Chitin is the most widespread amino polysaccharide in nature. The annual global amount
of chitin is believed to be only one order of magnitude less than that of cellulose.
It is a linear polymer composed of N-acetylglucosamines that are joined in a reaction
catalyzed by the membrane-integral enzyme chitin synthase, a member of the family
2 of glycosyltransferases. The polymerization requires UDP-N-acetylglucosamines as
a substrate and divalent cations as co-factors. Chitin formation can be divided into
three distinct steps. In the first step, the enzymes' catalytic domain facing the
cytoplasmic site forms the polymer. The second step involves the translocation of
the nascent polymer across the membrane and its release into the extracellular space.
The third step completes the process as single polymers spontaneously assemble to
form crystalline microfibrils. In subsequent reactions the microfibrils combine with
other sugars, proteins, glycoproteins and proteoglycans to form fungal septa and cell
walls as well as arthropod cuticles and peritrophic matrices, notably in crustaceans
and insects. In spite of the good effort by a hardy few, our present knowledge of
the structure, topology and catalytic mechanism of chitin synthases is rather limited.
Gaps remain in understanding chitin synthase biosynthesis, enzyme trafficking, regulation
of enzyme activity, translocation of chitin chains across cell membranes, fibrillogenesis
and the interaction of microfibrils with other components of the extracellular matrix.
However, cumulating genomic data on chitin synthase genes and new experimental approaches
allow increasingly clearer views of chitin synthase function and its regulation, and
consequently chitin biosynthesis. In the present review, I will summarize recent advances
in elucidating the structure, regulation and function of insect chitin synthases as
they relate to what is known about fungal chitin synthases and other glycosyltransferases.