Structural and biochemical analysis of a B12 superbinder.

A prominent bacterial family in the human gut is Bacteroidetes, for which cobalamin (vitamin B12) is an essential micronutrient. Here, we characterized the structural-function relationships underlying the role of three BtuG family proteins, BtuG1, BtuG2, and BtuG3, in cobalamin acquisition. We solved crystal structures (resolution between 2 and 1.5 Å) of these homologs bound to different cobalamin forms and the precursor cobinamide, and measured the binding affinities by grating coupled interferometry. We observed high binding affinities, with KD values in the low picomolar range, underlining their role in cobalamin scavenging. Mutations based on the structures resulted in altered binding affinities for cobalamin and its precursor. This study provides structural and quantitative insights into the B12 scavenging mechanism of BtuG proteins.