Cellulose

Chitin

       


      Cellulose and chitin are the most abundant biopolymers in the terrestrial and marine environments, respectively. Cellulose is a linear b-(1–4)-linked polymer of D-gluco-pyranose units in the 4C1 conformation and chitin is a cellulose derivative where the 2-hydroxy group has been substituted with an acetamido group. Different polymorphic forms of cellulose have been described where the glucan chains are believed to be packed in a parallel fashion in cellulose I (i.e. with the reducing ends pointing in the same direction) and in anti-parallel fashion in cellulose II. It is still not understood how it is possible to convert parallel cellulose chains into an anti-parallel packing in processes that are essentially solid-state transformations. Interestingly, chitin also exists in two main crystalline forms, where a-chitin is similar to cellulose II and bchitin has a parallel chain arrangement comparable to that of cellulose I. From crystal structure data for cellulose and chitin and the unit cell dimensions it is possible to calculate packing densities, which are estimated at 1.62 g/cm3 for cellulose and a significantly lower 1.46 g/cm3 for chitin. Little is known about the effect of this difference on the enzymatic degradation of insoluble cellulose or chitin. Other substrate parameters, such as the degree of crystallinity and the degree of heterogeneity of the samples, might be equally important. The insolubility of cellulose and chitin in water is a major obstacle for in-depth studies of enzymatic degradation, especially when complex issues such as enzyme processivity are addressed (see Box 2). Therefore, several chemically modified soluble cellulose and chitin forms have been developed. Soluble cellulose derivatives are obtained by modification of free hydroxyl groups. The most commonly used soluble cellulose form in research is carboxymethylcellulose, in which on average 0.5–0.9 hydroxyl groups per sugar (primarily O-2) have been modified. These relatively large extra groups pose steric limitations to the enzyme–substrate interaction that in turn might affect processivity. Chitosans, a family of well-characterized water-soluble chitin derivatives, are derived from chitins by removing varying fractions of the N-acetyl groups, which results in less bulky amino groups on the polymer. These polymers are very valuable substrates for in-depth studies of processivity in family 18 chitinases. [1]

[1] Vincent G.H. Eijsink, Gustav Vaaje-Kolstad, Kjell M. Varum and Svein J. Horn Towards new enzymes for biofuels: lessons from chitinase research, Trends in Biotechnology (2008) 26 (5) 228-235.