Signatures of activation parameters reveal substrate-dependent rate determining steps in polysaccharide turnover by a family 18 chitinase

Glycoside hydrolases play an important role in the degradation of biomass such as cellulose and chitin. Many of these enzymes act by a processive mechanism, which is generally considered favorable because it improves substrate-accessiblity. Recently we showed that this only applies to insoluble substrates. Towards more soluble and accessible substrates, processivity may in fact reduce the catalytic activity. Here, we describe kinetic studies showing how the type of substrate, insoluble or soluble, affects the activation parameters and rate determining steps for catalysis by the processive two-domain chitinase A (ChiA) from Serratia marcescens. The activation parameters show a large entropic activation barrier, indicative of a bimolecular (associative) rate determining step, for the degradation of insoluble crystalline β-chitin. For the water-soluble polymeric chitin-derivative chitosan, the rate determining step is associated with product-displacement and release. Furthermore, the degree of processivity is reflected in the activation parameters for chitosan hydrolysis; increase in processivity results in increase in activation enthalpy change and decrease in activation entropy change.