Chitin - The Sugar Code - page 222

12.3.1
Fungal Cell Walls

The cell walls of fungi consist mainly of polysaccharides (glucans, mannans,
chitin) that account for more than 90% of the cell wall and only minor amounts
of glycoproteins [10]. The common skeletal core structure is composed of branched

β 1,3 - and β 1,6-glucans to which chitin is linked via β 1,4 - linkages. In addition,

amorphous polysaccharides ( 1,3-glucans and mannans) are present, which

vary between the different species as much as the chitin content. While in Sac-
charomyces cerevisiae chitin accounts for only 1-2% of the cell wall's dry weight,
the cell walls of Aspergillus fumigatus or Neurospora crassa contain up to 20% chitin.
Chitin is predominantly deposited close to the plasma membrane, whereas the

β 1,6 - , α

β 1,3-glucans extend throughout the cell wall. Most of the proteins found in the

cell wall are in transit towards the extracellular milieu and only part of them are
real cell wall proteins. The latter proteins are frequently linked to the plasma
membrane via glycosylphosphatidylinositol anchors (see Chapter 9), but are not
considered to be structural components. Rather, they appear to be involved in
remodeling or become covalently bound to polysaccharides in order to fulfi ll their
biological functions at the cell wall' s surface.

12.3.2
Arthropod Cuticles and Shells

The major components of arthropod cuticles are chitin, which is predominately
in the

-form, a variety of different cuticle proteins, which signifi cantly determine
the physical properties (soft or hard cuticles), and calcium salts (mainly calcium
carbonate present as calcite), which particularly harden crustacean shells. The
relative amounts of chitin and protein vary considerably between the different
arthropods. The shells from crabs and shrimps, for instance, consist of 30- 40%
protein, 20-30% chitin, 30-50% calcium carbonate, and less than 1% lipids and
astaxanthin, the red pigment of crustaceans (values are given on the basis of dry
weight, [11]). Soft insect cuticles with low stiffness (1 kPa to 1 MPa) commonly
contain chitin and protein in equal amounts, and 40-75% water. Hard cuticles (up
to 60 MPa) contain higher amounts of protein, 15-30% chitin and only about 10%
water [12]. Cuticles are initially formed as rather soft structures, but become rigid
after sclerotization, a process that leads to the nonenzymatic cross- linking of
cuticle proteins by ortho -chinones, which are generated from N - acylated catechol-
amines in an enzymatic reaction catalyzed by diphenoloxidases. Chitin is present
in the procuticle, but not in the epicuticle, which instead contains sclerotized
proteins impregnated with lipids, lipoproteins, cements and waxes. In the procu-
ticle chitin is deposited in about 3- nm thick microfi brils of indefi nite length, each
containing about 20 single sugar chains. Parallel- aligned microfi brils are embed-
ded in a proteinous matrix forming single layers. The procuticle is built of many
of these layers, which are horizontally arranged. Frequently, the layers are slightly
rotated against each other resulting in typical helicoidal textures (Figure 12.3 a)

α