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3.4.1 Chitinases
Being composed of β-1,4-linked N-acetylglucosamine units, chitin is a modified
polysaccharide abundantly present in fungal cell walls. The β-1,4 linkage is broken
by the enzyme (Poly {1, 4-N-acetyl D-glucosaminide} glucano hydrolase) commonly designated as chitinase. The cleavage site of chitinase is the bond between C
and C of two consecutive N-acetylglucosamine monomers (Flach et al. 1992). The
chitinase enzyme has the potential to degrade the chitin-containing cell wall of
pathogens. The exoskeleton of insect pests is also affected by chitinases. Thus in
controlling the insect and fungal pathogens chitinases are powerful biological tools.
As enzymes are not persistent, the pathogens fail in developing resistance against
them.
Chitinases form a diverse group of enzymes and are divided majorly into two
classes based on the mode of action. The International Union of Biochemistry and
Molecular Biology recommends them as exochitinases and endochitinases.
Endochitinases (EC 3.2.1.14) cleave randomly at internal sites, generating lowmolecular mass multimers of N-acetylglucosamine (NAG) such as chitotriose and
chitotetraose. Exochitinases are further divided into two subcategories, β-(1,4)
N-acetyl hexosaminidases (EC 3.2.1.52) and chitobiosidases (EC 3.2.1.30).
Chitobiosidases catalyse the release of diacetyl chitobiose from the nonreducing
end of chitin monofibril. The oligomers obtained by endochitinases and chitobiosidases are cleaved by β-(1,4) N-acetyl glucosaminidases to monomers of N-acetyl
glucosamine (Dahiya et al. 2006). Some chitinases also show lysozyme activity and
in the cell wall of bacteria they can lyse the β-(1,4) linkage of the peptidoglycan
layer (Gokul et al. 2000).
Henrissat and Bairoch (1993) report three families of chitin-degrading enzymes
which have related sequences of amino acids and they are the glycosyl hydrolase
families of 18, 19 and 20. In a family, the members show differences in amino acid
sequences, three-dimensional structure, domain fold, reaction mechanisms and
specificity of substrate. In family 18, which is multifaceted, animal, plant, bacterial,
fungal and viral chitinases are represented. Family 19 consists of chitinases from
Streptomyces and some members of plant chitinases. Certain chitinases of human
origin and streptomyces origin belong to family 20. The reactions at the active site
of chitinases of family 18 suggested a substrate-assisted catalysis mode (Tews et al.
1997). Eight α-helices and eight β-strands are oriented to form the active site of this
family. This provides a (β/α) barrel appearance (Perrakis et al. 1994). Family 19
chitinases exhibit an acid base mechanism during catalysis (Hart et al. 1995) and
their active site domains are composed of α- helica structures (Hart et al. 1995;
Monzingo et al. 1996).
3 Enzymes for Bioremediation and Biocontrol
3.4.1 Chitinases
Being composed of β-1,4-linked N-acetylglucosamine units, chitin is a modified
polysaccharide abundantly present in fungal cell walls. The β-1,4 linkage is broken
by the enzyme (Poly {1, 4-N-acetyl D-glucosaminide} glucano hydrolase) commonly designated as chitinase. The cleavage site of chitinase is the bond between C
and C of two consecutive N-acetylglucosamine monomers (Flach et al. 1992). The
chitinase enzyme has the potential to degrade the chitin-containing cell wall of
pathogens. The exoskeleton of insect pests is also affected by chitinases. Thus in
controlling the insect and fungal pathogens chitinases are powerful biological tools.
As enzymes are not persistent, the pathogens fail in developing resistance against
them.
Chitinases form a diverse group of enzymes and are divided majorly into two
classes based on the mode of action. The International Union of Biochemistry and
Molecular Biology recommends them as exochitinases and endochitinases.
Endochitinases (EC 3.2.1.14) cleave randomly at internal sites, generating lowmolecular mass multimers of N-acetylglucosamine (NAG) such as chitotriose and
chitotetraose. Exochitinases are further divided into two subcategories, β-(1,4)
N-acetyl hexosaminidases (EC 3.2.1.52) and chitobiosidases (EC 3.2.1.30).
Chitobiosidases catalyse the release of diacetyl chitobiose from the nonreducing
end of chitin monofibril. The oligomers obtained by endochitinases and chitobiosidases are cleaved by β-(1,4) N-acetyl glucosaminidases to monomers of N-acetyl
glucosamine (Dahiya et al. 2006). Some chitinases also show lysozyme activity and
in the cell wall of bacteria they can lyse the β-(1,4) linkage of the peptidoglycan
layer (Gokul et al. 2000).
Henrissat and Bairoch (1993) report three families of chitin-degrading enzymes
which have related sequences of amino acids and they are the glycosyl hydrolase
families of 18, 19 and 20. In a family, the members show differences in amino acid
sequences, three-dimensional structure, domain fold, reaction mechanisms and
specificity of substrate. In family 18, which is multifaceted, animal, plant, bacterial,
fungal and viral chitinases are represented. Family 19 consists of chitinases from
Streptomyces and some members of plant chitinases. Certain chitinases of human
origin and streptomyces origin belong to family 20. The reactions at the active site
of chitinases of family 18 suggested a substrate-assisted catalysis mode (Tews et al.
1997). Eight α-helices and eight β-strands are oriented to form the active site of this
family. This provides a (β/α) barrel appearance (Perrakis et al. 1994). Family 19
chitinases exhibit an acid base mechanism during catalysis (Hart et al. 1995) and
their active site domains are composed of α- helica structures (Hart et al. 1995;
Monzingo et al. 1996).
3 Enzymes for Bioremediation and Biocontrol