Refolding process of ovalbumin from urea-denatured state. Evidence for the involvement of nonproductive side chain interactions in an early intermediate.

Ovalbumin contains one cystine disulfide (Cys73-Cys120) and four cysteine sulfhydryls (Cys11, Cys30, Cys367, and Cys382) in a single polypeptide chain of 385 amino acid residues. The refolding mechanism of ovalbumin was investigated under disulfide-bonded and disulfide-reduced conditions using the denatured protein state, DA, as the starting protein sample. For the preparation of DA, the disulfide-intact and disulfide-reduced forms of ovalbumin were denatured by protein incubation in 9 M urea at pH 2.2. When DA was placed in a refolding buffer, pH 8.2, an intermediate state IN was produced in either the disulfide-bonded or the disulfide-reduced condition; IN showed about 60% of the native CD ellipticity at 222 nm and the intrinsic tryptophan fluorescence with the native spectrum peak but with decreased intensity. The formation of IN as detected by far UV CD ellipticity was quite rapid and finished within a mixing dead time of 20 ms. When DA was diluted with an acidic buffer, pH 2.2, a partially folded equilibrium intermediate IA with the structural characteristics equivalent to those of IN was formed. After the formations of IN and IA, the regains in CD ellipticity and tryptophan fluorescence at pH 8.2 followed biphasic kinetics in the disulfide-bonded condition but monophasic kinetics in the disulfide-reduced condition. As unexpected findings, the native disulfide in DA and IA underwent nonproductive disulfide rearrangements in the disulfide-bonded condition at an early refolding stage and then was recovered during the subsequent refolding. The integrity of overall refolding was confirmed by the observation that the proteins refolded for 20 h in the disulfide-bonded and disulfide-reduced conditions showed, on differential scanning calorimetry analyses, almost exactly the same denaturation temperatures as their native protein counterparts. These results were consistent with a refolding process for ovalbumin which includes nonproductive side chain-side chain interactions in the early intermediate IN, which requires subsequent reorganization for the correct refolding.