Proteins are large biomolecules consisting of more than 50 amino acids connected by multiple peptide bonds, while peptides are small biomolecules consisting of less than 50 amino acids. Proteins fulfill a variety of functions, including regulating physiological activity and providing structure to cells, and their functions are closely tied to their conformation. After ingestion, dietary proteins are denatured by gastric acid and subsequently cleaved by pepsin and proteases into monopeptides, dipeptides, tripeptides, and tetrapeptides. These end products are absorbed in the small intestine via proton symporter and Na+-coupled carrier proteins. Intracellularly, endogenous proteins are degraded by the ubiquitin proteasome system, while endocytosed dietary proteins are degraded by the lysosome. Accumulation of damaged or misfolded proteins/peptides has been observed in many neurological diseases such as Alzheimer disease, Parkinson disease, Huntington disease, Creutzfeldt-Jakob disease, and myotonic muscular dystrophy.
- Composition: Proteins consists of a chain of ≥ 50 amino acids (AAs) that are connected through multiple peptide bonds (polypeptide chain).
Structure: broken down into 4 levels
- Primary structure: the sequence of AAs in the polypeptide chain
- Secondary structure: structural folding that occurs based on patterns of H+-bonds between parts of the same polypeptide chain
- Tertiary structure: 3-dimensional structure of secondary structures
- Quaternary structure: 3-dimensional structure of ≥ 2 individual polypeptide chains (subunits) = multimer
- For both tertiary and quarternary, folding driven by: hydrophobic interactions, H+-bonds, salt bridges, disulfide bonds
- Proper protein folding must occur for a protein to be functional (see learning card on translation and protein synthesis)
- Protein synthesis: See learning card on .
- Denaturation: the undoing of correct protein structure
- Duodenum: further cleavage from pancreatic and intestinal proteases
- Absorption of di-, tri-, and tetrapeptides: likely via a proton symporter
- Absorption of single amino acids: via Na+-coupled carrier proteins for specific AA groups (neutral, branched-chain, aromatic, acidic, basic)
- AAs enter bloodstream → liver (via portal vein)
- Zymogens: proteases that are first secreted in an inactive form to avoid damage to the immediate surrounding tissue
|Important proteases of the gastrointestinal tract|
|Endopeptidases: split peptide bonds within the polypeptide chain|| |
|Pancreatic elastase|| |
|Exopeptidases: split peptide bonds from end AAs||Carboxypeptidases: split unspecific end AAs from C-terminal||Carboxypeptidase A|| || |
|Carboxypeptidase B|| |
|Aminopeptidase|| || || |
|Dipeptidase|| || || |
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).
- Description: Proteins are targeted via ubiquitination for degradation in proteasomes.
- Ubiquitination: addition of ubiquitin to the ε-amino group of lysine residues of a substrate protein; it consists of 3 parts
Examples of diseases associated with aberrant proteolysis
There are many diseases associated with aberrant proteolysis; this list is not exhaustive.
- Conditions that lead to increased tissue protein breakdown
Conditions caused by increased protein breakdown
- ; and
- Pancreatitis and possibly resulting
- Malignancy induced cachexia
Conditions caused by accumulation of damaged or misfolded proteins/peptides (see in the learning card on for more details)
- Age-related neurological diseases/neurodegenerative diseases (e.g., Alzheimer disease, Parkinson disease, Huntington disease)
- Prion-related conditions (e.g., Creutzfeldt-Jakob disease)
- Myotonic muscular dystrophy
- Cardiovascular diseases
- Inflammatory responses and autoimmune diseases