Photocobilins integrate B 12 and bilin photochemistry for enzyme control.

Photoreceptor proteins utilise chromophores to sense light and trigger a biological response. The discovery that adenosylcobalamin (or coenzyme B 12 ) can act as a light-sensing chromophore heralded a new field of B 12 -photobiology. Although microbial genome analysis indicates that photoactive B 12 -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 B 12 and biliverdin (BV) to sense light across the visible spectrum. Crystal structures reveal close juxtaposition of the B 12 and BV chromophores, an arrangement that facilitates optical coupling. Light-triggered conversion of the B 12 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 B 12 photobiology. Their characterisation provides inspiration for the design of broad-spectrum optogenetic tools and next generation bio-photocatalysts.