THE EFFECT OF LOW-TEMPERATURES ON ENZYME-ACTIVITY

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.