From the protein–polypeptide model system to the interaction between physiological partners using electrochemistry

The process and experimental conditions of protein–protein complex formation are studied at a graphite electrode using voltammetry techniques. Several situations are examined including protein–polypeptide complexes and physiological partner proteins. Electrochemical reversible responses are observed for different acidic proteins such as ferredoxins and tetraheme Desulfovibrio africanus cytochrome c3 only in the presence of poly(l-lysine). This polycationic polypeptide is able to promote the electron exchanges between the electrode and proteins by lowering repulsion of electrostatic origin. In a second step, interactions have been examined in the case of two proteins of opposite charge. It is remarkable that Desulfovibrio vulgaris Hildenborough cytochrome c3, a typical basic protein (pI 10.5), is able to promote the electrochemical response of a variety of acidic proteins such as ferredoxins not necessarily involved in the electron-carrier chain of the DvH organism. Such results highlight the role played by electrostatic forces in the interaction between proteins of opposite global charges. Nevertheless, the electrostatic factor is not enough to explain the efficient interaction between the two physiological partners cytochrome c3 and hydrogenase from Desulfomicrobium norvegicum. Despite relatively close global charges, it is shown that electron transfer occurs between the two proteins, as is revealed from the well-developed catalytic currents observed by cyclic voltammetry. Other examples studied here suggest that several factors including polar interactions and hydrophobic contacts must be involved in more specific interactions.