Monday, April 30, 2007 - 8:00 AM

On the roles of accessory proteins and processivity in enzymatic degradation of chitin

Svein J. Horn1, Gustav Vaaje-Kolstad1, Pawel Sikorski2, Morten Sørlie1, Kjell M. Vårum3, and Vincent G.H. Eijsink1. (1) Department of Chemistry, Biotechnology and Food Science, Norwegian University of Life Sciences, P.O. Box 5003, Aas, N-1432, Norway, (2) Department of Physics, Norwegian University of Science and Technology, Trondheim, N-7491, Norway, (3) Department of Biotechnology, Norwegian University of Science and Technology, Trondheim, N-7491, Norway


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.


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