Vitamin B12 (cobalamin) is a complex organometallic cofactor and the most complex vitamin1, consisting of a corrin ring containing a cobalt atom in the centre, coordinated with an upper ligand (such as adenosyl or methyl group) and a lower ligand anchored to the ring through a nucleotide loop. The upper ligand contains the chemical reactivity, directly participating in reactions, while the lower ligand provides functional specificity. There are three different families of lower ligands; benzimidazoles, purines and phenolics. The nature of this ligand is important as enzymes belonging to different species can require different lower ligands to be active. Vitamin B12 is involved in a wide variety of metabolic processes, and for many organisms it is an essential cofactor for the final enzymatic reaction of the l-methionine biosynthesis pathway in the cytoplasm. Despite its many roles in eukaryotic and prokaryotic cells, only a small group of microorganisms is able to produce vitamin B12. However, its synthesis is energetically expensive, requiring ~30 enzymatic steps. As a result, microorganisms have developed mechanisms to take up exogenous cobalamins, named the salvage route. In Gram-negative bacteria, the translocation of cobalamins presents challenges given that three different compartments need to be crossed: the outer membrane (OM), the periplasm and the inner membrane (IM). The best-characterised B12 transport system in Gram-negatives is that of Escherichia coli, comprising the OM TonB-dependent transporter (TBDT) BtuB, the periplasmic binding protein BtuF, and the BtuCD ABC transporter located in the IM. Here you can see a crystal structure of the Bacteroides thetaiotaomicron B12 TonB dependent transporter in complex with a surface lipoprotein (PDB code: 8BMX)

#molecularart ... #immolecular ... #vitamin ... #b12 ... #bacteroides ... #transport ... #membrane ... #tonB ... #xray

Structure rendered with @proteinimaging and depicted with @corelphotopaint