Protein Digestion and Absorption

Protein Digestion and Absorption

* Dietary proteins, with few exceptions, are not absorbed.
* Dietary proteins, with few exceptions, are not absorbed.
* They must be digested first into amino acids or di- and tri-peptides.
* Dietary proteins, with few exceptions, are not absorbed.
* They must be digested first into amino acids or di- and tri-peptides.
* Through the action of gastric and pancreatic proteases, proteins are digested within the lumen into medium and small peptides (oligopeptides).


Digestion of protein - hydrolysis
Protein digestion begins in stomach
Pepsin - inactive precursor pepsinogen
Active @ pH 2-3, inactive pH>5
Secretion stimulated by acetylcholine or acid
Only protease which can break down collagen
Action terminated by neutralisation by bicarbonate in duodenum.
N.B. **All proteases (stomach & pancreatic) secreted as inactive precursors. Most protein digestion occurs in the duodenum/jejunum

Activation of pancreatic proteases
Trypsinogen
Trypsin
Enterokinase
Trypsinogen
Chymotrypsinogen
Proelastase
Procarboxypeptidase
Trypsin
Chymotrypsin
Elastase
Carboxypeptidase
Active proteases inactivated by trypsin
peptidases
aminopolypeptidase
transporters
amino acids
Di/tri peptides
Cytoplasmic peptidase

Pancreatic enzymes
Essential for digestion
essential for life
Proteases
Inactive form
Activated in gut
Acinar cells
Lipases Amylases
Active enzymes

Pancreatic Enzymes
* The bulk of protein digestion occurs within the intestine due to the action of pancreatic proteases.

Pancreatic Proteases
* The two primary pancreatic proteases are trypsin and chymotrypsin.
* They are synthesized and packaged within secretory vesicles as inactive proenzymes:
trypsinogen chymotrypsin
* The two primary pancreatic proteases are trypsin and chymotrypsin.
* They are synthesized and packaged within secretory vesicles as inactive proenzymes:

trypsinogen chymotrypsin
The secretory vesicles also contain a trypsin inhibitor to serve as a safeguard against trypsinogen converted to trypsin.

Other Pancreatic Proteases
* Procarboxypeptidase  carboxypeptidase
* Proelastase  elastase

Trypsin
* Trypsinogen is converted to trypsin by the enzyme enterokinase (enteropeptidase) secreted by cells lining duodenum.
* Trypsinogen is converted to trypsin by the enzyme enterokinase (enteropeptidase) secreted by cells lining duodenum.
* Trypsin then activates the conversion of other zymogens from their inactive to active forms.
* Trypsinogen is converted to trypsin by the enzyme enterokinase (enteropeptidase) secreted by cells lining duodenum.
* Trypsin then activates the conversion of other zymogens from their inactive to active forms.
* Inhibition of trypsin will slow activation of other proteases.
* Trypsin catalyzes the splitting of peptide bonds on the carboxyl side of lysine and arginine residues.
* It has a pH optimum of 7.6 to 8.0 (alkaline).
* Classified as a serine protease (serine and histidine at the active site.

Trypsin, Chymotrypsin
* Similar chemical compositions
* Chief differences are specificity of action:
trypsin – lysine, arginine
chymotrypsin – tyrosine, phenylalanine, tryptophan, methionine,leucine
(aromatic or large hydrophobic side chains)

Lock and Key Model of Enzyme Activity
Visualization of the Lock and Key Model of Enzyme Function

Lock Key Enzyme Catalysis
* The “Active Site” contains:
* A shape that fits a specific substrate(s)
* Side chains that attract (chemically) the substrate
* Side chains that are positioned to speed the reaction

Enzyme Catalysis
* -OH of serine 195 attacks C=O of peptide bond. Histidine 57 donates a proton to the N of the peptide bond leading to cleavage and acylation of the enzyme. Asp-102 is also involved.

Carboxypeptidase COO- terminal peptide bond
* Hydrolysis occurs most readily if the COO- terminal residue has an aromatic or bulky aliphatic side chain.
* Binding of a typical substrate results in a rearrangement of the active site (induce fit). Glutamate-270, Arginine-145, Arginine-127, Tyrosine-248 are important at the active site.

Trypsin Inhibitors
* Trypsin (protease) inhibitors are found naturally in many seeds, particularly legumes such as soy, peas, other beans.
* heat labile, heat stable
* Osborne and Mendel (1917) – soybeans need to be heated to support growth in rats
* Kunitz inhibitor, Bowman-Birk inhibitor
* Both are inactivated during moist heat treatment.
* Protease inhibitors are proteins which bind to the enzyme, rendering them inactive.
* Symptoms include pancreatic hypertrophy due to stimulated secretory activity.

Absorption of peptides and amino acids
Transport at the brush border
1. Active transport by carrier.
2. Mostly dependent on Na+ gradient - co-transport similar to that for glucose
3. Some amino acids (basic, and neutral with hydrophobic side chains) are absorbed by facilitated diffusion
Protein assimilation affected by - Pancreatitis, congenital protease deficiencies, deficiencies of specific transporters

Absorption of Amino Acids
* The transporters bind amino acids only after binding sodium.
* The fully loaded transporter undergoes a conformational change that dumps Na+ and the amino acid into the cytoplasm. The transporter then reorients back to its original form.

Absorption of Amino Acids
* Absorption of amino acids is dependent on the electrochemical gradient of Na+ across the epithelium.
* The basolateral membrane of the enterocyte contains additional transporters which export amino acids from the cell into the blood (not dependent on sodium gradients).

Absorption of Peptides
* There is virtually no absorption of peptides longer than three amino acids but there is abundant absorption of di- and tri-peptides, probably by a single transport molecule.
* The vast bulk of di- and tri-peptides are digested into amino acids by cytoplasmic peptidases.

Absorption of Intact Proteins
* Absorption of intact proteins occurs rarely.
* Very few proteins can get through the gauntlet of soluble (lumen) and membrane-bound proteases intact.
* “Normal” enterocytes do not have the transporters neededt to carry proteins across the plasma membrane and they can’t permeate tight junctions.

Absorption of Intact Proteins
* Shortly after birth, neonates can absorb intact proteins.
Absorption of Intact Proteins
* Shortly after birth, neonates can absorb intact proteins.
* Most of these intact proteins are immunoglobulins which can be absorbed from the very first milk (colostrum) and this imparts early neonatal passive immunity.

Absorption of Intact Proteins
* Shortly after birth, neonates can absorb intact proteins.
* Most of these intact proteins are immunoglobulins which can be absorbed from the very first milk (colostrum) and this imparts early neonatal passive immunity.
* “Closure” is when the small intestine loses the capacity to absorb intact proteins.

Protein Requirements
* Maintenance = nutritional requirements to stay alive (does not require positive BW gain)
* Growth = positive tissue accretion
* Reproduction = tissue specific growth related to reproduction, reproductive function (milk, eggs, reproductive tissue)

How do you express a protein requirement ?
* Protein percent of the diet
* Amino acid percent of the diet
Growth Will Dictate Feed Intake
Intake Will Dictate Actual Requirement
* Protein percent of the diet
* Amino acid percent of the diet
* Amino acid percent of total protein
How do you express a protein requirement ?

* Protein percent of the diet
* Amino acid percent of the diet
* Amino acid percent of total protein
* Digestible protein percent of the diet

Digestible Protein Estimates
Digestible Amino Acid Determination
How do you express a protein requirement ?
* Protein percent of the diet
* Amino acid percent of the diet
* Amino acid percent of total protein
* Digestible protein percent of the diet
* Ideal Protein ratios (relationships among amino acids)
Economics of Protein Nutrition
Caloric cost of protein deposition
Amino Acid Balance
Dietary Protein/Amino Acid Balance
Protein Quality Evaluation
Protein Efficiency Ratio
Comparison of Protein Sources
Commercial Application of PER

Protein Digestion and Absorption.ppt