Food Chemistry and Fish in Nutrition

8.1 Digestion and absorption of proteins

Ingested proteins are enzymatically hydrolyzed into amino acids in the gastrointestinal tract. When protein enters the stomach, it stimulates the secretion of hydrochloric acid by the parietal cells of the gastric glands and pepsinogen by the chief cells.

Action of HCl

Due to the secreted HCl the gastric juice has a pH between 1.5 and 2.5. The acidity of gastric juice acts as an antiseptic and kills most bacteria and other cells. In addition, it causes globular proteins to undergo denaturation or unfolding at this low pH, rendering their internal peptide bonds more accessible to enzymatic hydrolysis.

Action of pepsin

Pepsinogen, an interactive precursor or zymogen, is converted into active pepsin in the gastric juice by the enzymatic action of pepsin itself. In this process, 42 amino acid residues as a mixture of small peptides is cutoff. The rest of the pepsinogen molecule, which remains intact, is the enzymatically active pepsin.

In the stomach, pepsin hydrolyzes the peptide bonds of ingested proteins involving the aromatic amino acids tyrosine, phenylalanine, and tryptophan, thus cleaving long polypeptide chain into a mixture of smaller peptides.

Action of Hormones

Secretin: As the acid stomach contents pass into the small intestine, the low pH triggers the secretion of the hormone, secretin into the blood. Secretin stimulates the pancreas to secrete bicarbonate into the small intestine to neutralize the gastric HCl. The pH then rises abruptly from between pH 1.5 to 2.5 to about pH 7. In the small intestine the digestion of proteins continues.

Cholecystokinin: The entry of amino acids into the duodenum releases the hormone cholecystokinin, which stimulates secretion of several pancreatic enzymes, whose optimum pHs are near 7. Three of these, trypsin, chymotrypsin, and carboxypeptidase, are made by the exocrine cells of the pancreas as their respective enzymatically inactive zymogens, trypsinogen, chymotrypsinogen and procarboxy peptidase, synthesis of these enzymes as interactive pre cursors protects the exocrine cells from destructive proteolytic attack.

Action of trypsin:

After trypsinogen enters the small intestine, it is converted into its active form trypsin by enterokinase, a specialized proteolytic enzyme secreted by intestinal cells. Once some free trypsin has been formed, it also can catalyze the conservation of trypsinogen into trypsin. The formation of free trypsin is brought about by removal of a hexa peptide from the amino-terminal end of the trypsinogen chain. Trypsin hydrolyzes those peptide bonds whose carbonyl groups are contributed by lysine and arginine residue.

Action of chymotrypsin

Chymotrypsinogen has a single polypeptide chain with a number of intra chain disulfide bonds. When it reached the small intestine, it is converted into chymotrypsin by trypsin, which cleaves the single long polypeptide chain of chymotrypsinogen at two points by excision of dipeptides. The three segments formed from the original chymotrypsinogen chain are still held together, however, by disulfide cross-linkages. Chymotrypsin hydrolyzed those peptide bonds involving phenylalanine, tyrosine, and tryptophan residues. Trypsin and chymotrypsin thus hydrolyze the polypeptides resulting from the action of pepsin in the stomach into smaller peptides.

Action of peptidases

Degradation of the short peptides in the small intestine is now completed by other peptidases. The first is carboxypeptidase, a zinc containing enzyme, which the pancreas make as its inactive zymogen, procarboxypeptides.

Carboxypeptidase removes successive carboxyl-terminal residues from peptides.

The small intestine also secrets an amino peptidase, which can hydrolyze off successive amino-terminal residues of peptides. ingested proteins are ultimately hydrolyzed to yield a mixture of free amino acids, which are then transported across the epithelial cells lining the small intestine. The free amino acids enter the blood capillaries in the villi and are transported to the liver.

Enzymes involved in protein digestion and their peptide bond specificity

Table 8.1.1

Pepsin	Tyr, Phe, Trp; Leu, Glu, Gln
Trypsin	Lys , Arg
Chymotrypsin	Tyr, phe, Trp
Carboxypeptidase	Successive carboxy terminal residues
Aminopeptidase	Successive amino terminal resides(except proline)

Absorption

Ingested proteins are ultimately hydrolyzed to yield a mixture of free amino acids, which are then transported across the epithelial cells lining the small intestine. The free amino acids enter the blood capillaries in the villi and are transported to the liver.