Do cellulose binding domains increase substrate accessibility?

This article provides an overview of various theories proposed during the past five decades to describe the enzymatic hydrolysis of cellulose highlighting the major shifts that these theories have undergone. It also describes the effect of the cellulose-binding domain (CBD) of an exoglucanase/xylanase from bacterium Cellulomonas fimi on the enzymatic hydrolysis of Avicel. Pretreatment of Avicel with CBDCex at 4 and 37 degrees C as well as simultaneous addition of CBDCex to the hydrolytic enzyme (Celluclast, Novo, Nordisk) reduced the initial rate of hydrolysis owing to irreversible binding of CBD proteins to the substrate's binding sites. Nonetheless, near complete hydrolysis was achieved even in the presence of CBDCex. Protease treatment of both pure and CBDCex-treated Avicel reduced the substrates' hydrolyzability, perhaps owing to proteolysis of the hydrolyzing enzyme (Celluclast) by the residual Proteinase K remaining in the substrate. Better protocols for complete removal of CBD proteins from the substrate need to be developed to investigate the effect of CBD adsorption on cellulose digestibility.