88
Proteases are subdivided into two major groups based on the position of the peptide bond cleavage: exopeptidases and endopeptidases. The exopeptidases act only
near the ends of polypeptide chains. Exopeptidases cleave the peptide bond proximal to the amino or carboxy terminal of the substrate. Based on their site of action
at the N or C terminus, they are classified as aminopeptidases and carboxypeptidases, respectively. The former act at a free N terminus of the polypeptide chain and
liberate a single amino acid residue, a dipeptide or a tripeptide while the latter act at
C terminals of the polypeptide chain and liberate a single amino acid or a dipeptide.
Endopeptidases cleave peptide bonds distant from the termini (Barrett and
McDonald 1986). The peculiar characteristic of endopeptidases is their preferential
action at the peptide bonds in the inner regions of the polypeptide chain away from
the N and C termini.
Based on the pH optimal for their functioning, proteolytic enzymes can be classified as alkaline, neutral or acidic proteases.
3.4.3.1 Acid Proteases
Acid proteases are proteases which are active in the pH range of 2–6 (Rao et al.
1998) and are mainly fungal in origin (Aguilar et al. 2008). Common examples in
this subclass include aspartic proteases of the pepsin family. Some of the metalloprotease and cystein proteases are also categorised as acidic proteases.
3.4.3.2 Neutral Proteases
Neutral proteases are highly stable enzymes with broad substrate specificity. They
cleave peptide bonds of hydrophobic amino acids from the aminoterminal end. The
enzyme works best at neutral pH and is activated in the presence of calcium, magnesium and manganese ions. The main sources are plant, fungi and bacteria (Aguilar
et al. 2008).
3.4.3.3 Alkaline Proteases
Alkaline proteases are an enzymatic group that is exploited in the industrial field.
They can also be used to restrict some pests and pathogens. They show maximum
activity at a pH of 8–13. Halophilic bacteria as well as alkaline bacteria are dependable sources of alkaline proteases. The presence of this hydrolytic enzyme make
these organisms suitable for exploiting the biocontrol property. In most cases the
active site consists of a serine residue, though some alkaline proteases may have
other amino acid residue in their active site (Rao et al. 1998).
Four main groups of proteases are observed. They are: (1) serine proteases, (2)
aspartic proteases, (3) cysteine proteases and (4) metalloproteases. This division is
based on the mechanism of catalysis and the nature of the functional group present
at the active site.
3.4.3.4 Serine Proteases
Serine proteases are identified as biocontrol agents and isolated from both prokaryotes and eukayotes. Viruses also exhibit their presence. The antinematode activity
I.C. Nair and K. Jayachandran
Proteases are subdivided into two major groups based on the position of the peptide bond cleavage: exopeptidases and endopeptidases. The exopeptidases act only
near the ends of polypeptide chains. Exopeptidases cleave the peptide bond proximal to the amino or carboxy terminal of the substrate. Based on their site of action
at the N or C terminus, they are classified as aminopeptidases and carboxypeptidases, respectively. The former act at a free N terminus of the polypeptide chain and
liberate a single amino acid residue, a dipeptide or a tripeptide while the latter act at
C terminals of the polypeptide chain and liberate a single amino acid or a dipeptide.
Endopeptidases cleave peptide bonds distant from the termini (Barrett and
McDonald 1986). The peculiar characteristic of endopeptidases is their preferential
action at the peptide bonds in the inner regions of the polypeptide chain away from
the N and C termini.
Based on the pH optimal for their functioning, proteolytic enzymes can be classified as alkaline, neutral or acidic proteases.
3.4.3.1 Acid Proteases
Acid proteases are proteases which are active in the pH range of 2–6 (Rao et al.
1998) and are mainly fungal in origin (Aguilar et al. 2008). Common examples in
this subclass include aspartic proteases of the pepsin family. Some of the metalloprotease and cystein proteases are also categorised as acidic proteases.
3.4.3.2 Neutral Proteases
Neutral proteases are highly stable enzymes with broad substrate specificity. They
cleave peptide bonds of hydrophobic amino acids from the aminoterminal end. The
enzyme works best at neutral pH and is activated in the presence of calcium, magnesium and manganese ions. The main sources are plant, fungi and bacteria (Aguilar
et al. 2008).
3.4.3.3 Alkaline Proteases
Alkaline proteases are an enzymatic group that is exploited in the industrial field.
They can also be used to restrict some pests and pathogens. They show maximum
activity at a pH of 8–13. Halophilic bacteria as well as alkaline bacteria are dependable sources of alkaline proteases. The presence of this hydrolytic enzyme make
these organisms suitable for exploiting the biocontrol property. In most cases the
active site consists of a serine residue, though some alkaline proteases may have
other amino acid residue in their active site (Rao et al. 1998).
Four main groups of proteases are observed. They are: (1) serine proteases, (2)
aspartic proteases, (3) cysteine proteases and (4) metalloproteases. This division is
based on the mechanism of catalysis and the nature of the functional group present
at the active site.
3.4.3.4 Serine Proteases
Serine proteases are identified as biocontrol agents and isolated from both prokaryotes and eukayotes. Viruses also exhibit their presence. The antinematode activity
I.C. Nair and K. Jayachandran