Proteolysis
It is also important in the regulation of some physiological and cellular processes including apoptosis, as well as preventing the accumulation of unwanted or misfolded proteins in cells.
Proteolysis can also be used as an analytical tool for studying proteins in the laboratory, and it may also be used in industry, for example in food processing and stain removal.
This may involve removal of the N-terminal methionine, signal peptide, and/or the conversion of an inactive or non-functional protein to an active one.
[2] In both prokaryotes and eukaryotes, the exposed N-terminal residue may determine the half-life of the protein according to the N-end rule.
Many viruses also produce their proteins initially as a single polypeptide chain that were translated from a polycistronic mRNA.
The latter name refers to the fact that a slippery sequence in the mRNA that codes for the polypeptide causes ribosomal frameshifting, leading to two different lengths of peptidic chains (a and ab) at an approximately fixed ratio.
Proteases in particular are synthesized in the inactive form so that they may be safely stored in cells, and ready for release in sufficient quantity when required.
The complement system of the immune response also involves a complex sequential proteolytic activation and interaction that result in an attack on invading pathogens.
For example, limitation for major elements in proteins (carbon, nitrogen, and sulfur) induces proteolytic activity in the fungus Neurospora crassa[3] as well as in of soil organism communities.
It also serves to regulate cellular processes by removing enzymes and regulatory proteins that are no longer needed.
In contrast, other proteins like actin and myosin have a half-life of a month or more, while, in essence, haemoglobin lasts for the entire life-time of an erythrocyte.
[6] Other factors suspected to affect degradation rate include the rate deamination of glutamine and asparagine and oxidation of cystein, histidine, and methionine, the absence of stabilizing ligands, the presence of attached carbohydrate or phosphate groups, the presence of free α-amino group, the negative charge of protein, and the flexibility and stability of the protein.
The degradation of cyclins is the key step that governs the exit from mitosis and progress into the next cell cycle.
The precursors of caspase, procaspase, may be activated by proteolysis through its association with a protein complex that forms apoptosome, or by granzyme B, or via the death receptor pathways.
Autoproteolysis takes place in some proteins, whereby the peptide bond is cleaved in a self-catalyzed intramolecular reaction.
Examples include cleavage of the Asp-Pro bond in a subset of von Willebrand factor type D (VWD) domains[14][15] and Neisseria meningitidis FrpC self-processing domain,[16] cleavage of the Asn-Pro bond in Salmonella FlhB protein,[17] Yersinia YscU protein,[18] as well as cleavage of the Gly-Ser bond in a subset of sea urchin sperm protein, enterokinase, and agrin (SEA) domains.
Chronic inflammatory diseases such as rheumatoid arthritis may involve the release of lysosomal enzymes into extracellular space that break down surrounding tissues.
Abnormal proteolysis may result in age-related neurological diseases such as Alzheimer's due to the generation and ineffective removal of peptides that aggregate in cells.
Protein backbones are very stable in water at neutral pH and room temperature, although the rate of hydrolysis of different peptide bonds can vary.
[28] Certain chemicals cause proteolysis only after specific residues, and these can be used to selectively break down a protein into smaller polypeptides for laboratory analysis.
[27][30] Protein may also be broken down without hydrolysis through pyrolysis; small heterocyclic compounds may start to form upon degradation.
Above 500 °C, polycyclic aromatic hydrocarbons may also form,[31][32] which is of interest in the study of generation of carcinogens in tobacco smoke and cooking at high heat.
[33][34] Proteolysis is also used in research and diagnostic applications: Proteases may be classified according to the catalytic group involved in its active site.
