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 35000 ± 1000) has been previously described as ‘unusually heatstable’ [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°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 (< 10 s in the temperature range from 24.6°C to 7.7°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.