Biochemical Characterization and Crystal Structures of a Fungal Family 3 {beta}-Glucosidase, Cel3A from Hypocrea ȷecorina [Enzymology]

August 27th, 2014 by Karkehabadi, S., Helmich, K. E., Kaper, T., Hansson, H., Mikkelsen, N. E., Gudmundsson, M., Piens, K., Fujdala, M., Banerjee, G., Scott-Craig, J. S., Walton, J. D., Phillips, G. N., Sandgren, M.

Cellulase mixtures from H. jecorina are commonly used for the saccharification of cellulose in biotechnical applications. The most abundant β-glucosidase in the mesophilic fungus Hypocrea jecorina is HjCel3A, which hydrolyzes the β-linkage between two adjacent molecules in dimers and short oligomers of glucose. It has been shown that enhanced levels of HjCel3A in H. jecorina cellulase mixtures benefit the conversion of cellulose to glucose. Biochemical characterization of HjCel3A shows that the enzyme efficiently hydrolyzes (1,4)- as well as (1,2)-, (1,3)-, and (1,6)- β-D linked disaccharides. For crystallization studies, HjCel3A was produced in both H. jecorina (HjCel3A) and Pichia pastoris (Pp-HjCel3A). While the thermostabilities of HjCel3A and Pp-HjCel3A are the same, Pp-HjCel3A has a higher degree of N-linked glycosylation. Here, we present X-ray structures of HjCel3A with and without glucose bound in the active site. The structures have a three-domain architecture as previously observed for other glycoside hydrolase family 3 β-glucosidases. Both production hosts resulted in HjCel3A structures that have N-linked glycosylations at Asn208 and Asn310. In H. jecorina-produced HjCel3A a single N-acetylglucosamine is present at both sites, while in Pp-HjCel3A the P. pastoris-produced HjCel3A enzyme the glycan chains consist of 8 or 4 saccharides, respectively. The glycosylations are involved in intermolecular contacts in the structures derived from either host. Due to the different sizes of the glycosylations, the interactions result in different crystal forms for the two protein forms.