Protein absorption and digestion

Let's talk about the digestion of protein, through polypeptides, all the way to the final product, the aminoacids.

The digestion of the protein starts in the stomach, where the enzyme called pepsin will break the polypeptides into smaller parts. The highly acid medium will help this to happen. The digestion continue in the duodenum and upper small intestine, where the pancreas will secrete another enzyme called chymotripsin, with the role of breaking the small polypeptides into even smaller polypeptides. The final work is done in the lower small intestine, where carboxypeptidase and aminopeptidase enzymes will break everything into aminoacids, and they will be absorbed into the blood stream. About 5% of the proteins will leave the body undigested, through feces.

The single molecules of aminoacids are taken up into the blood stream and distributed across the human body, used to synthesize the so much needed proteins (like albumin in liver or muscle proteins in muscle) and they can have extracellular functions, such as hormones and cell adhesion, or intracellular, such as generating fuel, cell structure, signal transduction. You can find more than 100.000 proteins in our body.

The protein digestion starts in the stomach, when the acid environment causes proteins to unfold, allowing pepsin to access the dietary protein easier. The pepsin enzyme is produced by the Chief cells lining the stomach, and it is present as an inactive pro-enzyme called pepsinogen, being activated by the high acidity. The pepsin will cleave the peptide bonds, creating small polypeptide from the protein. The process is further going into the small intestine (upstream), with the polypeptides being cleaved into even smaller polypeptides under the influence of trying and chymotripsin (which are also created by pancreas as inactive pro-enzymes, being activated when they reach the intestine). In the downstream on the small intestine, the polypeptides are finally transformed into individual aminoacids, under the action of aminopepsidase and carboxypepsidase (both of them produced by the intestinal cells). Those last mentioned enzymes will remove single aminoacids from each end of the peptide. The single aminoacids are taken to the liver, after they were being absorbed via portal circulation, and to the rest of the body, used as building block for the synthesis of the body proteins. In the liver, they are mainly used to synthesize the main serum protein called albumin.

In the next post I will talk about the protein functions (enzymes, structural, hormones, transporter, antibodies and so on).