Protease

Classification

Proteases are currently classified into six groups:

• Serine proteases
• Threonine proteases
• Cysteine proteases
• Aspartic acid proteases
• Metalloproteases
• Glutamic acid proteases

The threonine and glutamic acid proteases were not described until 1995 and 2004, respectively. The mechanism used to cleave a peptide bond involves making an amino acid residue that has the cysteine and threonine (peptidases) or a water molecule (aspartic acid, metallo- and glutamic acid peptidases) nucleophilic so that it can attack the peptide threonine as a nucleophile.

Occurrence

Proteases occur naturally in all organisms and constitute 1%-5% of the gene content. These enzymes are involved in a multitude of physiological reactions from simple digestion of food proteins to highly-regulated cascades (e.g., the amino acid sequence of a protein, or break down a complete peptide to amino acids (unlimited proteolysis). The activity can be a destructive change, abolishing a protein's function or digesting it to its principal components; it can be an activation of a function, or it can be a signal in a signaling pathway.

Proteases are also a type of exotoxin, which is a virulence factor in bacteria pathogenesis. Bacteria exotoxic proteases destroy extracellular structures. Protease enzymes are also found used extensively in the bread industry in Bread improver.

Proteases, also known as proteinases or proteolytic enzymes, are a large group of hydrolysis of various bonds with the participation of a water molecule.

Proteases are involved in digesting long protein chains into short fragments, splitting the carboxypeptidase A); the others attack internal peptide bonds of a protein (endopeptidases, such as trypsin, chymotrypsin, pepsin, papain, elastase).

Proteases are divided into four major groups according to the character of their catalytic active site and conditions of action: serine proteinases, cysteine (thiol) proteinases, aspartic proteinases, and metalloproteinases. Attachment of a protease to a certain group depends on the structure of catalytic site and the amino acid (as one of the constituents) essential for its activity.

Proteases are used throughout an organism for various metabolic processes. Acid proteases secreted into the stomach (such as elastase, cathepsin G) and play several different roles in metabolic control. Proteases determine the lifetime of other proteins playing important physiological role like hormones, antibodies, or other enzymes -- this is one of the fastest "switching on" and "switching off" regulatory mechanisms in the physiology of an organism. By complex cooperative action the proteases may proceed as cascade reactions, which result in rapid and efficient amplification of an organism's response to a physiological signal.

Inhibitors

The function of peptidases is inhibited by protease inhibitor enzymes. Examples of protease inhibitors are the class of coagulation, fibrinolysis) and the recently discovered neuroserpin.

Natural protease inhibitors include the family of protease inhibitors are developed as antiviral means.

Degradation

Proteases, being themselves proteins, are known to be cleaved by other protease molecules, sometimes of the same variety. This may be an important method of regulation of peptidase activity.

Protease research

The field of protease research is enormous. Barrett and Rawlings estimated that approximately 8000 papers related to this field are published each year. For a look at current activities and interests of protease researchers, see the International Proteolysis Society web page.

References

• Barrett A.J., Rawlings ND, Woessner JF. The Handbook of Proteolytic Enzymes, 2nd ed. Academic Press, 2003. ISBN 0-12-079610-4.
• Hedstrom L. Serine Protease Mechanism and Specificity. Chem Rev 2002;102:4501-4523.
• Southan C. A genomic perspective on human proteases as drug targets. Drug Discov Today 2001;6:681-688.
• Hooper NM. Proteases in Biology and Medicine. London: Portland Press, 2002. ISBN 1-85578-147-6.
• Puente XS, Sanchez LM, Overall CM, Lopez-Otin C. Human and Mouse Proteases: a Comparative Genomic Approach. Nat Rev Genet 2003;4:544-558.
• Ross J, Jiang H, Kanost MR, Wang Y. Serine proteases and their homologs in the Drosophila melanogaster genome: an initial analysis of sequence conservation and phylogenetic relationships. Gene 2003;304:117-31.
• Puente XS, Lopez-Otin C. A Genomic Analysis of Rat Proteases and Protease Inhibitors. Genome Biol 2004;14:609-622.