Lec. 34 Digestion and absorption - Protein and lipid

protein digestion begins with __

enzymatic hydrolysis

where does hydrolysis for breaking down proteins start

stomach

what happens as protein digestion starts with hydrolysis

* pepsin
* released by gastric mucosa (as pepsinogen)
* hydrolyzes proteins at specific amino acid residues
* yields peptides
* rennin (chymosin) important in abomasum of young ruminants

summary of how protein is digested

* hydrolysis starts in stomach
* hydrolysis continues in small intestine with trypsin and chymotrypsin released by pancreas
* hydrolysis is completed by enzymes released by pancreas and small intestine lining (brush border)
* hydrolytic enzymes are secreted as inactive precursors (zymogens)
* they are cleaved to active forms only later

what happens as hydrolysis for protein digestion continues in small intestine with trypsin and chymotrypsin released by pancreas

* hydrolyze at different amino acid residues
* yield shorter peptides

what happens as protein is digested and hydrolysis is completed by enzymes released by pancreas and small intestine lining (brush border)

* released by pancreas
* carboxypeptidase A
* carboxypeptidase B
* released by small intestine
* aminopeptidase
* dipeptidases
* yield amino acids and very short (di- and tri-) peptides

pepsin

* secreted as pepsinogen
* cleaved to pepsin in low pH of stomach

trypsin

* secreted as trypsinogen by pancreas
* cleaved to trypsin in small intestine

what happens as protein is digested and hydrolytic enzymes are secreted as inactive precursors (zymogens)

protects stomach, pancreas, and small intestine from themselves

what happens in pancreatitis

* pancreatic duct is blocked
* by gallstones or heavy alcohol use
* zymogens are activated prematurely inside pancreas
* pancreas becomes inflamed
* painful and may be fatal

__ are absorbed in the small intestine

amino acids

absorption is carried out by __

three broad system

what are the three broad systes

* acidic AA


* neutral AA
* basic AA

most of the three systems are __ but some are __

* 1st: symporters (Na+- or H+- dependent)
* 2nd: uniporters

T/F: amino acids compete with each other for absorption

true

why do amino acids compete with each other

* occurs with amino acids sharing same transporter
* e.g, leucine inhibits isoleucine transport
* e.g., lysine inhibits arginine transport

what happens when amino acids compete with each other

outcompeted amino acids absorbed less efficiently, increasing its dietary requirement

what is the problem with amino acids compete in swine and poultry

* purified (crystalline) amino acids are common ingredients
* accidental overfeeding of amino acid will create deficiency in another

what is the difference between long peptides/proteins and short peptides

short peptides can be transported and long peptides and proteins normally cannot be absorbed

what happens with short peptides

short epeptides can be transported

* peptide transporter 1 (PepT1)
* H+ dependent
* transports di- and tri- peptides

what happens with long peptides and proteins

long peptides and proteins normally cannot be absorbed

* but absorption can occur in neonate (<24 h after birth)

how are long peptide and proteins absorption occuring in neonate

* carried out by pinocytosis
* important in immunoglobulin transfer
* early milk (colostrum) contains immunoglobulins

__ are emulsified before hydrolysis

lipids

digestion of __ begins in small intestine

lipids

how are lipids digested

* digestions begins in the small intestine
* bile enters duodenum from gallbladder and emulsifies lipid
* digestion continues with hydrolysis of triacylglycerols

what happens has bile enters duodenum from gallbladder and emulsifies lipid

* decreases lipid droplet size
* increases surface area for enzymatic digestion
* solves problem that enzymes are soluble in water but lipid is not

lipids are emulsified before __

hydrolysis

what happens as lipid digestion continues with hydrolysis of triacylglycerols

* involves lipases released by pancreas and small intestine
* yields
* free FA
* glycerol
* monoacylglycerols

lipids aggregate into __ before __

* 1st: micelles
* 2nd: absorption

what happens as lipids aggregate into micelles before absorption

* free FA and monoacylglycerols pack into micelles
* micelles also include other lipids

what are other lipids do micelles include

* cholesterol
* fat-soluble vitamins
* bile salts

where does absorptions occur for lipids

absorption occurs in the jejunum and ileum

what happens as lipid digestion occurs in the jejunum and ileum

* fatty acids and monoacylglycerol move out of micelles
* they are absorbed by diffusion

__ are hydrolyzed in the stomach and small intestine

proteins

what are the results of proteins hydrolyzed in the stomach and small intestine

amino acids and short peptides

__ and __ are absorbed in the small intestine

amino acids and short peptides

__ must be emulsified before hydrolysis, which occurs in the small intestine

lipid

what happens to lipids after hydrolysis

lipids move into micelles and are then absorbed