What makes the difference between a cryptochrome and DNA photolyase? A spectroelectrochemical comparison of the flavin redox transitions.

Cryptochromes and DNA photolyases are highly homologous flavoproteins that accomplish completely different tasks. While plant cryptochrome1 functions as blue light photoreceptor that triggers various morphogenic reactions, photolyases repair UV-induced DNA damages. Both enzymes share the photoactive cofactor, noncovalently bound FAD. For photolyase, the reaction mechanism involves electron transfer to the substrate from the excited-state of fully reduced flavin. For cryptochrome, photoexcitation of the oxidized flavin leads to formation of the semireduced radical FADH(*). Key parameters for the redox state of the flavin in the cell are the midpoint potentials E(1) and E(2) for the oxidized/semireduced and semireduced/fully reduced transitions, respectively. A link between cryptochrome function and its cofactor's redox states has been suggested early on, but no reliable determinations of midpoint potentials have been available. Here we report spectroelectrochemical titrations of cryptochrome1 from Arabidopsis thaliana and photolyases from both E. coli and Anacystis nidulans at pH 7.4. For the cryptochrome, we obtained E(1) approximately E(2) approximately -160 mV vs NHE, strongly deviating from the photolyases where FADH(*) could not be oxidized up to 400 mV, and E(2) approximately -40 mV. Functional and evolutionary implications are discussed, highlighting the role of an asparagine-to-aspartate replacement close to N5 of the flavin.

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