CHARACTERIZATION AND AFFINITY APPLICATIONS OF CELLULOSE-BINDING DOMAINS

Cellulose-binding domains (CBDs) are discrete protein modules found in a large number of carbohydrolases and a few nonhydrolytic proteins. T o date, almost 200 sequences can be classified in 13 different familie s with distinctly different properties. CBDs vary in size from 4 to 20 kDa and occur at different positions within the polypeptides; N-termi nal, C-terminal and internal. They have a moderately high and specific affinity for insoluble or soluble cellulosics with dissociation const ants in the low micromolar range. Some CBDs bind irreversibly to cellu lose and can be used for applications involving immobilization, others bind reversibly and are more useful for separations and purifications . Dependent on the CBD used, desorption from the matrix can be promote d under various different conditions including denaturants (urea, high pH), water, or specific competitive ligands (e.g. cellobiose). Family I and IV CBDs bind reversibly to cellulose in contrast to family II a nd III CBDs which are in general, irreversibly bound. The binding of f amily II CBDs (CBD,,,) to crystalline cellulose is characterized by a large favourable increase in entropy indicating that dehydration of th e sorbent and the protein are the major driving forces for binding. In contrast, binding of family IV CBDs (CBD,,) to amorphous or soluble c ellulosics is driven by a favourable change in enthalpy which is parti ally offset by an unfavourable entropy change. Hydrogen bond formation and van der Waals interactions are the main driving forces for bindin g. CBDs with affinity for crystalline cellulose are useful tags for cl assical column affinity chromatography. The affinity of CBD,, for solu ble cellulosics makes it suitable for use in large-scale aqueous two-p hase affinity partitioning systems. (C) 1998 Elsevier Science BN. All rights reserved.