PROBING PROTEIN ELECTROSTATIC INTERACTIONS THROUGH TEMPERATURE REDUCTION POTENTIAL PROFILES/

The change in the equilibrium reduction potentials of the iron-sulfur proteins, Pyrococcus furiosus rubredoxin and P. furiosus ferredoxin, a nd heme protein, horse cytochrome c, has been calculated as a function of temperature using a numerical solution to the Poisson-Boltzman equ ation. Working curves for different internal dielectric constants were generated to best reproduce experimental, observation. Based on a com parison of the experimental and simulated change in reduction potentia l with temperature, it is concluded that the dielectric constant of pr oteins is temperature-dependent and varies from protein to protein. Fo r example, the temperature-dependent reduction potential of cytochrome c can only be simulated using a different temperature-dependent diele ctric constant for each oxidation state, but this was not the case for rubredoxin or ferredoxin. The role of changes in ionization states of cytochrome c at alkaline pHs, where the reduction potential is known to be pH-dependent at room temperature, is also discussed in terms of electrostatic interaction energies as a function of temperature. It ap pears that temperature/reduction potential profiles may provide a dire ct method for measuring relative changes in internal protein dielectri c constants.