By altering the interface between myoglobin (Mb) and cytochrome b 5 to strengthen their interaction, researchers have sped up long-range electron transfer between the two proteins by several orders of magnitude (Science, DOI: 10.1126/ science.1197054). Long-range, interprotein electron transfer plays a role in a variety of biological redox processes, including keeping the iron atom in Mb reduced so it can bind to O2 in muscle cells. The research team, led by Brian M. Hoffman and Michael R. Wasielewski at Northwestern University, used Brownian dynamics simulations to determine the optimal Mb residues—two aspartates and a glutamate—to replace with lysines to promote complex formation between Mb and cyt b 5. The researchers then used femtosecond transient absorption spectroscopy to probe the rate of electron transfer in the altered system. They found a distribution of rate constants, on the order of 109 to 1010 s–1, indicating an ensemble of complexes and reactions. Typical interprotein electron transfer rates are 106 s–1, although the ultrafast transfers in photosystems I and II in photosynthesis run at 1011 to 1012 s–1. Further study should improve understanding of the interplay between protein-protein binding and electron transfer, Hoffman says.