Femtosecond conformational changes in a photolyase guide long-range electron transfer as resolved by serial crystallography

Abstract Charge transfer reactions in proteins are fundamentally important for life, but it is currently not clear how protein structural dynamics control these electron transfer reactions. Photolyases/cryptochromes, which repair DNA and signal in all kingdoms of life, have a paradigm electron transfer cascade. Here, photoreduction of the flavin cofactor initiates charge transfer along a chain of four conserved tryptophans. We report femtosecond X-ray crystallographic snap-shots for the Drosophila melanogaster (6-4) photolyase, revealing protein structural changes while electron transfer occurs. At femto- and picosecond delays, photoreduction of the flavin by the first tryptophan causes directed structural responses at key residue asparagine 403, at a conserved salt bridge, and by rearrangements of nearby water molecules. Along the tryptophan cascade, we detect charge-induced protein structural changes close to the second tryptophan from 1 ps to 20 ps, thereby identifying methionine 408 as an active participant in the redox chain, and from 300 ps around the fourth tryptophan. The data reveal that the protein undergoes highly directed and carefully timed adaptations of its structure to facilitate electron transfer. This suggests that evolution has optimized fast protein fluctuations for optimal function.