The stability of two enzymes from extreme thermophiles (glutamate dehy
drogenase from Thermococcales strain ANI and beta-glucosidase from Cal
docellum saccharolyticum expressed in Escherichia coli) has been explo
ited to allow measurement of activity over a 175 degrees C temperature
range, from +90 degrees C to -85 degrees C for the glutamate dehydrog
enase and from +90 degrees C to -70 degrees C for the beta-glucosidase
. The Arrhenius plots of these enzymes, and those for two mesophilic e
nzymes (glutamate dehydrogenase from bovine liver and beta-galactosida
se from Escherichia coli), exhibit no downward deflection correspondin
g to the glass transition, found by biophysical measurements of severa
l non-enzymic mesophilic proteins at about -65 degrees C and reflectin
g a sharp decrease in protein flexibility as the overall motion of gro
ups of atoms ceases.