Substrate Binding by the Catalytic Domain and Carbohydrate Binding Module of <i>Ruminococcus flavefaciens</i> FD-1 Xyloglucanase/Endoglucanase

Abstract

Binding and thermodynamic properties of a carbohydrate binding module (CBM) and a glycoside hydrolase family 44 xyloglucanase/endoglucanase catalytic domain (CD) from <i>Ruminococcus flavefaciens</i>, both when separate and when linked to each other, have been quantified when binding various β-1,4-linked glucans and xylans. The three constructs bind cellotetraose, cellopentaose, and cellohexaose with association constants that increase with chain length. The CBM does not bind xylotetraose, xylopentaose, or xylohexaose. The CD appears to bind carboxymethylcellulose (CMC) and xylan only weakly, while the CBM and the CD/CBM bind them much more strongly than they bind the cellooligosaccharides. CMC is bound to a much greater degree than is xylan. Association constants for the cellooligosaccharides are in the order CBM ≪ CD < CD/CBM, while those on CMC and xylan are CD ≪ CBM ≪ CD/CBM. A synergistic effect was observed for the association constants of cellopentaose and cellohexaose with the CD/CBM when compared to the CD and CBM alone. Binding of all ligands by all three constructs is energetically favorable, enthalpy-driven, and subject to enthalpy–entropy compensation