163

Quantum Compass of Migratory Birds

FIGURE 4.10 Photoexcitation of Arabidopsis thaliana cryptochrome 1 (AtCRY1) (Liu et al., 2011). (a) Five possible

redox forms of favins. Te two diferent forms of semiquinone radicals: anion radical (e.g., FAD⁻), neutral radical

(e.g., FADH^•), two forms of reduced favins: protonated hydroquinone (e.g., FADH2), and anionic hydroquinone

(e.g., FADH⁻) are shown. R: side groups of favins. (b) Te photolyase-like cyclic electron shuttle model of CRY pho­

toexcitation. In this model, the resting state of a CRY contains the anion radical semiquinone (FAD⁻). Upon photon

absorption, the excited FAD⁻ transfers an electron to ATP, triggering phosphotransfer and autophosphorylation

of the CRY. Te electron is subsequently transferred back to favin to complete the cycle. Te putative locations of

phosphorous group and electron transfer path are indicated. (Reproduced with permission from Liu et al., 2011,

Copyright 2011, Elsevier.) It is licensed under the Creative Commons Attribution 4.0 International.

trigger states: the favin radical FADH^•, the tyrosyl radical TyrO^•, the oxidized form Dox of an external

electron donor that reduced TyrO^•, and the reduced form Ared of an external electron acceptor that

oxidized FADH^• (Figure 4.9).

Te FAD is a two electron carrier that can exist in one of the three diferent redox states or fve

diferent protonated forms: oxidized favin (FAD), semireduced favin (anion radical FAD^•− or neutral

radical FADH^•), and fully reduced favin (FADH⁻ or FADH2) in AtCRY1 (Liu et al., 2011, Figure 4.10a).

Here, FADH^• is the semiquinone form of FAD and a radical species in which proton transfer occurs

following electron transfer. Among the diferent redox forms, only the oxidized favin, FAD, and anion

radical semiquinone favin, FAD^•− (FAD* in Figure 4.9), absorb signifcant amounts of blue light (~400–

500 nm). It has been proposed that the oxidized favin, FAD may be the ground state chromophore of

AtCRY1, because it absorbs blue light most efectively.

Te CRY contains two domains, the N-terminal PHR (Photolyase-Homologous Region) domain

of about 500 residues, and the CRY C-terminal extension (CCE) domain of various lengths and

sequences (Liu et al., 2011, Figure 4.10b). PHR is the chromophore-binding domain of CRYs that bind