What are the classifications of proteases?

Proteases are classified into six categories: aspartic, cysteine, serine, threonine, metalloproteases and glutamic proteases. 

1. Aspartic proteases use two highly conserved aspartic acid residues in the active site for catalytic cleavage of their peptide substrates and are optimally active at acidic pH. They use an activated water molecule bound to one or more aspartate residues for catalysis of their substrate. Almost all known aspartic proteases are inhibited by pepstatin. These proteases do not form a covalent intermediate during cleavage. 
2. Cysteine proteins are enzymes that degrade proteins. These enzymes possess a common catalytic mechanism which involves a nucleophilic cysteine thiol in a catalytic dyad or triad manner assembly. 
3. Serine proteases are enzymes which cleave peptide bonds in proteins. Serine acts as the nucleophilic amino acid at the active site. 
4. Threonine proteases are a family of proteolytic enzymes containing a threonine residue within its active site. These residues are crucial active sites in multiple catalytic subunits of the proteasome. Threonine proteases are activated by primary amines. 
5. Metalloproteases are a family of proteases which contain a metal ion at their active site which acts as a catalyst in the hydrolysis of peptide bonds. The metallic core of each enzyme is the location of the specific reaction carried out by the enzyme. The most commonly used metal ion in metalloproteases is a zinc ion. 
6. Glutamic proteases are a class of proteolytic enzymes including glutamic acid residue in their active site. These enzymes function well at a pH of 2.0.