Protein engineering of cellulases

Cellulases are enzymes which hydrolyse the beta -1,4-glucosidic linkages of cellulose. They fall into 13 of the 82 glycoside hydrolase families identi fied by sequence analysis, but they are traditionally divided into two clas ses termed 'endoglucanases' (EC 3.2.1.4) and 'cellobiohydrolases' (3.2.1.91 ). Both types of cellulases degrade soluble cellodextrins and amorphous cel lulose but, with a few notable exceptions, it is only the cellobiohydrolase s which degrade crystalline cellulose efficiently. Site-directed mutagenesi s has been central to the characterisation of cellulases, ranging from the identification and characterisation of putative catalytic and binding resid ues, the trapping of enzyme-substrate complexes by crystallography through to the construction of new and improved biocatalysts including 'glycosyntha ses'. Whilst studies on soluble substrates and substrate analogues have pro vided a wealth of information, understanding the mechanism of degradation o f the natural substrate, crystalline cellulose, remains a great challenge. (C) 2000 Elsevier Science B.V. All rights reserved.