Lecture 11 Protein Digestion Monogastric Flashcards
How much of protein load is received by the human gut? -Dietary -Endogenous proteins(and where do these ones come from?)
- 70-100 g = dietary.
- 35-200 g = endogenous proteins from :
– Secreted enzymes.
– Cell turnover
Are proteins digestible?
-Highly digestible (especially animal protein)
What is Peptidases?
Hydrolysis of peptide bonds
What types of Protein hydrolysis by peptidases are there?
- Endopeptidases = internal peptide bonds leading from large polypeptides to small oligopeptides.
- Exopeptidases = act on small oligopeptides to produce: – Amino acids. – Di and tripeptides.
Where does protein digestion in monogastric begin?
- begins in stomach with pepsin
-pancreatic enzymes, inactive when secreted(because it would digest themselves if it was active) *
-brush border enzymes
What is protein broken down into?
– Tripeptides
– Dipeptides
– Free amino acids
What is included in peptidases?
-proteases,
-proteolytic enzymes
Where is the exopeptidases bonds next to?
peptide bonds next to N- or C-terminus
* Contribution to hydrolysis increases as proteins become more digested
Where is the endopeptidases bonds next to?
bonds not next to N- or C-terminus (bonds in the middle)
What are digestive enzymes initially produced as?
-Zymogens - these are inactivated and they are activated when needed. (– Zymogens and proenzymes are the same thing)
Where is most of the digestive enzymes synthesized and stored?
- Most of them are synthesized and stored in
the pancreas, then secreted into the SI
where they are activated by removal of a
small peptide section - Must be stored as a “zymogen” because
they would damage the synthesizing cell
Is the pH in your stomach always really low?
-No its not always low, it is only low when you eat something
What is the first step of protein digestion in the stomach?
- Initiated in stomach
– HCl from parietal cells - Stomach pH 1.6 to 3.2
- Denatures 4 0, 3 0, and 2 0 structures (breaks H and electrostatic
bonds so the protein “unfolds” or “uncoils”
– Pepsinogen from chief cells
What happens to Pepsinogen in the stomach?
Pepsinogen, synthesised by chief cells secretes HCL and then this turns pepsinogen into pepsin.
Tell me about pepsin?
- Pepsin also self-activates (initiates conversion of pensinogen)
- Pepsin accounts for 10-20% of protein digestion
- Protein leaves stomach as mix of indigested protein, denatured
protein, peptides and amino acids
What is Denaturation?
A process in which a protein uncoils and loses its shape, causing it to lose its availability to function, it can be caused by high temperatures, whipping, acids, bases, and a high salt concentration. EG, frying an egg
What does the pancreas do for digestion?
- Produces enzymes responsible for
– 50% of carbohydrate digestion
– 50% of protein digestion
– 90% of lipid digestion - Produces sodium bicarbonate for
neutralization of chyme in duodenum
What does the pancreas synthesizes?
– Trypsinogen
– Chymotrypsinogen
– Procarboxypeptidase
What does the duodenum recognize?
Duodenum recognizes proteins and secretes
CCK which stimulates the pancreas to secrete
protein digesting enzymes in an “inactive state”
* Not “active” in this proenzyme form
* Must be chemically converted into their active
enzymes that are capable of substrate hydrolysis
Activation of Pancreatic Enzymes:
- Enterokinase (aka enteropeptidase)
– Secreted from by brush-boarder (microvilli)
– Trypsinogen trypsin - Trypsin then converts:
– Chymotrypsinogen chymotrypsin
– Procarboxypeptidase carboxypeptidase
Monogastric digestion in the small intestine:
- Zymogens must be converted to active form
- Trypsinogen to Trypsin (by enteropeptidase/trypsin)
- Endopeptidase
*Chymotrypsinogen to Chymotrypsin (by trypsin) - Endopeptidase
*Procarboxypeptidase to Carboxypeptidase (by trypsin) - Exopeptidase
– Removes carboxy terminal amino acid
What happens initially with brush border enzymes?
- The intestinal brush broader produces
peptidases which continue to hydrolyze (i.e.
split/break/digest) the remaining
polypeptides into tripeptides, dipeptides,
and some amino acids
What is the process for brush border protein digestion?
*Small intestine (brush border)
– Enterokinase (or enteropeptidase)
* Trypsinogen trypsin
* Trypsin then activates all the other enzymes
*Aminopeptidases
* Most abundant
* Cleave at N-terminal AA
*Dipeptidases
* Cleave dipeptides
Whats important to know about aminopeptidases ?
It is the most abundant and it cleaves at N terminal AA
Whats the small intestine enzymes that complete digestion of proteins?
– Endopeptidases.
– Dipeptidases.
– Aminopeptidases.
Whats the product of protein hydrolysis:
Products.
– Free amino acids.
– Di- and tripeptides.
– Absorbed into enterocyte by specific carrier-mediated transport.
– Further digestion to amino acids inside enterocytes.
– Free amino acids inside enterocyte transferred across the
basolateral membrane into the portal blood system.
Where does absorption happen in the body of proteins?
- Most protein absorption takes place in the
duodenum and jejunum - Most amino acids are absorbed into the
bloodstream, but some remain in the
enterocytes and are used to synthesize
enzymes and new cells - > 99% of protein enters the bloodstream as
amino acids - Absorption of whole protein can cause a
severe allergic reaction
Absorption of peptides and amino acids from GI lumen? -what happens?
- amino acids
– Na+(sodium gradient) gradient-driven symports (co-transport) mechanism same as glucose–Na+ symport. Sodium gradient generated by the Sodium/potassium pump on basolateral membrane
– at least 4 kinds of symport proteins
– specificities by amino acid types
examples: Neutral/small (polar side chains (ala, ser, thr)
Basic (cationic side chains; lys, arg)
Acidic (anionic side chains: asp, glu)
What is the process of free amino acid absorption?
- Free amino acids
– Carrier systems - Neutral AA
- Basic AA
- Acidic AA
– Entrance of AA is via
active transport - Requires energy by
sodium/potassium pump
What’s the absorption of peptides and amino acids like?
- Small peptides are absorbed
– mostly di- & tripeptides
– H + gradient-driven symport - Requires pH in lumen to be lower than pH w/i enterocyte
– absorbed peptides hydrolysed to amino acids by
cytosolic peptidases
Explain the process of peptide absorption:
- Form in which the majority of protein is absorbed
- More rapid than absorption of free amino acids
- Active transport
– Energy required - Peptides further hydrolyzed into free amino acids in enterocyte
- Only free amino acids absorbed into blood
In a basolateral membrane what transport occurs?
- Transport of free amino acids only*
– Peptides are hydrolysed within the enterocyte - Transport mainly by diffusion and Na-independent carriers
Tell me how free amino acids and di and tri peptides are brought into the enterocyte*** exam MCQ
What is protein transport in blood like?
- Amino acids diffuse across the basolateral
membrane
– Enterocytes > portal blood > liver > tissues
– Transported mostly as free amino acids - Liver
– Breakdown of amino acids
– Synthesis of non-essential amino acids
What does the cell use first in the enterocytes?
- First cells that can use the amino acids
– Transport into portal blood
– Protein synthesis - Digestive enzymes
- Structure and growth
– Energy
Absorption of intact proteins :
*New-borns
– First 24 hours after birth
– Immunoglobulins
* Passive immunity…..colostrum
*Adults
– Paracellular routes
* Tight junctions between cells
– Intracellular routes
* Endocytosis
* Pinocytosis
*Of little nutritional significance…
– Affects health (allergies and passive immunity)
Explain 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.
What are trypsin inhibitors?
- Trypsin (protease) inhibitors are found
naturally in many seeds, particularly
legumes such as soy, peas, other beans.
But these inhibitors are heat labile - Osborne and Mendel (1917) – soybeans
need to be heated to support growth in rats
What are factors that affect protein digest and metabolism? (Trypsin Inhibitors) -only for monogastric
– Enzyme inhibitors and other anti-nutritional factors in the feed
* trypsin and chymotrypsin inhibitors in some feeds
– decrease the activity of these enzymes
– moderate heat treatment is effective in reducing the effects of these inhibitors
* Inhibition of trypsin will slow activation of other proteases
and therefore decrease protein digestion.
* Inhibitors are naturally inactivated by rumen microflora. Thus, heat treating soy is not necessary in ruminant diets
What’s celiac disease? explain it:
- Also called gluten-sensitive enteropathy
- Mediated by the immune system
- A permanent intestinal intolerance to
dietary gluten - Gluten is not a protein itself but rather a
protein composite, which means it is
composed of several different proteins. - GIT lesions in genetically susceptible
people
What is Gluten?
- A protein found in wheat, barley, rye, and
some other grains (not corn, oats or rice)
Give a background on celiac disease:
- Gallen first described celiac disease in 250 AD
- 1950, role of wheat and rye flour in the pathogenesis of the disease
- 1954, first biopsy (surgical)
How many people have it in the US?
- Nearly 3 million people in the United States
have celiac disease. - But many more are gluten intolerant.
- Genetic component. Western Ireland has the
highest prevalence in the world.
What are the 3 categories of gluten intolerance?
- Three categories:
– Celiac Disease
(Immune activation, inflammation
,Villi atrophy
,Malabsorption)
– Non-Celiac Gluten Sensitivity
(Silent celiac)
– Wheat Allergy.
What is gluten intolerance and celiac disease a result of?
- Is the result of an autoimmune reaction that
causes villous atrophy.
– Immunologically mediated small intestinal
enteropathy.
– Immunologic overstimulation. - Gluten-dependent activation of mucosal
immunity.
When a person has celiac disease, what does the small intestine look like?
-No villi, becomes smooth and cannot absorb nutrients , these end up (all carbs) end up in the large intestine. all these macronutrients end up in the LI, they have simple sugars, proteins, and fats, the microbes are rapidly fermenting and causing the person to have bloat and diarrhoea etc.
What’s the symptoms of Celiac disease
- Vomiting
- Anorexia
- Chronic diarrhea
- Weigth loss
- Irritability
- “Failure to thrive”
- Abdominal distention
- Muscle wasting
- Death
What does a gluten free diet look like?
-Not necessarily a “healthy” diet.
-Not the same as a low-carb diet.
-Only needed for people with gluten intolerance.
