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Proteases (Proteinases, Peptidases or Proteolytic enzymes) are enzymes that break peptide bonds between amino acids of proteins. The process is called proteolytic cleavage, a common mechanism of activation or inactivation of enzymes especially involved in blood coagulation or digestion. They use a molecule of water for this and are thus classified as hydrolases.

There are currently six classes of proteases:
  • Serine proteases
  • Threonine proteases
  • Cysteine proteases
  • Aspartic acid proteases (e. g. plasmepsin)
  • Metalloproteases
  • Glutamic acid proteases

The threonine and glutamic proteases were not described until 1995 and 2004, respectively. The mechanism used to cleave a peptide bond involves making an amino acid residue (serine, cystein and threonine peptidases) or a water molecule (aspartic, metallo and glutamic peptidases) nucleophilic so that it can attack the peptide carbonyl group. One way to make a nucleophile is by a catalytic triad, where a histidine residue is used to activate serine, cysteine or threonine as a nucleophile.

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 blood clotting cascade, the complement system, apoptosis pathways, and the invertebrate prophenoloxidase activating cascade). Peptidases can break either specific peptide bonds (limited proteolysis), depending on 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 signalling pathway.
Protease split peptide bonds with water ,they attack proteins via two modes,yielding different end products.Therefore ,your choice of enzyme may be dictated by what you require as hydrolysis product.

On there mode of attack proteases are classified into two:

  1. Exoprotease:
    These are proteases which can cleave off single amino acids from either end of the protein chain.Eventually under right conditions a protein can be reduced down to a single amino acids.

  2. Endoproteases:
    These are proteases which attack peptide bonds on the interior of the protein chain.The hydrolysis products are usually smaller polypeptides and peptides. Therefore most endoproteins will not produce a great deal of free amino acids as end products.

There are many different protease, each with a different specificity toward protein substrates.

  • Protease is a commercially important enzyme, it has a wide industrial application some of which are
  • The greatest industrial use of protease is for laundry detergents where they help to remove protein based stains (such as    blood and egg) from clothing.
  • The second largest use of protease is for cheese making. Enzymes from calf stomach and microbial sources are used to clot    milk-one of the first steps in cheesemaking.
  • Proteases are also used for bating(softening) leather, modifying food ingredients(e.g. soy protein whipping agents),meat    tenderizers, and flavor development.
  • Proteases have also been studied for their role in blood clotting and inflammatory diseases.

Proteases have been classified on different criteria it can be on the bases of their sources, biochemical activity, and pH. On basis of pH Proteases are classified as:

  • Acid Protease: These are proteases that show maximum activity at acidic PH.
  • Microbial Rennets: Higly specific endoproteases used to clot milk in cheese making.
    Source: Mucor miehei.M. pusillus, Endothia parasitica
  • Renin: Highly specific endoprotease used to clot milk in cheesemaking
    Source: Calf stomach
  • Pepsin: An endoprotease that will hydrolyze a broad range of synthetic peptides. Commercial products have exoprotease and esterase side activities.Pepsin prefers to cleave bonds near phenylalanine and leucine residues.
    Source: Swine and bovine stomach
  • Fungal Acid Proteases: Hydrolyzes a wide range of peptide bonds. Preparations usually exhibit endo- and exo- protease    activity.
    Source: Molds- Aspergillus, Rhizopus
  • Neutral Protease: These are proteases that show maximum activity at Neutral pH.
  • Trypsin: Highly specific endoprotease which prefers to cleave bonds next to arginine and lysine residues.
    Sources:Bovine and swine pancrease glands
  • Papain: Endoprotease that hydrolyzes a wide range of peptide bonds. It is a sulfhydryl protease requiring mild reducing    agents.
    Souces: Papaya latex
  • Bromelain/Ficin: Endoprotease that hydrolyze a wide range of peptide bonds and are similar to papain. Usually sold as a    mixture of proteases.
    Source: Pineapple and fig respectively
  • Bacterial Neutral Protease: An endoprotease that hydrolyses a wide range of peptide bonds.
    Source: Bacillus subtilis
  • Alkaline Protease: These are proteases that show maximum activity at Alkaline pH.
  • Subtilisin (and related protease): An endoprotease that hydrolyze a wide range of peptide bond preferences.
    Source: Bacillus subtilis and others in this gene.