Protein digestion and AA absorption Flashcards
Utilization of AA
-unique pathway for each AA but they are mostly directed towards protein synthesis
-Excess AA= carbon chain are catabolized for fuel (TCA cycle, glycolysis, gluconeogenesis), and the amino group is excreted as urea
Conditionally essential
-under some conditions, the rate of utilization are greater than the rates of production
Essential amino acids
-can’t be synthesized from materials normally in the diet at a rate meeting the requirements for normal growth and production
Zwitterions
-amino acids have opposite charges so an AA can act as an acid or base by accepting or donating H+
-pKa of zwitterions
Carboxyl group: between 1.5-2.5
Amino group: between 8.5-10.5
How is Dietary protein measured?
-measured as crude protein, which is determined by analyzing Nitrogen
**therefore if you have protein that is heavy in AA but low in nitrogen, then crude protein will be low
D or L configurations of AA
-AA can be in either D or L configurations
-Within animal cells, all proteinogenic AA exist in L configuration
Taurine
-neutral, sulfur containing AA
*can be essential sometimes
What does crude protein assume?
assumes that protein averages 16% N (factor of 6.25) AND all nitrogen found is from the protein
**but Nitrogen content in AA varies (ex. Arg 32%, Phe, Tyr, Met 0.8-0/8%)
Melamine adulteration
**Melamine added to pet foods
-Melamine is very high in nitrogen, making it appear that there were much higher protein present than there was
-Melamine and cyanuric acid is toxic causing many cats and dogs dying
what effects Nutritional value of protein?
-AA content and composition
-molecular structure (impacts enzyme access)
Digestibility of protein
The N content or AA content does not provide information on bioavailability (ability to be used for protein synthesis)
Digestibility relationship with solubility
Solubility is the opening up of the structures whereas digestibility is the breakdown
Factors affecting protein digestibility
1.source or protein/solubility
2.Feed processing can either decrease or increase digestibility
3.increased dietary fibre will lead to decreased protein digestibility
4.Trypsin inhibitors decrease protein digestibility
5.diet acidification helps breakdown
Feed processing impact on amino acid digestibility
-grinding decreases particle size which increases breakdown
-heat treatment inactivates trypsin inhibitors which increases breakdown
-overheating can cause Maillard product formation and therefore decreased breakdown
How is nitrogen removed?
Urea
Glutamate
95% uses by small intestine
5% enters portal circulation
Protein digestion in the stomach
Based on secretion of HCl
-H ions and bicarbonate generated from dissociation of water and CO2 by carbonic anhydrase
-bicarbonate out, Cl in
-H+ out, K in
What stimulates HCl secretion?
-gastrin
-acetylcholine
-histamine
Factors causing a decrease in HCl secretion
-High H+ concentration in gastric and duodenal contents stimulatig secretin, GIP, VIP
-high concentrations of fat digestion products, activating CCK
-deficiencies in protein, zinc and B-vitamins
-large dietary particle size
-small dietary particle size due to prolonged retention
Role of Gastric HCl and gastric proteases
-secretion begins in cephalic phase of digestion
>Pepsinogen secreted as a zymogen and activated to pepsin when pH is low
-protein digestion initiated in the stomach (protein to oligopeptides)
Pepsin
-produced by chief cells
-recognizes aromatic and hydrophobic AA
Renin
-produced by chief cells
-recognizes Phe and Met (clotting milk)
Digestion of proteins and oligopeptides in small intestine
-Proteins resistant to pepsin breakdown will enter the duodenum
-stimulating the release of CCK and secretin from enteroendocrine cells AND exocrine pancreas to release HCO3-, enzymes (zymogens) and proenzymes through pancreatic duct
-Proenzyme activation initiated by enteropeptidase (from bile salts) and trypsinogen
Trypsin inhibition
inhibits all pancreatic proteases
Why are pancreatic enzymes released as zymogens?
to avoid their activation until they are in the duodenal lumen
They are only activated when trypsinogen cleaved by enterokinase/peptidase to become trypsin
Trypsin inhibitor classes
1.Kunitz trypsin inhibitors (soybeans)
2.Bowman-Birk trypsin/chymotrypsin inhibitors (field peas, lentils, soybeans)
**they bind trypsin and chymotrypsin forming inactive complexes
Result of trypsin inactivation
-reduce AA digestibility
-negative feedback mechanism resulting in increased secretion of these enzymes
-enlarged pancreas
-high energy expenditure
-trypsin inhibitors increase CCK output, inhibit feed intake
Heat treatment of oil seeds
-heat treated oilseeds results in decrease in trypsin inhibitors
Small intestine Luminal digestion of peptides
1.large peptides from gastric proteolysis are cleaved in the small intestinal lumen by endoproteases that cut chains into smaller peptides with neutral or basic AA at the C terminus
2.The AA at C terminus are acted on carboxypeptidase A or carboxypeptidase B
3.Brush border endo and ectopeptidases also yield free AAs
**60-70% of dietary protein in form of small oligopeptides following luminal hydrolysis. The other is in the form of AAs
Absorption of free AAs and small peptides
-Absorption of free AAs, and di and tripeptides through a variety of transport mechanisms to ensure efficient N intake
Peptides in enterocytes
Di and tri peptides are transported more efficiently than free AAs. They will be taken up by enterocytes and broken down (hydrolyzed) into free AAs
Free AAs in enterocytes
-pass unchanged through cytosol and exit the cell
>they are used for enterocyte protein synthesis
>partially or completely oxidized
>N leaves the cell as alanine, proline, citrulline, ammonia and they undergo intermediary metabolic conversion into other AAs or metabolites
Secondary active transport during AA and peptide absorption
-ion coupled
-move against concentration gradients
-includes Na/K ATPase which is responsible for maintaining other gradients
Facilitated transport during AA and peptide absorption
-non active
-move AA down their chemical or electrical gradients
Where are peptide transporters located?
luminal side
**no evidence of any on basolateral membrane
Specificity of AA transporters
Almost all of them are specific for the L isomers
**methionine D and L isomers are similar
Factors affecting absorption of AA and % appearance in protal vein
-excessive amount of proteins (decreases absorption)
-decreased amount of protein (decreases absorption)
-improved quality of protein (increases absorption)
-slowing rate of protein digestion (increases absorption)
-adding carbohydrates to meal (increases absorption because can use CHO for energy)
What happens to protein that escapes small intestine?
AA are used by bacteria resulting in bacterial metabolites
**bad bacteria grows more, good bacteria grows less
