Chemistry Reference
In-Depth Information
Info Box 2
The general route for incorporating sugar units into polysaccharides starts with
their conversion into activated sugar nucleotides. In the case of chitin the acti-
vated sugar nucleotide is UDP-GlcNAc, the sugar portion of which is trans-
ferred to the nonreducing end of the growing polymer in the transglycosylation
step catalyzed by the enzyme chitin synthase (EC 2.4.1.16). The biosynthetic
steps to produce UDP-GlcNAc follow a variant of the Leloir pathway, which
was named after Louis Federico Leloir (1906-1987), who received the 1970
Nobel Prize in Chemistry. One of Leloir's major discoveries was the fi nding
that the biosynthesis of basically all glycosylated molecules including glycopro-
teins, glycolipids and polysaccharides requires an activated sugar nucleotide as
donor for the transglycosylation reaction. In eukaryotes UDP- GlcNAc is syn-
thesized from Fru-6-P in a sequence of four enzymatic reactions: (i) conversion
of Fru - 6 - P into GlcN - 6 - P catalyzed by GlcN - 6 - P synthase (EC 2.6.1.16), which
transfers the ammonia from the cosubstrate
L
- glutamine to Fru - 6 - P and isom-
erizes the resulting fructosi mine - 6 - phosphate to GlcN - 6 - P; (ii) transfer of an
acetyl group from coenzyme A by GlcN-6-P acetyltransferase (EC 2.3.1.4) to
obtain GlcNAc- 6 - P; (iii) isomerization of GlcNAc - 6 - P to GlcNAc - 1 - P catalyzed
by phoshoacetylglucosamine mutase (EC 5.4.2.3) which is autophosphorylated
and dephosphorylated during the reaction cycle in an ping-pong mechanism;
(iv) uridylation of GlcNAc- 1 - P by UDP -GlcNAc pyrophosphorylase (EC 2.7.7.23)
resulting in the end product UDP-GlcNAc, which inhibits the fi rst step of the
pathway in a negative-feedback loop in eukaryotes.
stereochemical point of view chitin synthesis follows an inverting reaction mecha-
nism, in which the nucleophilic attack by the acceptor hydroxyl group leads to an
inversion of the anomeric carbon of the donor substrate [19]. The underlying cata-
lytic mechanism for inverting glycosyltransferases was deduced from crystal struc-
tures of bacterial enzymes. The reaction cycle involves an oxocarbenium ion-like
transition state and requires a catalytic base, which deprotonates the incoming
nucleophilic acceptor facilitating S
N
2 displacement of the nucleoside diphosphate
(Figure 12.4c). Frequently, a divalent metal ion acts as a Lewis acid catalyst in the
reaction cycle by stabilization of the leaving nucleoside diphosphate. Next to their
catalytic function the chitin synthase may exhibit also a transport function. As the
catalytic site of the chitin synthase faces the cytoplasm, the growing polymer has
to be translocated across the membrane to reach the extracellular space where it
is deposited. It is likely that the extended transmembrane regions typically found
in family II glycosyltransferases are involved in this transport process [22].
Chitin is rapidly degraded in Nature by three different types of chitinolytic
enzymes: (i) endochitinases splitting chitin into oligosaccharides of different chain
length, (ii) exochitinases splitting oligosaccharides into diacetylchitobiose and (iii)
chitobiases splitting diacetylchitobiose into GlcNAc monomers. Chitinases do not
