Ultrafast Oxidation of a Tyrosine by Proton-Coupled Electron Transfer Promotes Light Activation of an Animal-like Cryptochrome

Abstract

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.

Structure–activity relationship and mechanism of four monostilbenes with respect to ferroptosis inhibition

“…[57][58][59][60] Herein, hydrogen donation is not limited to the typical hydrogen-atom-transfer; other antioxidant pathways could also result in hydrogen donation, such as proton-coupled electron transfer. 54,55,[61][62][63] The hydrogen donation and subsequent dimerization reactions are shown in Fig. 4, where the isorhapontigenin was linked at 2,5 0 in agreement with the results of previous studies [57][58][59][60][61][62][63][64][65][66][67] and our MS analysis (Fig.…”

“…14,15,[51][52][53] However, electron donation is always accompanied by proton donation in cells. 54,55 The net result of electron and proton donation is generally equivalent to that of whole hydrogen donation.…”

Characterization of Light-Induced, Short-Lived Interacting Radicals in the Active Site of Flavoprotein Ferredoxin-NADP+ Oxidoreductase

“…21 Similar reaction products have later also been observed upon excitation of oxidized flavin in the related cryptochrome blue-light sensors. 16,22,23 More recently, the flavin excited state quenching by tryptophan has been examined in detail in artificial protein maquettes. 24,25 In contrast to tryptophan, for a long time it had been assumed that the oxidation of tyrosine is necessarily accompanied by a concerted deprotonation due to its extremely low pKa (~−2).…”

Photoactivation of Cryptochromes Invokes Competing Inter- and Intramolecular Electron Transfer

“…Chlamydomonas reinhardtii (400-480 fs) 47,81 and two flavoproteins of the cryptochrome photolyase family from the green algae Ostreococcus tauri (390-590 fs). 124 Note that the absorption spectra of adenine and the adenine radical cation have not been included in the experimental analysis, and the fs-ps time-resolved spectroscopy experiments have been conducted on the different proteins, not on the one from Arabidopsis thaliana used in our simulations.…”

“…45 The tyrosyl radical, for which the hyperfine couplings are as strong as for the tryptophan radical, 27,46 is experimentally found to be formed on the microsecond timescale in the plant cryptochrome. 41 Moreover, it has been recently reported that the tyrosine Y373 serves as the fourth electron donor in the ET/proton-coupled ET chain in the cryptochrome from Chlamydomonas reinhardtii with a radical pair lifetime of ≈ 50 ms. 47 Therefore, despite thorough and extensive theoretical and experimental studies of cryptochromes and their homologs, mechanistic features of the PET specifically for cryptochromes are still missing. The present work revisits and sheds new light on the role of FAD itself, and cellular metabolites, in photoactivation of cryptochromes.…”