• catalytic base;
  • exocellulase;
  • glycoside hydrolase;
  • hydrolysis mechanism;
  • proton-transferring network

Thermobifida fusca exocellulase Cel6B acts by an inverting hydrolysis mechanism; however, the catalytic acid and base residues for this enzyme have not been confirmed. Site-directed mutagenesis and kinetic studies were used to show that Asp274 is the catalytic acid, which is consistent with what is found for other members of family-6 glycoside hydrolases; however, a single catalytic base was not identified. Mutation of all putative catalytic base residues, within 6 Å of the −1/+1 glucose subsites, including the highly conserved Asp226, Asp497 and Glu495, as well as Ser232 and Tyr220, did not reveal a catalytic base, although these residues are all important for activity. We propose a novel hydrolysis mechanism for T. fusca Cel6B involving a proton-transferring network to carry out the catalytic base function.