Catalysis of the refolding of urea denatured creatine kinase by peptidyl-prolyl cis-trans isomerase.

The effect of peptidyl-prolyl cis-trans isomerase (PPIase) on the refolding and reactivation courses of urea-denatured creatine kinase was followed by fluorescence emission, ultraviolet difference spectra and recovery of activity. PPIase is shown to accelerate the slow-phasic reaction of the refolding of urea-denatured creatine kinase. The results suggest that the prolyl peptide bond isomerization may be one of the rate-determining steps in the refolding of creatine kinase.

Salt-induced folding of alkaline denatured creatine kinase under high pH conditions.

The conformational changes of creatine kinase during alkaline unfolding and salt-induced folding at high pH have been followed by fluorescence emission and circular dichroism spectra. The results obtained show that at low ionic strength, with increasing pH value, creatine kinase denatured gradually to reach the ultimate unfolded conformation in the vicinity of pH 12.7. With the increase of pH from 9.0 to 12.7, the fluorescence emission maximum red shifted from 337 to 355 nm, indicating complete exposure of the buried tryptophan residues to the solvent. The far-UV CD spectra show that even at pH 12.7, the apparently fully denatured enzyme retains a great part of ordered secondary structure. At pH 12.7 by adding the salt, the relatively unfolded state of denatured enzyme changes into a compact conformational state by hydrophobic collapsing. Folded state induced by salt bound ANS strongly, indicating the existence of increased hydrophobic surface. The above results suggest that the salt-induced folded state at high pH may be the folded intermediate which exists in the general protein folding, and the larger residual ordered secondary structure might become folded being point on the salt-induced folding.

Alkaline unfolding and salt-induced folding of yeast alcohol dehydrogenase under high pH conditions.

The conformational changes of yeast alcohol dehydrogenase during unfolding at alkaline pH have been followed by fluorescence emission and circular dichroism spectra. A result of comparison of inactivation and conformational changes shows that much lower values of alkaline pH are required to bring about inactivation than significant conformational change of the enzyme molecule. At pH 9.5, although the enzyme has been completely inactivated, no marked conformational changes can be observed. Even at pH 12, the apparently fully unfolded enzyme retains some ordered secondary structure. After removal of Zn2+ from the enzyme molecule, the conformational stability decreased. At pH 12 by adding the salt, the relatively unfolded state of denatured enzyme changes into a compact conformational state by hydrophobic collapsing. Folded states induced by salt bound ANS strongly, indicating the existence of increased hydrophobic surface. More extensive studies showed that although apo-YADH and holo-YADH exhibited similar behavior, the folding cooperative ability of apo-enzyme was lower than that of holo-enzyme. The above results suggest that the zinc ion plays an important role in helping the folding of YADH and in stabilizing its native conformation.