Characterization of pre-molten globule state of yeast iso-1-cytochrome c and its deletants at pH 6.0 and 25°C

To understand the role of five extra N-terminal residues, we prepared wild type (WT) yeast iso-1-cytochrome c (y-cyt-c) and its deletants by subsequently deleting these residues. Denaturation of all these proteins induced by LiCl was followed by observing changes in molar absorption coefficient at 405nm (δε₄₀₅), the mean residue ellipticity at 222nm ([θ]₂₂₂), and the difference mean residue ellipticity at 409nm (δ[θ]₄₀₉) near physiological pH and temperature (pH 6.0 and 25°C). It was observed that in each case LiCl induces biphasic transition, N (native) state↔X (intermediate) state↔D (denatured) state. The intermediate (X) was characterized by the far-UV, near-UV and Soret circular dichroism, ANS (8-anilino-1-naphthalenesulfonic acid) binding and dynamic light scattering measurements. These measurements led us to conclude that X state of each protein has structural characteristics of PMG (pre-molten globule) state. Thermodynamic stability of all proteins was also determined. It was observed that the N-terminal extension stabilizes the native WT protein but it has no effect on the stability of PMG state. Another state was observed for each protein, in the presence of 0.33M Na₂SO₄ at pH 2.1, which when characterized showed all structural characteristics of MG (molten globule) state.