STABILITY OF ARTHROBACTER D-XYLOSE ISOMERASE TO DENATURANTS AND HEAT

There was no inactivation of Mg2+-containing Arthrobacter D-Xylose isomerase up to 1 h in 0-8 m-urea at 22-degrees-C, but over this range there was rapid reversible dissociation into fully active dimers with a midpoint around 4 m-urea, as shown by gradient urea gels with an activity stain, and by ion-exchange chromatography and gel filtration in urea buffers. These dimers must have the A-B* conformation, since the tetramer could dissociate into A-A*, A-B or A-B* dimer conformations, but only residues across the A-B* interface contribute to the active site. The kinetics of inactivation of the Mg2+-containing enzyme in 8 m-urea at higher temperatures suggest a partially unfolded Mg-A-B* dimer intermediate with 50 % activity, followed by irreversible inactivation coincident with the appearance of unfolded monomer. In 0-4 m guanidinium chloride, a similar reversible dissociation into active dimers occurs, but activity falls, suggesting that A-A* and/or A-B dimers might be part of the mixture. Low concentrations of SDS also give active dimers leading to unfolded monomers, but SDS above 1 % (w/v) provides relative stabilization. The apoenzyme is least thermostable (t1/2 at 80-degrees-C, pH 7, = 0.06 h) but Mg2+ stabilizes strongly (t1/2 = 5.5 h) and Co2+ even more so. Competitive inhibitors or substrates provide a small further stabilization, but this effect is more marked at 80-degrees-C, pH 5.5. Together with a marked decrease in optimum pH with temperature, this allows batch isomerizations of glucose under these conditions that produce clean but sweeter syrups.