10.5 Breakdown of polysaccharide: chitin
Chitin, in which the repeating unit is the same as that in cellulose except that the hydroxyl group at C-2 is replaced by an acetamido group (Fig. 2), is the second most abundant polymer on Earth as it occurs in the exoskeletons of arthropods and, of course, in fungal cell walls. Polysaccharides which contain amino sugars or their derivatives are called mucopolysaccharides. Chitin is degraded by chitinase, a glucan hydrolase which attacks the β1→4 glycosidic bonds, eventually producing the disaccharide chitobiose which is then converted to the monosaccharide N-acetylglucosamine by chitobiase (Seidl, 2008). Chitinase may also be involved in fungal wall synthesis (see Chapter 6).
Fig. 2. Structural formulae of chitin and its constituents. Chitin structure is similar to that of cellulose (Fig. 1), except that acetylamino groups replace the hydroxyl groups at carbon position 2 of the glucose molecule. Approximately 16% of naturally occurring chitin units are deacetylated. Modified from Moore, 1998.