Serratia marcescens produces three family 18 chitinases and a chitin-binding protein, CBP21, that convert the recalcitrant insoluble polysaccharide chitin to chitobiose. Using the unique experimental possibilities provided by the soluble chitin-derivative chitosan (partially deacetylated chitin), we have studied the properties of the three chitinases. ChiC is a non-processive endo-acting enzyme, whereas ChiA and ChiB act in a processive fashion after initial endo-binding, presumably in opposite directions [1,2]. Systematic mutations of aromatic residues lining sugar-binding sites close to the catalytic centre of ChiB revealed that some of these residues are essential for processivity [3]. ChiB variants displaying reduced processivity were less effective in degrading solid chitin, presumably because detached single polymer chains are no longer kept from re-associating with the solid material in between catalytic events. Most remarkably, these same mutants showed a large increase in the degradation rate for non-solid substrates, such as the single soluble polymer chains of chitosan [3]. Thus, processivity comes at a cost in terms of enzyme speed. Recently, we discovered that CBP21 adds to chitin degradation by increasing the accessibility of the substrate for chitinases [4]. Helper proteins such as CBP21 are a potentially valuable tool for biomass turnover and may provide an alternative for processivity as a mechanism for improving substrate disruption and accessibility.

[1] Horn SJ et al., 2006, FEBS J. 273:491-503.

[2] Sikorski P et al., 2006, Biochemistry 45:9566-9574.

[3] Horn SJ et al., 2006, Proc. Natl. Acad. Sci. USA 103:18089-18094.

[4] Vaaje-Kolstad G et al., 2005, J. Biol. Chem. 280:28492-28497.