Protein digestion and AA absorption

Question 1

Utilization of AA

Answer 1

-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

Question 2

Conditionally essential

Answer 2

-under some conditions, the rate of utilization are greater than the rates of production

Question 3

Essential amino acids

Answer 3

-can’t be synthesized from materials normally in the diet at a rate meeting the requirements for normal growth and production

Question 4

Zwitterions

Answer 4

-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

Question 5

How is Dietary protein measured?

Answer 5

-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

Question 6

D or L configurations of AA

Answer 6

-AA can be in either D or L configurations
-Within animal cells, all proteinogenic AA exist in L configuration

Question 7

Taurine

Answer 7

-neutral, sulfur containing AA
*can be essential sometimes

Question 8

What does crude protein assume?

Answer 8

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%)

Question 9

Melamine adulteration

Answer 9

**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

Question 10

what effects Nutritional value of protein?

Answer 10

-AA content and composition
-molecular structure (impacts enzyme access)

Question 11

Digestibility of protein

Answer 11

The N content or AA content does not provide information on bioavailability (ability to be used for protein synthesis)

Question 12

Digestibility relationship with solubility

Answer 12

Solubility is the opening up of the structures whereas digestibility is the breakdown

Question 13

Factors affecting protein digestibility

Answer 13

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

Question 14

Feed processing impact on amino acid digestibility

Answer 14

-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

Question 15

How is nitrogen removed?

Answer 15

Urea

Question 16

Glutamate

Answer 16

95% uses by small intestine

5% enters portal circulation

Question 17

Protein digestion in the stomach

Answer 17

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

Question 18

What stimulates HCl secretion?

Answer 18

-gastrin
-acetylcholine
-histamine

Question 19

Factors causing a decrease in HCl secretion

Answer 19

-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

Question 20

Role of Gastric HCl and gastric proteases

Answer 20

-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)

Question 21

Pepsin

Answer 21

-produced by chief cells
-recognizes aromatic and hydrophobic AA

Question 22

Renin

Answer 22

-produced by chief cells
-recognizes Phe and Met (clotting milk)

Question 23

Digestion of proteins and oligopeptides in small intestine

Answer 23

-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

Question 24

Trypsin inhibition

Answer 24

inhibits all pancreatic proteases

Question 25

Why are pancreatic enzymes released as zymogens?

Answer 25

to avoid their activation until they are in the duodenal lumen

They are only activated when trypsinogen cleaved by enterokinase/peptidase to become trypsin

Question 26

Trypsin inhibitor classes

Answer 26

1.Kunitz trypsin inhibitors (soybeans)
2.Bowman-Birk trypsin/chymotrypsin inhibitors (field peas, lentils, soybeans)

**they bind trypsin and chymotrypsin forming inactive complexes

Question 27

Result of trypsin inactivation

Answer 27

-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

Question 28

Heat treatment of oil seeds

Answer 28

-heat treated oilseeds results in decrease in trypsin inhibitors

Question 29

Small intestine Luminal digestion of peptides

Answer 29

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

Question 30

Absorption of free AAs and small peptides

Answer 30

-Absorption of free AAs, and di and tripeptides through a variety of transport mechanisms to ensure efficient N intake

Question 31

Peptides in enterocytes

Answer 31

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

Question 32

Free AAs in enterocytes

Answer 32

-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

Question 33

Secondary active transport during AA and peptide absorption

Answer 33

-ion coupled
-move against concentration gradients
-includes Na/K ATPase which is responsible for maintaining other gradients

Question 34

Facilitated transport during AA and peptide absorption

Answer 34

-non active
-move AA down their chemical or electrical gradients

Question 35

Where are peptide transporters located?

Answer 35

luminal side

**no evidence of any on basolateral membrane

Question 36

Specificity of AA transporters

Answer 36

Almost all of them are specific for the L isomers

**methionine D and L isomers are similar

Question 37

Factors affecting absorption of AA and % appearance in protal vein

Answer 37

-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)

Question 38

What happens to protein that escapes small intestine?

Answer 38

AA are used by bacteria resulting in bacterial metabolites

**bad bacteria grows more, good bacteria grows less