Photocobilins integrate B₁₂ and bilin photochemistry for enzyme control

Photoreceptor proteins utilise chromophores to sense light and trigger a biological response. The discovery that adenosylcobalamin (or coenzyme B₁₂) can act as a light-sensing chromophore heralded a new field of B₁₂-photobiology. Although microbial genome analysis indicates that photoactive B₁₂-binding domains form part of more complex protein architectures, regulating a range of molecular–cellular functions in response to light, experimental evidence is lacking. Here we identify and characterise a sub-family of multi-centre photoreceptors, termed photocobilins, that use B₁₂ and biliverdin (BV) to sense light across the visible spectrum. Crystal structures reveal close juxtaposition of the B₁₂ and BV chromophores, an arrangement that facilitates optical coupling. Light-triggered conversion of the B₁₂ affects quaternary structure, in turn leading to light-activation of associated enzyme domains. The apparent widespread nature of photocobilins implies involvement in light regulation of a wider array of biochemical processes, and thus expands the scope for B₁₂ photobiology. Their characterisation provides inspiration for the design of broad-spectrum optogenetic tools and next generation bio-photocatalysts.