Flavocytochrome b2: an ideal model system for studying protein-mediated electron transfer.

Introduction Most of the recent intense scientific effort towards providing an understanding of electron transfer in proteins has focused on intraprotein electron transfer and on the pathway between the donor (D) and acceptor (A) redox centres. These centres are usually fixed within the protein matrix as in the case of the photosynthetic reaction centre [ l ] , or involve attaching an artificial redox centre onto the surface of a protein at a fixed distance from the natural centre, e.g. in ‘ruthenated’ proteins [Z]. Such studies usually reduce to an analysis of whether the electron travels from D to A directly through space (i.e. treating the intervening protein medium as homogeneous like an ‘organic glass’), or whether it travels through a distinct o-tunnelling pathway involving specific covalent bonds, hydrogen bonds, etc. (i.e., treating the protein medium as heterogeneous). As well as the distance between D and A, the rate of electron transfer is also influenced by the driving force of the reaction, A G O , and the reorganization energy, i [ 13. In addition to intraprotein electron transfer, in which the redox centres are fixed within one protein, there is also intense interest in bimolecular reactions between proteins which result in interprotein electron transfer. Here, one must consider the dynamics of the interactions between the two proteins involved. Do the two proteins form one defined complex with a specific electron transfer path between redox centres? Are there a number of possible sites on the proteins where binding followed by electron transfer can occur? An interesting example is the complex between cytochrome c and cytochrome c peroxidase for which there is now a crystal structure [3]. Based on this structure a o-tunnelling pathway linking the two haem groups has been proposed [3]. However an NMR study of the dynamics of the cytochrome c-cytochrome c peroxidase interaction indicates that the complex in solution is highly mobile and probably does not have a discrete architecture with one specific electron transfer pathway [4].