Structure-Based Demystification of Radical Catalysis by a Coenzyme B12 Dependent Enzyme-Crystallographic Study of Glutamate Mutase with Cofactor Homologues

The Rhodium Analogue of Coenzyme B12 as an Anti-Photoregulatory Ligand Inhibiting Bacterial CarH Photoreceptors

Coenzyme B12 (AdoCbl; 5'-deoxy-5'-adenosylcobalamin), the quintessential biological organometallic radical catalyst, has a formerly unanticipated, yet extensive, role in photoregulation in bacteria. The light-responsive cobalt-corrin AdoCbl performs this nonenzymatic role by facilitating the assembly of CarH photoreceptors into DNA-binding tetramers in the dark, suppressing gene expression. Conversely, exposure to light triggers the decomposition of this AdoCbl-bound complex by a still elusive photochemical mechanism, activating gene expression. Here, we have examined AdoRhbl, the non-natural rhodium analogue of AdoCbl, as a photostable isostructural surrogate for AdoCbl. We show that AdoRhbl closely emulates AdoCbl in its uptake by bacterial cells and structural functionality as a regulatory ligand for CarH tetramerization, DNA binding, and repressor activity. Remarkably, we find AdoRhbl is photostable even when bound "base-off/His-on" to CarH in vitro and in vivo. Thus, AdoRhbl, an antivitamin B12, also represents an unprecedented anti-photoregulatory ligand, opening a pathway to precisely target biomimetic inhibition of AdoCbl-based photoregulation, with new possibilities for selective antibacterial applications. Computational biomolecular analysis of AdoRhbl binding to CarH yields detailed structural insights into this complex, which suggest that the adenosyl group of photoexcited AdoCbl bound to CarH may specifically undergo a concerted non-radical syn-1,2-elimination mechanism, an aspect not previously considered for this photoreceptor.

Target-Induced Photocurrent-Polarity-Switching PEC Sensing Platform Based on In Situ Generation of Oxygen Vacancy-Modulated Energy Band Structures

The polarity of the photocurrent can be modulated by tunable bipolar photoelectrochemical (PEC) behavior, which is anticipated to address the issues of high background signal caused by traditional unidirectional increasing/decreasing response and false-positive/false-negative problems. Here, a new approach is suggested for the first time, which employs a target-induced enzyme-catalyzed reaction and in situ oxygen vacancy (OV) generation to achieve heterojunction photocurrent switching for highly sensitive detection of alkaline phosphatase (ALP). Among them, the ALP can catalyze the decomposition of ascorbic acid phosphate to produce ascorbic acid, which not only acts as an electron donor to change the redox environment but also acts as a reducing agent to introduce OVs into BiOBr semiconductors in cooperation with illumination. The introduction of vacancies can effectively modulate the energy band structure of BiOBr, while with the change of redox conditions, the transfer path of photogenerated carriers is changed, thus realizing the switching of photocurrents, which leads to its use in the construction of a negative-background anti-interference PEC sensing platform, achieving a wide linear range from 0.005 to 500 U·L-1 with a low detection limit of 0.0017 U·L-1. In conclusion, the photocurrent switching operation of this system is jointly regulated by chemistry, optics, and carrier motion, which provides a new idea for the construction of a PEC sensing platform based on photocurrent polarity switching.

The inorganic chemistry of the cobalt corrinoids - an update

The inorganic chemistry of the cobalt corrinoids, derivatives of vitamin B₁₂, is reviewed, with particular emphasis on equilibrium constants for, and kinetics of, their axial ligand substitution reactions. The role the corrin ligand plays in controlling and modifying the properties of the metal ion is emphasised. Other aspects of the chemistry of these compounds, including their structure, corrinoid complexes with metals other than cobalt, the redox chemistry of the cobalt corrinoids and their chemical redox reactions, and their photochemistry are discussed. Their role as catalysts in non-biological reactions and aspects of their organometallic chemistry are briefly mentioned. Particular mention is made of the role that computational methods - and especially DFT calculations - have played in developing our understanding of the inorganic chemistry of these compounds. A brief overview of the biological chemistry of the B₁₂-dependent enzymes is also given for the reader's convenience.

Structural Insights into the Very Low Activity of the Homocoenzyme B12 Adenosylmethylcobalamin in Coenzyme B12 -Dependent Diol Dehydratase and Ethanolamine Ammonia-Lyase

The X-ray structures of coenzyme B12 (AdoCbl)-dependent eliminating isomerases complexed with adenosylmethylcobalamin (AdoMeCbl) have been determined. As judged from geometries, the Co-C bond in diol dehydratase (DD) is not activated even in the presence of substrate. In ethanolamine ammonia-lyase (EAL), the bond is elongated in the absence of substrate; in the presence of substrate, the complex likely exists in both pre- and post-homolysis states. The impacts of incorporating an extra CH2 group are different in the two enzymes: the DD active site is flexible, and AdoMeCbl binding causes large conformational changes that make DD unable to adopt the catalytic state, whereas the EAL active site is rigid, and AdoMeCbl binding does not induce significant conformational changes. Such flexibility and rigidity of the active sites might reflect the tightness of adenine binding. The structures provide good insights into the basis of the very low activity of AdoMeCbl in these enzymes.