New light on vitamin B12: The adenosylcobalamin-dependent photoreceptor protein CarH

Abstract

Adenosylcobalamin (AdoCbl), or coenzyme B12, is a cofactor for enzymes important in metabolism in humans (and other mammals) and bacteria. AdoCbl contains a Co-C bond and is extremely light sensitive, but, until recently, this light sensitivity appeared to have no physiological function. Recently, AdoCbl has been found to act as cofactor for a photoreceptor protein (CarH) that controls the expression of DNA coding for transcription of the proteins needed for synthesis of carotenes in certain non-photosynthetic bacteria. In 2015, the X-ray crystal structures of two dark states of the photoreceptor protein from the bacterium Thermus thermophilus were determined: CarH bound to AdoCbl and CarH bound to a large portion of the cognate DNA operator (and AdoCbl); a light state was also determined in which CarH was bound to cobalamin in which the Co-C bond had been broken. The breaking of the Co-C bond of Ado-Cbl acts as a trigger for the regulatory switch that allows the transcription of DNA. In the two dark states AdoCbl is bound to a conserved histidine from CarH, which displaces the lower 5,6-dimethylbenzimidazole ligand of AdoCbl. In the light state the 5’-deoxyadenosyl group of AdoCbl is replaced by a second histidine from CarH, giving a bis-histidine cobalamin and 4’,5’-anhydroadenosine. Genes for B12-dependent photoreceptors are widespread in bacteria. Control of DNA transcription may represent an evolutionarily ancient function of AdoCbl, possibly pre-dating its function as a protein cofactor.

Vitamin B 12 in the spotlight again

“…Instead of producing 5′-dAdo• upon light exposure, the nonreactive molecule 4′,5′-anhydroadenosine is formed (Figure 2d) [11]. 4′,5′-anhydroadenosine differs from 5′-dAdo• by one electron and one proton; its mechanism of formation awaits discovery [13]. …”

The Photoactive Excited State of the B12-Based Photoreceptor CarH

“…Natural biological systems use light as an energy source in sensing and feedback. Examples of light-responsive sensors used to produce action in biological systems range broadly from the well-studied rhodopsins to the newly discovered B 12 -dependent CarH photoreceptor family that is widespread in bacteria. − CarH functions as a transcription factor, whose activity is modulated by changes of its oligomeric state through the interaction with 5′-deoxyadenosylcobalamin (AdoCbl, coenzyme B 12 ) and light. In CarH (Thermus thermophilus), photoexcitation of AdoCbl triggers dissociation of its upper axial 5′-deoxyadenosyl group, which is replaced with a histidine ligand to form a bis-His cobalamin (Figure , see also Figure S1).…”

Probing the Excited State of Methylcobalamin Using Polarized Time-Resolved X-ray Absorption Spectroscopy

“…Photoactivated cobalamins (Cbls) provide the opportunity to exert spatial and temporal control over varied outcomes such as radical formation, drug delivery, , gene activation, − and B 12 bioavailability. , Photoinduced bond cleavage is central to both the engineered and the natural biological exploitations of cobalamin photochemistry. Ultrafast time-resolved X-ray spectroscopy with X-ray free electron lasers (XFELs) provides the opportunity to probe structural changes in short-lived states of molecular systems at room temperature and in complex environments without the radiation damage due to radical diffusion, characteristic of synchrotron source measurements.…”