Crystal structure of the beta-glycosidase from the hyperthermophilic archeon Sulfolobus solfataricus: resilience as a key factor in thermostability.

Enzymes from hyperthermophilic organisms must operate at temperatures which rapidly denature proteins from mesophiles. The structural basis of this thermostability is still poorly understood. Towards a further understanding of hyperthermostability, we have determined the crystal structure of the beta-glycosidase (clan GH-1A, family 1) from the hyperthermophilic archaeon Sulfolobus solfataricus at 2.6 A resolution. The enzyme is a tetramer with subunit molecular mass at 60 kDa, and crystallises with half of the tetramer in the asymmetric unit. The structure is a (betaalpha)8 barrel, but with substantial elaborations between the beta-strands and alpha-helices in each repeat. The active site occurs at the centre of the top face of the barrel and is connected to the surface by a radial channel which becomes a blind-ended tunnel in the tetramer, and probably acts as the binding site for extended oligosaccharide substrates. Analysis of the structure reveals two features which differ significantly from mesophile proteins; (1) an unusually large proportion of surface ion-pairs involved in networks that cross-link sequentially separate structures on the protein surface, and (2) an unusually large number of solvent molecules buried in hydrophilic cavities between sequentially separate structures in the protein core. These factors suggest a model for hyperthermostability via resilience rather than rigidity.