Torben Peter Frandsen

Structural Basis for Ligand Binding and Processivity in Cellobiohydrolase Cel6A from Humicola Insolens

The enzymatic digestion of cellulose entails intimate involvement of cellobiohydrolases, whose characteristic active-center tunnel contributes to a processive degradation of the polysaccharide. The cellobiohydrolase Cel6A displays an active site within a tunnel formed by two extended loops, which are known to open and close in response to ligand binding. Here we present five structures of wild-type and mutant forms of Cel6A from Humicola insolens in complex with nonhydrolyzable thio-oligosaccharides, at resolutions from 1.7-1.1 A, dissecting the structural accommodation of a processing substrate chain through the active center during hydrolysis. Movement of ligand is facilitated by extensive solvent-mediated interactions and through flexibility in the hydrophobic surfaces provided by a sheath of tryptophan residues.

Structure of the Humicola insolens cellobiohydrolase Cel6A D416A mutant in complex with a non-hydrolysable substrate analogue, methyl cellobiosyl-4-thio-β-cellobioside, at 1.9 Å

The enzymatic degradation of cellulose continues to be one of the most important enzyme-catalysed reactions. Glycoside hydrolases from family GH-6 hydrolyse cellulose with inversion of the configuration of the anomeric carbon. Whilst the catalytic proton donor has been clearly identified (Asp226 in Humicola insolens Cel6A), the identification and even the existence of a potential Brønsted base remains unclear. Equally controversial is the role of surface-loop flexibility. Here, the structure of the D416A mutant of the H. insolens cellobiohydrolase Cel6A in complex with a non-hydrolysable thiooligosaccharide methyl cellobiosyl-4-thio-beta-cellobioside at 1.9 A resolution is presented. Substrate distortion in the -1 subsite, to a (2)S(0) skew-boat conformation, is observed, similar to that seen in the analogous Trichoderma reesei Cel6A structure [Zou et al. (1999), Structure, 7, 1035-1045], but the active-centre N-terminal loop of the H. insolens enzyme is found in a more open conformation than described for previous structures.

Cloning, heterologous expression, and enzymatic characterization of a thermostable glucoamylase from Talaromyces emersonii

The gene encoding a thermostable glucoamylase from Talaromyces emersonii was cloned and, subsequently, heterologously expressed in Aspergillus niger. This glucoamylase gene encodes a 618 amino acid long protein with a calculated molecular weight of 62,827Da. T. emersonii glucoamylase fall into glucoside hydrolase family 15, showing approximately 60% sequence similarity to glucoamylase from A. niger. The expressed enzyme shows high specific activity towards maltose, isomaltose, and maltoheptaose, having 3-6-fold elevated k(cat) compared to A. niger glucoamylase. T. emersonii glucoamylase showed significantly improved thermostability with a half life of 48h at 65 degrees C in 30% (w/v) glucose, compared to 10h for glucoamylase from A. niger. The ability of the glucoamylase to hydrolyse amylopectin at 65 degrees C is improved compared to A. niger glucoamylase, giving a significant higher final glucose yield at elevated temperatures. The increased thermal stability is thus reflected in the industrial performance, allowing T. emersonii glucoamylase to operate at a temperature higher than the A. niger enzyme.