Limited solvation of an electron donating tryptophan stabilizes a photoinduced charge-separated state in plant (6-4) photolyase

(6-4) Photolyases ((6-4) PLs) are ubiquitous photoenzymes that use the energy of sunlight to catalyze the repair of carcinogenic UV-induced DNA lesions, pyrimidine(6-4)pyrimidone photoproducts. To repair DNA, (6-4) PLs must first undergo so-called photoactivation, in which their excited flavin adenine dinucleotide (FAD) cofactor is reduced in one or two steps to catalytically active FADH(-) via a chain of three or four conserved tryptophan residues, transiently forming FAD(center dot-)/FADH(-) MIDLINE HORIZONTAL ELLIPSIS TrpH(center dot+) pairs separated by distances of 15 to 20 angstrom. Photolyases and related photoreceptors cryptochromes use a plethora of tricks to prevent charge recombination of photoinduced donor-acceptor pairs, such as chain branching and elongation, rapid deprotonation of TrpH(center dot+) or protonation of FAD(center dot-). Here, we address Arabidopsis thaliana (6-4) PL (At64) photoactivation by combining molecular biology, in vivo survival assays, static and time-resolved spectroscopy and computational methods. We conclude that At64 photoactivation is astonishingly efficient compared to related proteins-due to two factors: exceptionally low losses of photoinduced radical pairs through ultrafast recombination and prevention of solvent access to the terminal Trp(3)H(center dot+), which significantly extends its lifetime. We propose that a highly conserved histidine residue adjacent to the 3rd Trp plays a key role in Trp(3)H(center dot+) stabilization.

Automated summary

(6-4) Photolyases are enzymes that use sunlight to repair carcinogenic DNA lesions. This study investigates the photoactivation of Arabidopsis thaliana (6-4) PL (At64) through various experiments and proposes that the efficiency of At64 photoactivation is due to low losses of radical pairs and limited solvent access to Trp(3)H(center dot+).