1. Proteins

2. Digestion and absorption of proteins
The protein load received by the gut is derived from two primary sources:
g dietary protein, and
g endogenous protein,
the latter either as secreted enzymes and proteins in the gut or from intestinal epithelial cell turnover
In healthy adults, only 1-2 g nitrogen, equivalent to 6-12g protein, are lost in the feces on a daily basis
Thus, the digestion and absorption of protein is extremely efficient

3. Peptidases hydrolyse proteins
Proteins, like other dietary macromolecules, are broken down by hydrolysis of specific peptide bonds and hence the enzymes involved are termed ‘peptidases’
These enzymes can either cleave internal peptide bonds (i.e. endopeptidases) or cleave off one amino acid at a time from either the –COOH or –NH2 terminal of the polypeptide (i.e. they are exopeptidases subclassified into carboxypeptidases , and aminopeptidases, respectively)

4. The endopeptidases cleave the large polypeptides to smaller oligopeptides, which can be acted upon by the exopeptidases to produce the final products of protein digestion, amino acids, di- and tripeptides, which are then absorbed by the enterocytes
Depending on the source of the peptidases, the protein digestive process can be divided into gastric, pancreatic and intestinal phases

5. A. Digestion of proteins by gastric secretion
The digestion of proteins begins in the stomach, which secretes gastric juice, a unique solution containing hydrochloric acid and the proenzyme pepsinogen:
Hydrochloric acid:
Stomach acid is too dilute (pH 2-3) to hydrolyze proteins; however, the acid functions to kill some bacteria and to denature proteins, making them more susceptible to subsequent hydrolysis by proteases

6. Overview of protein digestion

7. Pepsin:
This acid-stable endopeptidase is secreted by the serous cells of the stomach as an inactive zymogen (or proenzyme), pepsinogen
In general, zymogens contain extra amino acids in their sequences, which prevent them from being catalytically active
Note: Removal of these amino acids permits the proper folding required for an active enzyme

8. Pepsinogen is activated to pepsin either by HCL, or autocatalytically by other pepsin molecules that have already been activated
Pepsin releases peptides and a few free amino acids from dietary proteins

9. B. Digestion of proteins by pancreatic enzymes
On entering the small intestine, large polypeptides produced in the stomach by the action of pepsin are further cleaved to oligopeptides and amino acids by a group of pancreatic proteases
These enzymes, like pepsin, are synthesized and secreted as inactive zymogens

10. 1. Release of zymogens:
The release and activation of the pancreatic zymogens is mediated by the secretion of cholecystokinin and secretin, two polypeptide hormones of the digestive tract

11. 2. Activation of zymogens:
Enteropeptidase, an enzyme synthesized by and present on the luminal surface of intestinal mucosal cells of the brush border membrane, converts the pancreatic zymogen trypsinogen to trypsin by removal of a hexapeptide from the NH2-terminus of trypsinogen
Trypsin subsequently converts other trypsinogen molecules to trypsin

12. trypsin is the common activator of all the pancreatic zymogens
Enteropeptidase thus unleashes a cascade of proteolytic activity, because
trypsin is the common activator of all the pancreatic zymogens

13. 3. Specificity:
Each of these enzymes has a different specificity for the amino acids R groups adjacent to the susceptible peptide bond

14. Cleavage of dietary proteins by pancreatic proteases

15. Products of pancreatic digestion
Thus, the products of digestion by pancreatic enzymes are:
Oligopeptides
Amino acids

16. Endopeptidases, dipeptidases and aminopeptidases complete the digestion of proteins
The final digestion of di- and oligopeptides is dependent on membrane-bound small intestinal endopeptidases, dipeptidases and aminopeptidases
The end products of this surface enzyme activity are free amino acids, di- and tripeptides

18. Enzymes responsible for protein digestion

19. Absorption of amino acids di-, and tri-peptides
Free amino acids, di- and tripeptide are absorbed across the enterocyte membrane by specific carrier-mediated transport
Amino acids are transported by specific active transporters showing mechanisms which are similar to ones active in glucose transport

20. This is an indirect active process
These Na dependant symporters are located at the brush- border membrane
This is an indirect active process
At least six specific symporter systems have been identified as follows:
Neutral amino acid symporter for a.a. with short or polar side- chains(Ser, Thr, Ala)
Neutral amino acid symporter for aromatic or hydrophobic side chains (Phe, Tyr,Tryp, Met,Val,Leu,Ileu)
Imino acid symporter (Pro, OH-Pro)
Basic amino acid symporter (Lys, Arg, Cys)
Acidic amino acid symporter ( Asp, Glu)
β - amino acid symporter ( β- Ala, Tau)

21. These transporter systems are also present in the renal tubules and
defects in their constituent protein structure can lead to disease( e.g. Hartnup disease)
Pathologies can be produced in both the kidney and intestine

22. Symporters for di-, and tri-peptides
These are:
Present in the brush- border surface
H+- dependant

23. Di- and tripeptides are further hydrolyzed to their constituent amino acids inside the enterocyte
The final transfer is; therefore; of free amino acids across the contraluminal plasma membrane into the portal blood system
Na- independent transporters are present in the contraluminal surface, allowing a.a. facilitated transport to the portal vein

24. Digestion and absorption of proteins

25. After a protein-rich meal, protein digestion takes place in the small intestine
The amino acids released are absorbed by intestinal epithelial cells
A large proportion of amino acids are transaminated to alanine, which is released into the portal vein and taken to the liver
Therefore, alanine is the major amino acid secreted by the gut and the principal carrier of nitrogen in the plasma

26. Defects in protein digestion and absorption
For a number of reasons, protein absorption might be incomplete
Some of the proteins, because of their physical or chemical structure, are resistant to proteolytic attack and therefore pass through the small intestine relatively unmodified
Furthermore, the absorption of free amino acids and peptides may be less than 100%, particularly if gut function is impaired

27. This occurs in a number of clinical conditions, such as intestinal infection or injury, and when certain ‘antinutritional’ factors such as lectins or trypsin inhibitor proteins are present in the diet
This unabsorbed protein or amino acid then passes through into the colon

28. Metabolism by the colonic microflora then occurs, but
the amino acids are no longer available to the body, and
are excreted in the faeces, mainly in the form of bacterial protein.

29. 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,
the digestion and absorption of fat and protein is incomplete
This results in the abnormal appearance of lipids, called “steatorrhea” , and undigested protein in the feces

30. Coeliac Disease
Characterized by severe mal-absorption and specific diagnostic features exhibited by the intestinal mucosa
The histological changes are due to the interaction of gluten, the principal protein of wheat, with the epithelium
The deficit is located within the mucosal cells, and permits certain polypeptides; resulting from digestion of gluten; to exert local effects within the intestine, and to be absorbed, inducing an antibody response

31. The intolerance is for gliadin portion of gluten ( found in wheat and several other grains)

32. Medical history Diarrhea Abdominal bloating Weight loss
Laboratory Features:
Anemia, Hgb of 9g/dl
Low serum iron, ferritin and Vit. B12
Low RBC folate
Antibodies to gluten and its fractions are found in serum
Histopathologigal features:
Biopsy demonstrated flattening of the mucosal surface, villous atrophy and disappearance of microvilli( reduction of surface, thus greatly reduced absorption

33. Nutritional Management
All gluten containing products should be removed from diet permanently(gluten-free diet)
Avoid wheat, barley, rye, and all products made with these grains
Oats may be tolerated, but this needs testing
Read labels carefully, because gluten containing grains are added to many products

34. Hartnup’s disease
A rare inherited disease of neutral amino acids symporter for aromatic or hydrophobic side chains (Phe, Tyr,Tryp, Met,Val,Leu,Ileu ) leading to malabsorption of these a.a.s
Produces a clinical syndrome mimicking pellagra (niacin deficiency- the three Ds)
Protein synthesis is normal
Treated by niacin supplements

35. Digestion of carbohydrate using a diagram if possible
Absorption of carbohydrate using a diagram if possible
The effect of food preparation on carbohydrate digestion and absorption

36. Colonic fermentation of carbohydrate to include types of carbohydrate reaching the colon
Physiological effects of carbohydrates not digested in the small intestine
Abnormal degradation of disaccharides
Modification of fats during food processing

37. Effect of heating on dietary oil
Digestion of dietary lipids using a diagram if possible
Absorption of dietary lipids using a diagram if possible
Hormonal control of lipid digestion

38. Defects if fat digestion and absorption including causes and the nutritional management of fat mal-assimilation
Digestion of dietary proteins using a diagram if possible
Absorption of dietary proteins using a diagram if possible

39. Defects in protein digestion and absorption
Coeliac disease
Steatorrhea (definition – causes – dietary management)