Alternative Photoreduction Pathway Involving Electron Tunneling and Proton Transfer in a Bifunctional Cryptochrome

The significance of the photoreduction channel via the tryptophan triad in the cryptochrome (CRY) has been generally recognized. However, there is literature reporting the retention of biological function in the primary electron transfer (ET) pathway-impaired mutants in some CRY species. In this work, a secondary ET pathway is identified in the animal-like <i>Chlamydomonas reinhardtii</i> cryptochrome (<i>Cr</i>aCRY) in the opposite direction of the primary ET pathway, involving ultrafast electron tunneling and proton transfer. Through sequence alignment, it is found that the proximal Trp (W_1') from the secondary ET pathway is conserved across all CRY species, which could be absent in the ancestral protein photolyases. This alternative ET pathway presumably triggers photoinduced conformational changes when the primary pathway is shut down upon random missense mutation, contributing to functional robustness.