Chemistry Reference
In-Depth Information
Table 12.1
Functions of chitinous structures in various chitin- synthesizing organisms.
Organism
Localization
Function
Bacteria (Rhizobiacea)
Secreted, extracellular space
Nodulation of leguminous plant roots
Protozoa
Cyst
Physical and chemical resistance
Fungi
Cell wall
Compensation of the turgor pressure
Spore wall
Physical and chemical resistance
Septa
Stabilization of cell division zones
Nematodes
Pharynx
Mechanical breakdown of food
Egg shell
Physical and chemical resistance
Gut peritrophic matrix
Multiple protective functions
Arthropods
Epidermal cuticles, shells
Cuticle differentiation, skeletal functions
Tracheal cuticle
Tracheal development, skeletal functions
Gut peritrophic matrix
Multiple protective functions
Egg shells
Physical and chemical resistance, oogenesis
Bivalves
Shell
Shell and nacre formation
Gastropods
Snail shell, radula
Shell formation, grazing
Gut peritrophic matrix
Multiple protective functions
Cephalopods
Squid pen, cuttlefi sh bone
Endoskeleton
Stomach cuticle
Protective functions
Tunicates
Integument (test)
Reinforcement
Gut peritrophic matrix
Multiple protective functions
Vertebrates (bony fi sh)
Fin cuticle
Reinforcement
phylogenetic point of view it is remarkable that some bacteria belonging to the
Rhizobium
/
Agrobacterium
group possess the biosynthetic machinery to produce
chito-oligosaccharides, which are involved in the nodulation of leguminous plant
roots and thus have a morphogenic function (for further information on nodula-
tion factors and their sensing, please see Chapter 18). Since the NodC enzyme,
which is responsible for chito-oligosaccharide synthesis, is related to chitin-syn-
thesizing enzymes of eukaryotes, it is tempting to speculate that all chitin- synthe-
sizing systems are phylogenetically related and hence may originate from a
common ancestral progenitor. However, the sequence similarities are comparably
low and therefore may be the result of convergent evolution as well.
12.2
Structure
Chitin is regarded the functional equivalent of cellulose, as both are long,
unbranched sugar polymers used to support extracellular structures. Actually, they
are pretty similar in their basic structures: the monomeric sugar residues are
linked by
1,4-glycosidic bonds and rotated with respect to each other by 180°
(Figure 12.1). The proper repetitive units in cellulose and chitin are therefore
β
