Unfolding and refolding of the NAD(+)-dependent isocitrate dehydrogenase from yeast

Conformational changes at the active site of bovine pancreatic RNase A at low concentrations of guanidine hydrochloride probed by pyridoxal 5'-phosphate

The alpha-amino group of Lys-1 and the epsilon-amino groups of Lys-41 and Lys-7 were labeled with pyridoxal 5'-phosphate (PLP) separately. The effects of GdnHCl on the activities and the fluorescence properties of the derivatives were compared. Both the fluorescence intensity and anisotropy of the probe introduced at the active site Lys-41 decreased obviously during denaturation by low concentrations of GdnHCl indicating conformational change of the active site is a parallel event to the inactivation of the enzyme. Time-correlated fluorescence lifetime measurements revealed the existence of two conformational states of PLP-modified RNase A and a shift of the conformation in favor of the slow-decay component with increasing GdnHCl concentration. The decrease in activity of RNase A at low concentrations of GdnHCl is therefore due to partial unfolding of the molecule, particularly at the active site. The experimental results of this work support the suggestion that the conformation at the active site is more easily perturbed and hence more flexible than the molecule as a whole.

Dissociation and unfolding of the pyruvate dehydrogenase complex by guanidinium chloride

The effect of guanidinium chloride (GdnHCl) on the pyruvate dehydrogenase complex (PDC) from bovine heart and its constituent enzymes has been studied. The overall activity of the complex is lost reversibly at low levels of GdnHCl (0.2 M) which cause 40-50% inactivation but no loss of overall secondary or tertiary structures of the individual enzymes; the inactivation of the complex is shown to be caused by dissociation of the E1 and E3 components from the E2/X core assembly. This provides an improved procedure for controlled dissociation of the complex and efficient recovery of its component enzymes in their native states. Higher concentrations of GdnHCl (up to 4 M) lead to the unfolding and irreversible inactivation of the separate enzymes of the complex with the E2/X core proving the most resistant to GdnHCl-induced unfolding. Neither the 60-meric E2/X core assembly nor the dimeric E3 component are dissociated into monomers in the presence of 6 M GdnHCl; the latter enzyme forms higher-M(r) aggregates under these conditions.

Inactivation during denaturation of ribonuclease A by guanidinium chloride is accompanied by unfolding at the active site

Inactivation of pancreatic RNAase A occurs in guanidinium chloride (GdmCl) at low concentrations before the unfolding of the molecule as a whole can be detected [Liu and Tsou (1987) Biochim. Biophys. Acta 916, 455-464]. We have now shown that the rate of digestion of the RNAase molecule by either trypsin or proteinase K increases significantly at low concentrations of GdmCl where the enzyme is largely inactivated, but fluorescence and absorption measurements reveal no conformational changes. N-Terminal sequence analysis of the peptide fragments generated shows that proteolysis occurs primarily at or near the active site. The decrease in activity of RNAase at low concentrations of GdmCl is therefore due to partial unfolding of the molecule, particularly at the active site and not to an inhibition by the denaturant.