Elucidation of a distinct photoreduction pathway in class II photolyase.

Class II photolyases (PLs) are a distant subclade in the photolyase/cryptochrome superfamily, displaying a unique Trp-Tyr tetrad for photoreduction and exhibiting a lower quantum yield (QY) of DNA repair (49%) than class I photolyases (82%) [M. Zhang, L. Wang, S. Shu, A. Sancar, D. Zhong, , 209-213 (2016)]. Using layer-by-layer mutant design and femtosecond spectroscopy, we have successfully determined the rates of electron transfer and proton transfer, driving force, and reorganization energy for nine elementary steps involved in the initial photoreduction of class II photolyase (AtPL), thereby constructing the photoreduction network specific to class II PLs. Several dynamic features have been revealed including a slow-rise (172 ps) and fast-decay (26 ps) kinetics between the excited lumiflavin and adenine groups within the flavin adenine dinucleotide cofactor, a slower electron transfer (ET) (22 ps) between the excited lumiflavin and the nearest Trp in the Trp triad (W) as compared to reported class I PL (0.8 ps), and a rapid deprotonation of the distal Trp in the Trp triad (W). Most strikingly, we captured a slightly energetically unfavorable ET step between W and the center Trp (W), as opposed to the decreasing reduction potential observed in class I PL that drives the electron flow unidirectionally. Such an energetically uphill ET step leads to a lower photoreduction quantum yield (~34%) in class II AtPL compared to that of class I PL (~45%), raising an important question on the evolutionary implications of various photoreduction networks in photolyases and cryptochromes.