Chemistry Reference
In-Depth Information
gel-like protein/proteoglycan matrix, containing covalently bound proteins to a
minor extent, tightly associated proteins (so-called peritrophins) and weakly associ-
ated proteins. In contrast to cuticles, peritrophic matrices are frequently organized
in random felt-like or less highly-ordered structures (Figure 12.3c and d). Interest-
ingly, some beetles excrete peritrophic matrices to form cocoons. In contrast to
cocoons produced by glands such as silk cocoons, these cocoons are chitinous
structures [3] .
12.3.4
Other Functions
Next to chitin's well-documented structural function, it appears to act as a kind of
alarm molecule signaling the invasion by pathogens and triggering immune
responses in those organisms that do not produce chitin themselves like plants or
mammals. For this purpose chitin-binding proteins are expressed, including
lectins and chitinases, which are part of the natural defense against pathogenic
fungi or parasites. For instance, wheat germ lectin is toxic to a variety of insect
pests and the small chitin-binding protein hevein from the latex of the rubber tree
has a signifi cant antifungal activity [15, 16] (for further information on lectins
binding chitin and the hevein-like domain, please see Chapters 15 , 18 and 19 ).
Moreover, many plants synthesize chitinases predominately to protect themselves
from fungal growth. However, plant chitinases serve also physiological functions
and are involved in symbiotic interactions [17]. As chitin does not exist in mammals,
it had been assumed that chitinolytic enzymes are also restricted to lower life
forms. Hence, it came as a big surprise that chitinases and chitinase-like proteins
are even expressed in humans, where they appear to contribute to host antiparasite
responses and asthmatic T helper type 2 infl ammation (please see also Chapter
19 ).
12.4
Metabolism
Chitin synthesis is a sequence of metabolic reactions that requires different
enzyme activities. Certainly, the key enzyme in this process is the chitin synthase
(EC 2.4.1.16), a membrane-integral protein, which has been grouped into family
II of processive, polymerizing glycosyltransferases - a family that includes closely
related enzymes like hyaluronan synthases and cellulose synthases [18]. The chitin
synthase utilizes UDP-GlcNAc as the activated sugar donor, which is synthesized
in a sequence of enzymatic reactions following a variant of the Leloir pathway (see
Info Box 2). The chitin synthase transfers the sugar moiety of UDP-GlcNAc to the
nonreducing end of the growing polymer. Due to the lack of structural data on
chitin synthases the precise catalytic mechanism is not known to date. In general,
catalysis of glycosyltransferases occurs with two possible outcomes - retention and
inversion of the anomeric confi guration of the donor sugar (Figure 12.4). From a
