Long-range electron transfer in multisite metalloproteins
Electron-transfer (ET)' reactions play major roles in respiration and photosynthesis. In both of these processes, there are electron-transport chains associated with specific cell organelles. These chains consist of a number of ET complexes firmly bound to biological membranes. ET between the complexes is mediated by small, diffusible molecules. The ET components within the complexes are flavins, iron-sulfur clusters, heme-bound iron, and manganese and copper ions. The complexes span the
... xes span the membranes, and ET through them is coupled to the translocation of protons across the membranes. This leads to the creation of an electrochemical potential, which drives the synthesis of ATP according to the chemiosmotic mechanism of Mitchell (1 966). Biological ET reactions have a number of characteristics distinguishing them from most redox processes involving small metal complexes. First, they occur rapidly over large molecular distances ( > l o A). Second, the ET event is often accompanied by only minor changes in the structure of the redox site. Finally, many biological ET complexes are molecular ion pumps, whose operation requires a structural control of the electron flow. A good deal is known about the factors governing shortrange ET in metal complexes (Taube, 1984; Marcus & Sutin, 1985) , whereas the understanding is less complete concerning long-range ET in biological systems. In recent years, there has, however, been real progress in the understanding of ET involving small metalloproteins such as plastocyanin and cytochrome c (Sykes, 1985; Scott et al., 1985) . In addition, there has been a considerable amount of experimental work with artificial multisite metalloproteins, aimed at illuminating the effects of the driving force, the reorganization energy, the distance, and the intervening medium on the rate of long-range Our research on protein electron transfer is supported by grants from I Abbreviations: a, ammine; cyt, cytochrome; Mb, sperm whale myoglobin; cyt c-Zn, zinc derivative of horse heart cytochrome c; Mb-ZnP, zinc mesoporphyrin IX derivative of sperm whale myoglobin; ET, electron transfer; isn, isonicotinamide; P, mesoporphyrin IX; py, pyridine.