THERMAL STABILIZATION OF XYLOSE ISOMERASE FROM THERMOANAEROBACTERIUM-THERMOSULFURIGENES

The thermostability of D-xylose isomerase from Thermoanaerobacterium t hermosulfurigenes was enhanced by site-directed substitutions of aroma tic amino acids in the active site. This enhancement may be explained as the consequence of the reduction of the area of water-accessible hy drophobic surface. The kinetics of thermoinactivation of the enzyme in aqueous solution was also investigated, and we report that in additio n to the well known divalent cations, the monovalent cation, K+, also protects the enzyme against thermoinactivation. The kinetic data sugge st that the formation of incorrect conformations of the enzyme (''scra mbled structure'') is the dominant factor governing the process of the rmoinactivation at elevated temperature (80-90-degrees-C).