Download Digestion of Dietary Proteins

Digestion of
Dietary Carbohydrates
Main Carbohydrates of Diet
1- Monosaccharides:
mainly glucose & fructose
ABORBED with NO DIGESTION
2- Disaccharides:
Sucrose, lactose & maltose
DIGESTED into monosaccharides
3-Polysaccharides:
Starch (plant source e.g. rice, potato, flour)
& glycogen (animal source)
DIGESTED into monosaccharaides
Cellulose (fibers of vegetables & fruits)
NOT DIGESTED
Digestion of Carbohydrates of Diet
1- In the Mouth
The major dietary polysaccharides are of plant (starch, composed of amylose & amylopectin)
& animal (glycogen) origin.
During mastication, the enzyme salivary a-amylase acts on dietary starch and glycogen
hydrolysing some a(1-4) bonds to give dexrtin
N.B. humans do not have b-amylase, so they can not digest cellulose, which is a carbohydate
of plant origin containing b(1-4) glycosidic bonds between glucose molecules.
Salivary a-amylase action on carbohydrates stops in the stomach as the media is strongly
acidic.
Digestion of Carbohydrates of Diet
2- In the Lumen of small intestine
Pancreatic bicarbonate neutralizes the acidic juice of stomach.
pancreatic a-amylase continues the process of starch and glycogen digestion
Dextrin is cleaved into oligosachharides and disaccharides.
Digestion of Carbohydrates of Diet
3- At the Mucosal Lining of Small Intestine
Mucosa of small intestine secretes two types of enzymes:
1- Intestinal oligosaccharidases: digest oligosaccharides into disaccharides & monosaccharaides.
2- intestinal disaccharidases : digest disaccharides into monosaccharides.
Disaccharidases are:
Sucrase: cleaves sucrose into glucose and fructose
Maltase: cleaves maltose into glucose and glucose
Lactase: cleaves lactose into glucose and galactose
Finally, most carbohydrates of diet (polysaccharides & disaccharides) are hydrolysed (digested)
into monosaccarides (mainly glucose, fructose & galactose).
Only monosaccharides are absorbed ---- to blood
Cellulose of diet (in fibers of vegetables & fruits) are not digested.
Overview of Digestion of Carbohydrates of Diet
Absorption of Monosaccharaides
1- From lumen to inside cells:
Only monosaccharaides are absorbed in the small intestine
- The duodenum and upper jejunum absorb the bulk of the dietary sugars
- Insulin hormone is not required for uptake of glucose by intestinal cells.
i-
Galactose & glucose are transported into the mucosal cells by an active energy
requiring process that requires the concurrent uptake of sodium ions.
The transport protein is the sodium dependent glucose cotransporter-1
Ii- Frucose absorption requires a sodium-independent transporter (GLUG-5) for its
absorption (energy is not required)
2- From inside cells into blood:
Glucose, galactose & fructose are transported from intestinal mucosal cells into the
portal circulation (blood) by GLUT-2 transporter.
(energy is not required)
Disaccharidase Deficiencies
Deficiency of a disaccharidase of the
intestinal mucosa causes:
1- The disaccharide is not digested to
monosaccharaide.
2- The undigested disaccharide passes into
the large intestine
3- In large intestine, disaccharides which are
osmotically active draw water from the
mucosa into the large intestine lumen
causing osmotic diarrhea.
4- Diarrhea is increased by the fermentation
of the remaining carbohydrates to
two- and three-carbon compounds which
are also, osmotically active.
5- Large volumes of CO2 and H2 gases cause
abdominal cramps and flatulence.
Disaccharides Deficiencies cont.
Causes of deficiency of enzymes:
1- Hereditary deficiencies of a disaccharidase deficiency
For example:
Lactose intolerance:
Inability to digest lactose of milk due to deficiency of lactase enzyme
So, diarrhea will occur on ingestion of milk or milk products
For infants (up to two years old): are treated by lactose-free milk
2- Intestinal diseases or drugs that injure the mucosa of the small intestine
Digestion of Dietary Proteins
Proteins must be digested to yield amino acids which can be absorbed
1- In the Stomach
Digestion by gastric secretion
The gastric juice contains hydrochloric acid (HCL) & pepsinogen
•
Hydrochloric acid:
- Denatures proteins to make them more susceptible to hydrolysis by enzymes
- kills some bacteria.
•
Pepsin:
- An acid-stable endopeptidase secreted by stomach cells as inactive pepsinogen
- Pepsinogen is activated to pepsin either by HCL or by other pepsin molecules
- Action of pepsin: digests polypeptides into smaller polypeptides.
Pepsin
HCL
Secreted inactive
(Zymogen) =
pepsinogen & needs
HCL for starting
activation
pH 2
Attacks peptide bonds formed by
Digestion of Dietary Proteins cont.
2- In the Lumen of Small Intestine
Digestion by pancreatic enzymes
On entering the small intestine,
• Polypeptides produced in the stomach by the action of pepsin are further cleaved to
oligopeptides by pancreatic proteases :
Trypsin, chymotrypsin, elastase & carboxypeptidase A&B
•
These proteases are released from the pancreas as zymogens (inactive forms).
•
Release of zymogens is mediated by cholecystokinin & secretin (hormones of GIT)
•
Activation of zymogens is mediated by the enzyme enteropeptidase
i.e. Trypsinogen (zymogen) is converted to trypsin (active enzyme).
cholecystokinin & Secretin
(Hormones from intestinal Mucosa)
Stimulate Pancreatic Secretions
of Zymogens
Activation of Zymogens
by enteropeptidase secreted from
intestinal mucosa
Trypsin
PH 7-8
Secreted inactive
(zymogen)
& needs enterokinase
for activation
Enterokinase
Attacks peptide bonds formed by
Chemotrypsin
PH 7-8
Secreted as inactive
(zymogen) & needs
trypsin for activation
Attacks peptide bonds formed by
Elastase
Is an endopeptidase which acts on elastic fibers.
It is secreted as an inactive form (proelastase) & activated by
trypsin.
Carboxypeptidase
It is an exopeptidase attacking the peptide chain at its carboxylic
end liberating amino acids.
secreted as an inactive procarboxypeptidase activated by trypsin.
Digestion of Dietary Proteins cont.
3- On the mucosa of Small Intestine
digestion by intestinal aminopeptidases
Aminopeptidase is available on the luminal surface of the intestine
cleaves the N-terminal amino acids from oligopeptides to produce smaller
oligopeptide & free amino acids
Absorption of amino acids
• Free amino acids are taken up into the small intestinal
cells by sodium-dependent transport system.
• Amino acids are taken via blood to the liver
Abnormalities in Protein Digestion
In individuals with a deficiency in pancreatic
secretion (for example, due to chronic pancreatitis,
cystic fibrosis or surgical removal of the pancreas
Incomplete digestion & absorption of fat & protein
abnormal increase of lipids (steatorrhea) and
undigested protein in the feces