The animal-like cryptochrome of Chlamydomonas reinhardtii (CraCRY) is a recently discovered photoreceptor that controls the transcriptional profile and sexual life cycle of this alga by both blue and red light. CraCRY has the uncommon feature of efficient formation and longevity of the semi-reduced neutral form of its FAD cofactor upon blue light illumination. Tyrosine Y 373 plays hereby a crucial role by elongating as fourth member the electron transfer (ET) chain that comprises the tryptophan triad otherwise found in most other cryptochromes and DNA photolyases. Here, we report the full mechanism of light-induced FADH • formation in CraCRY using transient absorption spectroscopy from hundreds of femtoseconds to seconds. Electron transfer starts from ultrafast reduction of excited FAD to FAD •by the proximal tryptophan (0.4 ps) and is followed by delocalized migration of the produced WH •+ radical along the tryptophan triad (3.7 and 55 ps). Oxidation of Y 373 by coupled ET to WH •+ and deprotonation then proceeds in ~800 ps, without any significant kinetic isotope effect, nor a pH effect between pH 6.5 and 9.0. The FAD •-/Y 373 • pair is formed with high quantum yield (~60%); its intrinsic decay by recombination is slow (~50 ms), favoring reduction of Y 373 • by extrinsic agents and protonation of FAD •to form the long-lived, red-light absorbing FADH • species. Possible mechanisms of tyrosine oxidation by ultrafast proton-coupled ET in CraCRY, a process about 40 times faster than the archetypal tyrosine-Z oxidation in photosystem II, are discussed in detail.

Flavin coenzymes are universally found in biological redox reactions. DNA photolyases with their flavin chromophore (FAD) utilize blue light for DNA repair and photoreduction. The latter process involves two single-electron transfers to FAD with an intermittent protonation step to prime the enzyme active for DNA repair. Here we use time-resolved serial femtosecond X-ray crystallography to describe how light-driven electron transfers trigger subsequent nanosecond-to-microsecond entanglement between FAD and its Asn/Arg-Asp redox sensor triad. We found that this key feature within the photolyase-cryptochrome family regulates FAD re-hybridization and protonation. After first electron transfer, the FAD •isoalloxazine ring twists strongly when the arginine closes in to stabilize the negative charge. Subsequent breakage of the arginine-aspartate salt bridge promotes proton transfer from arginine to FAD •-. Our molecular movies demonstrate how the protein environment of redox cofactors organizes multiple electron/proton transfer events in an ordered fashion, which could be applicable to other redox systems such as photosynthesis.

Cryptochromes (CRYs) are an ubiquitously occurring class of photoreceptors, which are important for regulating the circadian rhythm of animals via a time-delayed transcription-translation feedback loop (TTFL). Due to their protein architecture and common FAD chromophore, they belong to the same superfamily as photolyases (PHLs), an enzyme class that repairs UV-induced DNA lesions upon blue light absorption. Apart from their different functions the only prominent structural difference between CRY and PHL is the highly variable C-terminal extension (CTE) of the former. The nature of the CTE is still unclear and highly speculated. In this study, we show by hydrogen/deuterium exchange and subsequent mass-spectrometric analysis that the CTE of the animal-like cryptochrome from the green algae Chlamydomonas reinhardtii ( Cr aCRY) binds to the surface of the photolyase homology region, which flanks the DNA binding site. We also compared the fully oxidized and fully reduced states of the flavoprotein and designed a tool, so called light chamber, for automated HDX-MS measurements of photoreceptors in defined photostates. We could observe some striking differences between the two photostates and propose a model for light-dependent switching of this bifunctional cryptochrome.

We have undertaken a spectral deconvolution of the three FADs of EtfAB/bcd to the spectral changes seen in the course of reduction, including the spectrally distinct anionic and neutral semiquinone states of electron-transferring and bcd flavins. We also demonstrate that, unlike similar systems, no charge-transfer complex is observed on titration of the reduced M. elsdenii EtfAB with NAD + . Finally, and significantly, we find that removal of the et FAD from EtfAB results in an uncrossing of the half-potentials of the bifurcating FAD that remains in the protein, as reflected in the accumulation of substantial FAD• − in the course of reductive titrations of the depleted EtfAB with sodium dithionite.