Involvement of molten globule state on the folding process of secretary proteins

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Human serum albumin(HSA), N-terminal half-molecule of ovotransferrin(N-OVT), and ovalbumin(OVA)were reduced and denatured in the presence of dithiothreitol and 8 M urea. The samples were diluted with a neutral buffer, and protein conformation was evaluated by CD-spectrum. With regards to HSA and N-OVT, the refolded, disulfide-reduced form was found to take a partially folded molten globule-like conformation. When oxidized form of gultathione(GSSG)was added to the state, the intrachain protein disulfide bonds were regenerated. These data indicated that HSA as well as N-OVT take a molten globulelike state during oxidative refolding as an intermediate. In contrast, OVA showed the native-like conformation in its disulfide-reduced state as evaluated by CD-spectrum. The reactivity of cysteine sulfhydryls, however, was significantly different between the native form and the refolded, disulfide-reduced form : no reactive sulfhydryl was detected in the former form, but two sulfhydryls were detected in the latter form. By the addition of GSSG to the reduced state, ovalbumin was transformed to the disulfide bonded form with native cyteine pairing(Cys73-CYsl2O). From these data, we concluded that a protein with many disulfide and domain structure, such as HSA and N-OVT, takes a molten globule-like conformation as a intermediate for oxidative refolding, while a protein with single disulfide and single domain, such as OVA, takes a naive-like conformation in the disulfide-reduced state.