19] Direct and indirect electrochemical investigations of metalloenzymes

Abstract

All redox metalloproteins can be investigated by electrochemical methods. There is no difficulty in achieving the electrochemistry of small redox proteins such as cytochromes, ferredoxins, blue copper proteins, and flavodoxins. Direct electrochemistry proceeds without the need for an electron transfer shuttle, or mediator, between the redox center of the protein and the electrode. However, at most metal electrodes the presence of a promoter is required. Such a compound binds to the electrode surface and, while not itself taking part in the electron transfer process, encourages electron transfer with the protein to proceed. Redox enzymes may be classified as being intrinsic or extrinsic in nature. To enable communication between an enzyme and the electrode, the surface of the latter must bind the enzyme and prevent denaturation. An intrinsic redox enzyme is one in which electron transfer associated with the catalyzed event is contained within the confines of the active site—that is, the enzyme lacks a natural long-range electron transfer pathway. Thus, achieving heterogeneous electron transfer with an electrode may require that (1) the site of the catalytic reaction be close to the enzyme surface, (2) the enzyme be able to deform without losing its activity, (3) the electrode surface (with or without a promoter) project into the enzyme, or (4) electron transfer pathways be introduced by modification. Indirect electrochemical communication between an electrode and a metalloenzyme may be achieved using an associated redox protein as a mediator, in other words, as an electron transfer shuttle between the enzyme and the working electrode.