Activity and stability of hyperthermophilic enzymes: a comparative study on two archaeal β-glycosidases

Sbgly and CelB are well-studied hyperther- mophilic glycosyl hydrolases, isolated from the Archaea Sulfolobus solfataricus and Pyrococcus furiosus, respec- tively. Previous studies revealed that the two enzymes are phylogenetically related; they are very active and stable at high temperatures, and their overall three-dimensional structure is very well conserved. To acquire insight in the molecular determinants of thermostability and thermoac- tivity of these enzymes, we have performed a detailed com- parison, under identical conditions, of enzymological and biochemical parameters of Sgly and CelB, and we have probed the basis of their stability by perturbations induced by temperature, pH, ionic strength, and detergents. The major result of the present study is that, although the two enzymes are remarkably similar with respect to kinetic parameters, substrate specificity, and reaction mechanism, they are strikingly different in stability to the different physical or chemical perturbations induced. These results provide useful information for the design of further experi- ments aimed at understanding the structure–function relationships in these enzymes.