Introduction to chemistry of proteins

1.6. Digestion and Absorption of Proteins

Digestion and Absorption of Proteins

Proteins are large polypeptide molecules coiled by weaker bonds in their tertiary structure, the digestion of proteins involves the gradual breakdown of this polypeptide by enzymatic hydrolysis into amino acid molecules which are absorbed in the blood stream. 

The protein load received by the gut is derived from two sources 70-100g dietary protein which is required daily and 35 - 200g endogenous protein (secreted enzymes and proteins in the gut or from intestinal epithelia cell turnover).

Only 1-2g of nitrogen equivalent to 6-12g of proteins are lost in the faeces on a daily basis. 

The process of protein digestion can be divided, depending on the sources of peptidases.

A. Gastric Digestion

Entry of a protein in to stomach stimulates the gastric mucosa to secrete a hormone gastrin which in turn stimulates the secretion of HCl by the parietal cells of the gastric glands and pepsinogen by the chief cells.

The HCL thus produced lower the pH of stomach to (pH 1.5 – 2.5) and acts as an antiseptic and kills most of the bacteria and other foreign cells ingested along with.

The acid denatures the protein and the whole protein susceptible to hydrolysis by the action other proteolytic enzymes.

Proteases are endopeptidases which attack the internal bonds and liberate large fragments of peptides.

Then pepsinogen having MW 40,000 an inactive precursor or zymogen is converted in to active pepsin in the stomach itself. In this process 44 amino acids gets removed from the amino terminal end and the portion of the molecule that remain intact is enzymatically active pepsin (MW. 33,000).

This active pepsin cleaves the ingested protein at their amino terminus of aromatic amino acids (Phe, Tyr, and Trp.). The major products of pepsin action are large peptide fragments and some free amino acids.

B. Pancreatic Digestion

Pancreatic zymogens proceed digestion as the acidic stomach contents pass in to the small intestine, A low pH triggers the secretion of a hormone Secretin in the blood. Secretin stimulates the pancreas to secrete HCO₃^- (bicarbonate), which in the small intestine neutralizes the gastric HCL and abruptly change the pH to 7.0.

The entry of large peptide fragments and some free amino acids in the upper part of the small intestine (Duodenum), excites the release of a hormone cholecystokinin (CCK).

C. Intestinal Digestion

Since pancreatic juice does not contain appreciable aminopeptidase activity final digestion of di and Oligopeptides depends on the small intestinal enzymes.

The lumenal surface of epithelial cells is rich in endopeptidase, and dipeptidase aminopeptidase activity.

The end products of the cell surface digestion are free amino acids and di and tripeptides. These are passed into the interior of the epithelial cell where other specific peptidases convert almost all of them to a single amino acid that are transported to the blood stream by the opposite side of the cell membrane and carried to liver (primarily) and other tissues for oxidative degradation. This process completes the absorption of 99% of digested proteins.