The three-dimensional X-ray structures of the oxidized and reduced forms of rubredoxin from Pyrococcus furiosus, determined at -161 degrees C, and the NMR structure of the zinc-substituted protein, determined in solution at 45 degrees C, are compared. The NMR and X-ray structures, which were determined independently, are very similar and lead to similar conclusions regarding the interactions that confer hyperthermostability.