Integrated structural dynamics uncover new modes of B ₁₂ photoreceptor activation

Photoreceptor proteins initiate, regulate and control fundamental biological processes such as vision, photosynthesis and circadian rhythms ¹ . A large photoreceptor subfamily uses vitamin B ₁₂ derivatives for light sensing ² , contrasting with the well-established mode of action of these organometallic derivatives in thermally activated enzymatic reactions ³ . The molecular mechanism of B ₁₂ photoreception and how this differs to the thermal pathways remain unknown. Here we provide a detailed spatio-temporal description of photoactivation in the prototypical tetrameric B ₁₂ photoreceptor CarH ^4,5 from nanoseconds to seconds by using an integrative approach, combining time- and temperature-resolved structural and spectroscopic methods with quantum chemical calculations. High resolution structural snapshots of key intermediates illustrate how photocleavage of a Co–C bond triggers a pathway of structural changes that propagate throughout CarH from the B ₁₂ chromophore, via a previously unknown adduct, to finally cause tetramer dissociation. These unique intermediates, which differentiate CarH from thermally-activated B ₁₂ enzymes, steer the photoactivation pathway and act as the molecular bridge between photochemical and photobiological timescales. Our results offer a spatio-temporal understanding of CarH photoactivation and pave the way for designing and optimising B ₁₂ -dependent photoreceptors for future optogenetic applications.