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Bioelectromagnetism

 

property: the anisotropy of the singlet yield is markedly (by an order of magnitude) larger in RPs com­

bining FAD with a partner radical with no signifcant hyperfne interactions, such as O^•

2

− , or better a

hypothetic variety of it not subject to fast spin relaxation, conventionally denoted Z^• (Lee et al., 2014).

Cryptochromes undergo forward light-induced reactions involving electron transfer to excited state

favin to generate radical intermediates, which correlate with biological activity. A mechanism for

the reverse reaction, namely dark reoxidation of protein-bound favin in Arabidopsis cryptochrome

(AtCRY1) by molecular oxygen, involves the formation of a spin-correlated FADH-superoxide RP (Muller

and Ahmad, 2011). Under conditions of illumination, the cryptochrome photoreceptors are constantly

cycling between inactive (oxidized) and activated (reduced) redox states, such that the net biological

activity results from the sum of the light-induced (activating) and reverse (de-activating) redox reac­

tions at any given timepoint. A model of the cryptochrome photocycle incorporating these elements

and an estimation of the quantum efciency of redox state interconversions both in vitro and in vivo

has been recently derived (Procopio et al., 2016). However, when light and dark intervals are given inter­

mittently, the plant MFE is observed even when the MF is given exclusively during the dark intervals

between light exposures. Tis indicates that the magnetically sensitive reaction step in the cryptochrome

photocycle must occur during favin reoxidation, and likely involves the formation of reactive oxygen

species (Pooam et al., 2019). A recent model of MFE on the cryptochrome photocycle involves activation

of cryptochrome by favin reduction which triggers conformational change leading to unfolding and

subsequent phosphorylation of the C-terminal domain. Te favin is subsequently reoxidized by reaction

with molecular oxygen that occurs independently of light (Ahmad, 2016). Te efect of an applied MF on

the cryptochrome photocycle occurs during the period of favin reoxidation. Te most likely efect is to

alter the rate constant of reoxidation of the reduced favin intermediates, and thereby alter the lifetime

of the activated state (Figure 5.11). As discussed above, theoretical considerations have argued against a

favin/superoxide radical pair, which is formed in the course of favin reoxidation as the magnetosensing

intermediate in cryptochromes (Hore and Mouritsen, 2016); however, cryptochrome localized within liv­

ing cells is in contact with many cellular metabolites, which, moreover, can move into the favin pocket

FIGURE 5.11 Model of MFE on the cryptochrome photocycle, see text for explanation. (Modifed from Pooam

et al. (2019).)