Curated Optogenetic Publication Database

Abstract: Photoreceptor proteins regulate fundamental biological processes such as vision, photosynthesis and circadian rhythms1. A large photoreceptor subfamily uses vitamin B12 derivatives for light sensing2, contrasting with the well-established mode of action of these organometallic derivatives in thermally activated enzymatic reactions3. The exact molecular mechanism of B12 photoreception and how this differs from the thermal pathways remains unknown. Here we provide a detailed description of photoactivation in the prototypical B12 photoreceptor CarH4,5 from nanoseconds to seconds, combining time-resolved and temperature-resolved structural and spectroscopic methods with quantum chemical calculations. Building on the crystal structures of the initial tetrameric dark and final monomeric light-activated states5, our structural snapshots of key intermediates in the truncated B12-binding domain illustrate how photocleavage of a cobalt-carbon (Co-C) bond within the B12 chromophore adenosylcobalamin triggers a series of structural changes that propagate throughout CarH. Breakage of the photolabile Co-C5' bond leads to the formation of a previously unknown adduct that links the C4' position of the adenosyl moiety to the Co ion and can subsequently be cleaved thermally over longer timescales to allow release of the adenosyl group, ultimately causing tetramer dissociation4,5. This adduct, which differentiates CarH from thermally activated B12 enzymes, steers the photoactivation pathway and acts as the molecular bridge between photochemical and photobiological timescales. The biological relevance of our study is corroborated by kinetic data on full-length CarH in the presence of DNA. Our results offer a spatiotemporal understanding of CarH photoactivation and pave the way for designing B12-dependent photoreceptors for optogenetic applications.

Abstract: The discovery of naturally occurring B12-depedent photoreceptors has allowed for applications of cobalamins (Cbls) in optogenetics and synthetic biology to emerge. However, theoretical investigations of the complex mechanisms of these systems have been lacking. Adenosylcobalamin (AdoCbl)-dependent photoreceptor, CarH, is one example and it relies on daylight to perform its catalytic function. Typically, in enzymes employing AdoCbl as their cofactor, the Co-C5' bond activation and cleavage is triggered by substrate binding. The cleavage of the Co-C5' bond is homolytic resulting in radical pair formation. However, in CarH, this bond is instead activated by light. To explore this peculiarity, the ground and first excited state potential energy surfaces (PESs) were constructed using the quantum mechanics/molecular mechanics (QM/MM) framework and compared with other AdoCbl-dependent enzymes. QM/MM results indicate that CarH is photolytically active as a result of the AdoCbl dual role, acting as a radical generator and as a substrate. Photo-cleavage of the Co-C5' bond and subsequent H-atom abstraction is possible because of the specific orientation of the H-C4' bond with respect to the Co(II) center. Comparison with other AdoCbl-dependent enzymes indicate that the protein environment in the CarH active center alters the photochemistry of AdoCbl by controlling the stereochemistry of the ribose moiety.