Characterization of a partially unfolded structure of cytochrome c induced by sodium dodecyl sulphate and the kinetics of its refolding

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Abstract
The mechanism of unfolding of ferricytochrome c induced by the surfactant sodium dodecyl sulfate has been studied by heme absorption, tryptophan fluorescence, circular dichroism, resonance Raman scattering, stopped‐flow and time‐resolved resonance energy transfer to obtain a comprehensive view of the whole process. Unfolding occurred at an almost specific molecular ratio of SDS/cytochrome c in the concentration range (20−50 μM) studied here. However there appears to be a point at ≈0.6 mM SDS where unfolding begins to occur for lower cytochrome c concentrations. The kinetics of unfolding revealed only a single transition with a rate constant of 33 s−1 (at 298 K, [SDS] = 8.7 mM) and activation energy barrier of ≈ 16 kJ/mol, indicating that other associated steps, if any, are too fast to be significantly populated. The free energy change (ΔG+) involved with the unfolding transition was estimated to be about 16.8 kJ/mol. The CD spectrum at 220 nm of SDS‐unfolded cytochrome c shows only a partial decrease (25 %), indicating that a significant amount of helical structure remains folded in contrast to a complete loss of helical structure in GdnHCl‐denatured cytochrome c. The heme structure in SDS‐unfolded cytochrome c, as deduced from heme absorption and resonance Raman spectra, shows a major population (≈95 %) of mis‐ligated histidine to the heme which acts as a kinetic trap in the folding process. The structural changes associated with cytochrome c unfolding were also monitored by time‐resolved resonance energy transfer which shows a drastic increase in tryptophan fluorescence lifetime from 12 ps in the native protein to 0.63 ns in the unfolded one, associated with a movement of Trp59 by 10 Å away from heme. The maximum entropy method analysis of fluorescence decay indicated the growth of various conformational substates in SDS‐unfolded cytochrome c in contrast to narrowly distributed conformations in the native protein. The refolding was comprised of three kinetic steps; the first was significantly fast (≈8 ms) and was assigned to the dissociation of His26 that paves the protein towards correct folding pathway. The other two slower steps probably arise from chain misorganization and prolyl isomerization. The absence of a burst‐phase amplitude supports the idea that the burst phase observed in the folding from completely unfolded cytochrome c corresponds to a molecular collapse that produces significant secondary structure. The partially unfolded state represents a unique intermediate state in the folding pathway.

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