down to polypeptides and amino acids)
what do bacteria do to bile salts within the small intestine?
remove glycine or taurine amino acid to form bile acid