Pressure-Controlled Voltammetry of a Redox Protein: An Experimental Approach to Probing the Internal Protein Dielectric Constant? Equilibrium reduction potentials of recombinant Pyrococcus furiosus

mental process by which virtually all organisms obtain energy. For this reason, biochemists are interested in electrochemically characterizing proteins that make up electron transport pathways. They are also interested in understanding at a molecular level how a protein’s structure influences its electron transfer function. In this study, the electron transfer protein rubredoxin (F1) from the deep-sea microorganism Pyrococcus furiosus was electrochemically characterized. Formal equilibrium reduction potentials (Eo ́s), which represent an important aspect in the thermodynamic analysis of biological electron transfer reactions, were measured as a function of pressure. We are interested in determining and rationalizing how electron transfer equilibrium properties are influenced by pressure. In an earlier study presented in this journal, it was suggested that temperature-controlled electrochemical studies provide useful information concerning electrostatic interaction energies in proteins (1). We suggest that such information can also be obtained from pressure-controlled electrochemical studies. Reduction potentials at ambient pressures have been measured for many electron transfer proteins using a variety of voltammetric methods and various modified and unmodified solid electrode surfaces (2). Advantages of direct electrochemical methods include low cost, rapidity and nondestructive nature of the measurements, and small sample volume and low concentration requirements. In order to further our understanding of biological electron transfer reactions, an electrochemical cell was specifically designed to measure equilibrium reduction potentials as a function of pressure (F2). Pressure-controlled electrochemical experiments provide important information concerning the role of protein/solvent interactions on electron transfer, as well as information concerning changes in electrostatic interaction energies of electron transport proteins. While pressure-dependent reduction potentials have been reported for an electron transfer protein (3), few studies have been done to understand these observed changes at a molecular level (4, 5). It has long been recognized that electrostatic interactions are an important determinant of enzymatic and electron transfer reactivity. In simple terms, coulombic interaction energies, which contribute to the magnitude of the reduction potential, are directly related to the magnitude of charge and inversely related to the distance between charges and the screening of the medium which separates the charges. Coulombic interaction energies between two charged particles i and j in a homogenous medium can be calculated as follows: E = qiqj / εijrij (E1) Pressure-Controlled Voltammetry of a Redox Protein: An Experimental Approach to Probing the Internal Protein Dielectric Constant?