Direct electrochemical evidence for an equilibrium intermediate in the guanidine-induced unfolding of cytochrome c

Electrochemical evidence on the molten globule conformation of cytochrome c

To explore a new approach for characterizing the molten globule conformation, cyclic voltammetric studies of salt induced transitions at acidic pH of cyt c have been carried out. The use of modified electrodes has made the observation of direct electrochemistry in native cyt c possible. However, most of these electrodes do not show reversible responses at acidic pH, due to the fact that, for this system, a deprotonated electrode surface is needed. In these studies, we have used a 6-mercaptopurine and cysteine-modified gold electrodes which are effective for direct rapid electron transfer to cyt c, even in acid solutions. The change in the absorption bands of cyt c are used to monitor the conformational states and, hence, to compare the voltammetric results. Under the experimental conditions where the A state of cyt c is obtained, a reversible voltammetric signal is observed. The midpoint peak potentials are found to be very close to the formal potential of native cyt c. Results are discussed in terms of a cooperative two-state transition between the acid unfolded and the globular acidic states of cyt c. This finding establishes, for the first time, the similarity of both the native and the molten globule-like conformations in terms of its redox properties.

The Soret circular dichroism spectrum as a probe for the heme Fe(III)-Met(80) axial bond in horse cytochrome c

A spectroscopic signal sensitive to the strength of the heme iron(III)-Met(80) bond in cytochrome c represents a useful tool that will provide valuable information on the heme pocket region and redox properties of the protein. At present, the 695-nm absorption band is perhaps the simplest diagnostic signal for the axial bond; this band disappears when the Fe(III)-Met(80) bond is disrupted. From the analysis of the Soret region circular dichroism spectrum of cytochrome c under conditions that gradually induce disruption of the Fe(III)-Met(80) bond, we present evidence that the 416-nm spectral dichroic band provides independent information addressing the strength of the axial bond between Fe(III) and Met(80) in cytochrome c. Further, this study demonstrates extension of the diagnostic application to very dilute protein samples based on the useful sample concentration of 5-10 microM vs 200-300 microM required for 695-nm absorbance measurements.