Using chitosan to understand chitinases and the role of processivity in the degradation of recalcitrant polysaccharides

Enzymatic depolymerization of abundant polysaccharides such as chitin and cellulose is hampered by the recalcitrant, crystalline nature of these materials. Nature musters a large variety of hydrolytic and oxidative enzymes with varying properties that, together, manage the task of saccharifying chitin and cellulose. Processivity, i.e. the ability to carry out multiple hydrolytic reactions while sliding along the polysaccharide chain, is considered a key property of hydrolytic enzymes acting on the most recalcitrant parts of the substrate. Due to the insoluble nature of the substrate, this phenomenon is difficult to study. For family GH18 chitinases, the combination of a catalytic mechanism depending on acetyl groups in the substrate and the possibility to produce partially deacetylated soluble single chitin chains (chitosan) provides a unique tool for studying processivity. Here, we review how the use of well-defined chitosans has helped unraveling crucial and otherwise difficult to study properties of multiple chitinases, including well studied chitinases from Serratia marcescens and human chitotriosidase (HCHT). These studies have yielded pioneering insights into the structural basis and functional implications of processivity that apply to both chitinases and cellulases. Recent studies of processive chitinases and cellulases confirm the insights originally derived from the work with chitosan and take this further, for example by providing kinetic and thermodynamic data for processive enzyme action.