Enhanced Activity of the Cellulase Enzyme β-Glucosidase upon Addition of an Azobenzene-Based Surfactant

beta-Glucosidases catalyze the hydrolysis of cellobiose to glucose, which is often the rate-limiting step in the conversion of cellulose into fermentable sugars during bioethanol production. Thus, the structure and function of beta-glucosidase from Aspergillus niger were examined in response to a photoresponsive azobenzene-based surfactant (4-ethyl-4'(trimethylamino-butoxy)azobenzene bromide, azoTAB) as a means to enhance the enzyme activity. Light and neutron scattering data indicate that pure beta-glucosidase exists as dimers or higher aggregates in solution that are progressively converted to monomers with an increasing azoTAB concentration. This transition is accompanied by a 60% increase in catalytic activity. In contrast, the enzyme is simply deactivated in the presence of conventional straight-chain hydrocarbon surfactants. Shape-reconstructed images obtained from SANS data demonstrate that azoTAB causes selective unfolding in the alpha/beta sandwich domain that comprises the crystallographic dimer interface, consistent with the observed transition to monomers. Furthermore, this domain forms one side of a long cleft that begins at the active site and facilitates the nonproductive binding of substrate or longer oligosaccharides, which at times can block the active site. Indeed, kinetic data indicate that the azoTAB-induced increase in beta-glucosidase activity is a result of diminished substrate inhibition, thus providing a unique means of obtaining glucose-tolerant beta-glucosidases.