Proteins and peptides

Protein structure


Digestion and absorption of dietary proteins

  • Zymogens: proteases that are first secreted in an inactive form to avoid damage to the immediate surrounding tissue
Important proteases of the gastrointestinal tract
Proteases Location Reaction Product
Endopeptidases: split peptide bonds within the polypeptide chain



  • Oligopeptides
  • Oligopeptides

Pancreatic elastase
  • Oligopeptides
Exopeptidases: split peptide bonds from end AAs Carboxypeptidases: split unspecific end AAs from C-terminal Carboxypeptidase A
  • Activated from zyinomogen procarboxypeptidase A by trypsin
  • Cleaves bonds involving aromatic AAs
  • AAs
Carboxypeptidase B
  • Activated from zymogen procarboxypeptidase B by trypsin
  • Cleaves bonds involving basic AAs
  • AAs
  • Intestinal mucosa
  • Cleaves unspecific end AAs from N-terminal
  • AAs
  • Intestinal mucosa
  • Cleaves dipeptides
  • AAs

Trypsinogen is first activated by enteropeptidase via proteolytic cleavage at the N-terminal. The resulting trypsin then activates other zymogens, including further trypsinogen (positive feedback loop).

The inactive zymogen pepsinogen is activated to pepsin by gastric acid.


Protein degradation and associated diseases

Protein degradation

Endogenous proteins (those synthesized in cells) are degraded by proteasomes. Exogenous proteins are degraded by lysosomes.

Ubiquitin proteasome system (UPS)

Some cases of Parkinson disease have been linked to defects in the ubiquitin proteasome system.


Examples of diseases associated with aberrant proteolysis

There are many diseases associated with aberrant proteolysis; this list is not exhaustive.


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last updated 12/26/2019
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