Proteolysis
Proteolysis is the breakdown of proteins into smaller polypeptides or amino acids. Uncatalysed, the hydrolysis of peptide bonds is extremely slow, taking hundreds of years. Proteolysis is typically catalysed by cellular enzymes called proteases, but may also occur by intra-molecular digestion.
Proteolysis in organisms serves many purposes; for example, digestive enzymes break down proteins in food to provide amino acids for the organism, while proteolytic processing of a polypeptide chain after its synthesis may be necessary for the production of an active protein. 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. Consequently, abnormality in the regulation of proteolysis can cause disease.
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.
Biological functions[edit]
Post-translational proteolytic processing[edit]
Limited proteolysis of a polypeptide during or after translation in protein synthesis often occurs for many proteins. 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. The precursor to the final functional form of protein is termed proprotein, and these proproteins may be first synthesized as preproprotein. For example, albumin is first synthesized as preproalbumin and contains an uncleaved signal peptide. This forms the proalbumin after the signal peptide is cleaved, and a further processing to remove the N-terminal 6-residue propeptide yields the mature form of the protein.[1]
Autoproteolysis[edit]
Autoproteolysis takes place in some proteins, whereby the peptide bond is cleaved in a self-catalyzed intramolecular reaction. Unlike zymogens, these autoproteolytic proteins participate in a "single turnover" reaction and do not catalyze further reactions post-cleavage. 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.[19] In some cases, the autoproteolytic cleavage is promoted by conformational strain of the peptide bond.[19]
Proteolysis and diseases[edit]
Abnormal proteolytic activity is associated with many diseases.[20] In pancreatitis, leakage of proteases and their premature activation in the pancreas results in the self-digestion of the pancreas. People with diabetes mellitus may have increased lysosomal activity and the degradation of some proteins can increase significantly. 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 many age-related neurological diseases such as Alzheimer's due to generation and ineffective removal of peptides that aggregate in cells.[21]
Proteases may be regulated by antiproteases or protease inhibitors, and imbalance between proteases and antiproteases can result in diseases, for example, in the destruction of lung tissues in emphysema brought on by smoking tobacco. Smoking is thought to increase the neutrophils and macrophages in the lung which release excessive amount of proteolytic enzymes such as elastase, such that they can no longer be inhibited by serpins such as α1-antitrypsin, thereby resulting in the breaking down of connective tissues in the lung. Other proteases and their inhibitors may also be involved in this disease, for example matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs).[22]
Other diseases linked to aberrant proteolysis include muscular dystrophy, degenerative skin disorders, respiratory and gastrointestinal diseases, and malignancy.
Non-enzymatic processes[edit]
Protein backbones are very stable in water at neutral pH and room temperature, although the rate of hydrolysis of different peptide bonds can vary. The half life of a peptide bond under normal conditions can range from 7 years to 350 years, even higher for peptides protected by modified terminus or within the protein interior.[23][24][25] The rate of hydrolysis however can be significantly increased by extremes of pH and heat. Spontaneous cleavage of proteins may also involve catalysis by zinc on serine and threonine.[26]
Strong mineral acids can readily hydrolyse the peptide bonds in a protein (acid hydrolysis). The standard way to hydrolyze a protein or peptide into its constituent amino acids for analysis is to heat it to 105 °C for around 24 hours in 6M hydrochloric acid.[27] However, some proteins are resistant to acid hydrolysis. One well-known example is ribonuclease A, which can be purified by treating crude extracts with hot sulfuric acid so that other proteins become degraded while ribonuclease A is left intact.[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.[29] For example, cyanogen bromide cleaves the peptide bond after a methionine. Similar methods may be used to specifically cleave tryptophanyl, aspartyl, cysteinyl, and asparaginyl peptide bonds. Acids such as trifluoroacetic acid and formic acid may be used for cleavage.
Like other biomolecules, proteins can also be broken down by high heat alone. At 250 °C, the peptide bond may be easily hydrolyzed, with its half-life dropping to about a minute.[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: