Photoinduced formation of flavin radicals in BLUF domains lacking the central glutamine.

Blue light receptors using FAD (BLUFs) facilitate blue light-induced signal transduction via light-induced rearrangement of hydrogen bonds between the flavin chromophore and a conserved glutamine side chain. Here, we investigated the photochemistry of the BLUF domain Slr1694 from Synechocystis sp. in which the glutamine side chain was removed. Without the glutamine, no red-shifted signaling state is formed, but light-induced proton-coupled electron transfer between protein and flavin takes place similarly as for the wild-type protein. However, the lifetime of the neutral flavin semiquinone-tyrosyl radical pair is greatly prolonged from < 100 ps to several nanoseconds, which indicates that the formation of radical intermediates drives the hydrogen bond rearrangement in BLUF photoactivation. Moreover, glutamine plays a central role in the molecular organization of the hydrogen bond network in the flavin-binding pocket, as its removal enhances electron transfer from tyrosine to the excited flavin, and enables competing electron transfer from a nearby tryptophan.