Determination of Radical–Radical Distances in Light‐Active Proteins and Their Implication for Biological Magnetoreception

Abstract

Light-generated short-lived radial pairs have been suggested to play pivotal roles in cryptochromes and photolyases.Cryptochromes are very probably involved in magnetic compass sensing in migratory birds and the magnetic-fielddependent behavior of insects.W ee xamined photo-generated transient states in the cryptochrome of Drosophila melanogaster and in the structurally related DNA-repair enzyme Escherichia coli DNAp hotolyase.U sing pulsed EPR spectroscopy, the exchange and dipolar contributions to the electron spin-spin interaction were determined in astraightforward and direct way.W itht hese parameters,r adical-pair partners maybeidentified and the magnetoreceptor efficiency of cryptochromes can be evaluated. We present compelling evidence for an extended electron-transfer cascade in the Drosophila cryptochrome,a nd identified W394 as ak ey residue for flavin photoreduction and formation of as pincorrelated radical pair with as ufficient lifetime for highsensitivity magnetic-field sensing.Biological magnetoreception remains enigmatic with very different concepts under discussion. Among them is aradical pair (RP) based mechanism, [1] which gained considerable attention [2] after the discovery of cryptochromes. [3] Cryptochromes are versatile proteins present in all kingdoms of life with intriguing functions, [2b, 4] such as the resetting of the circadian clock [5] or the sensing of magnetic fields to perceive direction.[6] Their photoreceptor function is most likely driven by ar eduction of the flavin adenine dinucleotide (FAD) cofactor. [7] Even though the exact role of cryptochromes in magnetoreception is still unclear,t heir ability to readily conduct electron transfer (ET) upon blue-light exposure makes them prime candidates for ar ealization of the RP mechanism. Thei ntraprotein ET forms ap air of correlated spins,o ne situated on the FAD, the electron acceptor in the protein core,and the other one on an amino acid residue,the ultimate electron donor, which, in most cases,isatryptophan (Trp) residue at the protein surface.[8] Thee volution of the spin multiplicity of the RP,n amely singlet (S) versus triplet (T), determines the fate of this excited state,w hich either undergoes charge recombination from the singlet configuration or generates longer-lived paramagnetic products from the triplet manifold because direct charge recombination is spin-forbidden from the triplet state.[1] In amagnetoreceptor, the interconversion between the two spin multiplicities,S,T, is governed by the strength of the external magnetic field and furthermore sensitively depends on the couplings of the magnetic dipole moments associated with the electron spins to those of close-by nuclear spins,a nd on their mutual spinspin interaction, which has contributions from magnetic dipolar coupling and electronic exchange.Whereas the magnetic parameters of the individual trapped radicals of FADa nd Trph ave been characterized rather extensively in terms of their hyperfine interactions and g-matrices [9] by electron paramagnet...

Magnetic sensitivity mediated by the Arabidopsis blue-light receptor cryptochrome occurs during flavin reoxidation in the dark

“…6 for model of the effect of the MF on the cryptochrome photocycle). It should be added that attempts in the literature to link biological magnetosensing to forward electron transfer events in cryptochrome (see, e.g., Solov'yov et al 2007;Maeda et al 2012;Mouritsen and Hore 2012;Dodson et al 2013;Biskup 2013;Evans et al 2015;Hong and Pachter 2015;Lüdemann et al 2015;Müller et al 2015;Bialas et al 2016;Kattnig et al 2016;Nohr et al 2016Nohr et al , 2017Worster et al 2016;Van der Straeten et al 2018) are not supported by the biological data neither in birds (Wiltschko et al 2016) nor (as shown in this study) in plants.…”

The biophysical, molecular, and anatomical landscape of pigeon CRY4: A candidate light-based quantal magnetosensor

“…S2C and Table 1). This yielded electron-electron interaction parameters of −8.2 and +0.03 MHz for D and J ex , respectively, from which a distance of 2.12 nm can be calculated (22). This distance closely mirrors that for FAD-TrpD (2.09 nm) by crystallography (20) and permits exclusion of the involvement of a [FAD•••TrpC] radical pair (1.75 nm) ( Table 1) in wild-type clCRY4.…”

On the optimal relative orientation of radicals in the cryptochrome magnetic compass

“…The effect has been attributed to the Radical Pair Mechanism (RPM), which relates reaction outcomes to hyperfine-driven spin dynamics of a photo-generated radical pair within cryptochrome [4][5][6]. The hyperfine mechanism applies here despite the immobilisation of the radicals in the protein matrix, because an efficient charge-separation pathway exists that yields a well-separated radical pair in which inter-radical interactions are weak [18] compared to the dominating hyperfine interactions. In the mentioned in vitro experiments, this pair consists of a flavin adenine dinucleotide co-factor (FAD, here abbreviated as F) anion and a cation derived from the third tryptophan (W) of a highly-conserved electron transfer chain (the so-called tryptophan triad/tetrad [19]; see Figure 1A), i.e.…”