Physicochemical characterization of a fast refolding monomeric class I fructose-1,6-bisphosphate aldolase from Staphylococcus aureus.

The class I fructose-1,6-bisphosphate aldolase from Staphylococcus aureus is proposed as a good candidate for thermodynamic and kinetic studies on protein folding. The monomeric enzyme (molecular weight 35 000 +/- 1000) has been previously described as 'unusually heat-stable' [F. Götz et al. (1980) Eur. J. Biochem. 108, 295-301]. In the present paper we show that the enzyme is reversibly denatured at relatively low temperature (26-39 degrees C), as determined by protein fluorescence and far ultraviolet circular dichroism; the van't Hoff enthalpy of the thermal unfolding is 355 +/- 63 kJ/mol. The dichroic absorption shows that the aldolase is extensively unfolded in 6 M guanidine/HCl. Complete reactivation of the guanidine-denatured enzyme in the test solution is extremely fast (less than 10 s in the temperature range from 24.6 degrees C to 7.7 degrees C). Reactivation ought to be much slower if isomerization reactions around at least some of the ten Xaa-Pro peptide bonds were rate-limiting for reactivation.