The average dietary intake of protein is well above that required by the body. In order for proteins to be absorbed, they must be degraded into smaller molecules. This can be either individual amino acids or short peptides (dipeptide or tripeptides). Digestion of proteins begins in the stomach with the activation of pepsin. Pepsinogen is secreted from the chief cells where it is exposed to a hydrogen ion. This exposure partially activates pepsinogen which goes on to act upon other pepsinogen molecules to fully activate them into pepsin. Once activated, pepsin begins to cleave proteins between specific amino acid sequences. This means that pepsin can not completely degrade proteins. Once the chyme has moved to the intestines, proteases from the pancreas and other proteases bound to the brush border begin to act on the proteins. The enzymes from the pancreas are secreted as
zymogens like
trypsinogen,
chymotrypsinogen, and
procarboxypeptidase. These zymogens are activated by an enterokinase on the brush border that activates trypsin. Once activated, trypsin activates chymotrypsin and carboxypeptidase. Both trypsin and chymotrypsin are similar to pepsin in that they target specific sequences and are therefore unlikely to produce a large amount of individual amino acids. However, the enzymes
carboxypeptidase and
aminopeptidase (located on the brush border) cleave the amino acid at the carboxy end and the amino acid at the amino terminus, respectively. Individual amino acids can then be absorbed by a sodium-linked secondary active transport across the apical membrane of the small intestine. The dipeptides and tripeptides are actively transported into the cells where they are further degraded into individual amino acids. All of the individual amino acids are transported across the basolateral membrane by facilitated diffusion where they can diffuse into capillaries.
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