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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 β-glycosidase (clan GH-1A, family 1) from the hyperthermophilic archaeon Sulfolobus solfataricus at 2.6 Å 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 (βα)₈ barrel, but with substantial elaborations between the β-strands and α-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 …