Digestion and Absorption of Proteins

Dietary protein — from pulses, milk, eggs, meat and fish — must be broken down all the way to its constituent amino acids before it can be absorbed, since intact protein molecules and even most peptide fragments are far too large to cross the intestinal epithelium. Unlike carbohydrate digestion, protein digestion does not begin in the mouth at all (saliva contains no protein-digesting enzyme); it begins only once food reaches the stomach.

In the stomach, the chief gastric enzyme is pepsin, which acts specifically on the peptide bonds of dietary proteins, breaking large protein molecules down into shorter chains called proteoses and peptones — still far too large to be absorbed, but considerably reduced from the intact protein. Pepsin itself, however, is not secreted in its active form; the chief (peptic) cells of the gastric glands secrete an inactive precursor, pepsinogen, which is converted to active pepsin only within the stomach lumen, by the action of the hydrochloric acid secreted by the neighbouring parietal (oxyntic) cells (and, once some active pepsin has formed, further pepsinogen is converted autocatalytically by pepsin itself). This delayed, site-specific activation is essential: were pepsin to exist in its active form inside the secretory cells or ducts of the stomach wall, it would begin digesting the very proteins that make up those cells.

Once the partially digested proteoses and peptones pass into the duodenum, a further and more thorough round of proteolysis is carried out by pancreatic enzymes. Trypsin and chymotrypsin, both secreted by the pancreas as inactive zymogens (trypsinogen and chymotrypsinogen respectively) for exactly the same protective reason that pepsin is secreted as pepsinogen, break the proteoses and peptones down further into much smaller peptide chains; trypsinogen is specifically activated to trypsin by enterokinase (enteropeptidase), an enzyme bound to the brush border of the duodenal epithelium, and the trypsin so formed then activates chymotrypsinogen (and procarboxypeptidase) in turn, so that a single activating step triggers the entire cascade. Carboxypeptidase, also secreted by the pancreas, works alongside trypsin and chymotrypsin by removing amino acids one at a time from the free end of each peptide chain, yielding a mixture of very short peptides and some free amino acids. …