Integrated structural dynamics uncover new modes of B₁₂ 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.