Speedier Electron Transfer

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.