60
3 Chitin
1990). Chitin degradation at physiological condition is predominantly carried out by
the enzymes within the physiologic environment. Chitin degradation under standard
room conditions is considered for products that are used under such conditions. The
degradation mechanism and rate depend on the condition of storage. Its degradation in soil has been observed to be predominant by bacteria; however, some fungi
also take part in chitin degradation. In the aquatic environment where degradation
of shells and exoskeleton of dead or worn-out aquatic chitinous aquatic organisms
occurs, bacteria have been shown to be the main degraders.
Diatoms, microalgae which have quite a sizeable presence in the oceans and
other aquatic environment, are also known to have the ability to hydrolyze chitin
oligomers (Vrba et al. 1997). Chitinous aquatic organisms and other non-aquatic
arthropods occasionally shed their outer shell in order to allow for growth such that
the old shell is removed and a new larger one is formed to better accommodate and
allow for increase in size of the organism. This process is referred to as molting,
and it is thought that the organism releases chitin degradation enzymes during this
process (Vrba and Mackacek 1994). It is however yet to be determined, the exact role
of these enzymes as they could be either involved in reactive breakdown of chitin or
used to hydrolyze dissolved chitin oligomers.
Although here we have considered the degradation of chitin in nature, degradation
of chitin when it comes to products which have been made from chitin such as
scaffolds, packaging films and water treatment membranes, the environment and
conditions within which the degradation is occurring varies significantly. Studies
which look at the development of such products have also considered the degradation
mechanisms of each of these products.
The process of degradation of chitin is referred to as chitinoclastic. The degradation process could be chitinolytic, and this refers to the breaking of the (1 →
4)-β-glycosidic bonds. The deacetylation of chitin to chitosan is also a form of chitin
degradation as it involves the breaking of the acetyl group. The main difference
between deacetylated chitin and glucose is the presence of an amine group in the
place of one of the hydroxyl groups on the glucose ring. Deaminization is also another
breakdown process where the deacetylated chitin is then converted to cellulose by the
removal of the amine functional group. The breakdown of the products of the initial
degradation of chitin (chitosan and glucose) then follows their respective degradation
pathways to smaller units of glucose, glucosamine and N-acetylglucosamine which
can then be returned to the carbon and nitrogen cycle. Enzymes involved in these
processes are chitinases, chitosanase and to a lesser extent, cellulases. Lysozyme, an
enzyme involved in bacteria cell death as an immune response in animals, has also
been shown to degrade chitin (Beier and Bertilsson 2014).
Chitin degradation by microorganisms occurs for the purpose of breaking down
the chitin for their metabolism or for the growth of the organism as seen during the
molting of arthropods. In the case of lysozyme, it could also occur as an immune
response or defense mechanism. The degradation pathway of chitin is illustrated in
the chart in Fig. 3.5.
The pathway of degradation of chitin depends on the mix of microbes within the
habitat. These have been shown to have more effect than temperature. End product of
3 Chitin
1990). Chitin degradation at physiological condition is predominantly carried out by
the enzymes within the physiologic environment. Chitin degradation under standard
room conditions is considered for products that are used under such conditions. The
degradation mechanism and rate depend on the condition of storage. Its degradation in soil has been observed to be predominant by bacteria; however, some fungi
also take part in chitin degradation. In the aquatic environment where degradation
of shells and exoskeleton of dead or worn-out aquatic chitinous aquatic organisms
occurs, bacteria have been shown to be the main degraders.
Diatoms, microalgae which have quite a sizeable presence in the oceans and
other aquatic environment, are also known to have the ability to hydrolyze chitin
oligomers (Vrba et al. 1997). Chitinous aquatic organisms and other non-aquatic
arthropods occasionally shed their outer shell in order to allow for growth such that
the old shell is removed and a new larger one is formed to better accommodate and
allow for increase in size of the organism. This process is referred to as molting,
and it is thought that the organism releases chitin degradation enzymes during this
process (Vrba and Mackacek 1994). It is however yet to be determined, the exact role
of these enzymes as they could be either involved in reactive breakdown of chitin or
used to hydrolyze dissolved chitin oligomers.
Although here we have considered the degradation of chitin in nature, degradation
of chitin when it comes to products which have been made from chitin such as
scaffolds, packaging films and water treatment membranes, the environment and
conditions within which the degradation is occurring varies significantly. Studies
which look at the development of such products have also considered the degradation
mechanisms of each of these products.
The process of degradation of chitin is referred to as chitinoclastic. The degradation process could be chitinolytic, and this refers to the breaking of the (1 →
4)-β-glycosidic bonds. The deacetylation of chitin to chitosan is also a form of chitin
degradation as it involves the breaking of the acetyl group. The main difference
between deacetylated chitin and glucose is the presence of an amine group in the
place of one of the hydroxyl groups on the glucose ring. Deaminization is also another
breakdown process where the deacetylated chitin is then converted to cellulose by the
removal of the amine functional group. The breakdown of the products of the initial
degradation of chitin (chitosan and glucose) then follows their respective degradation
pathways to smaller units of glucose, glucosamine and N-acetylglucosamine which
can then be returned to the carbon and nitrogen cycle. Enzymes involved in these
processes are chitinases, chitosanase and to a lesser extent, cellulases. Lysozyme, an
enzyme involved in bacteria cell death as an immune response in animals, has also
been shown to degrade chitin (Beier and Bertilsson 2014).
Chitin degradation by microorganisms occurs for the purpose of breaking down
the chitin for their metabolism or for the growth of the organism as seen during the
molting of arthropods. In the case of lysozyme, it could also occur as an immune
response or defense mechanism. The degradation pathway of chitin is illustrated in
the chart in Fig. 3.5.
The pathway of degradation of chitin depends on the mix of microbes within the
habitat. These have been shown to have more effect than temperature. End product of