Vitamin B12 is a complex organometallic cofactor utilized by some enzymes to catalyze chemically challenging reactions. In mammals, it is an essential micronutrient that is obtained from the diet and whose absence is incompatible with life. Clinical genetics studies on patients with hereditary disorders of vitamin B12 metabolism led to the identification of nine genes, which suggested the existence of a complex B12 trafficking pathway. Biochemical studies in our laboratory have been focused on the chaperones that escort B12 from its point of entry into the cell to its two targets: cytoplasmic methionine synthase and mitochondrial methymalonyl-CoA mutase. In the mitochondrial branch of the B12 trafficking pathway, adenosyltransferase is the enzyme that converts inactive B12 into the coenzyme form and transfers it to the mutase. Prior to this study, little was known about how the adenosyltransferase signals to the mutase when the cofactor is ready for transfer. We have obtained crystallographic snapshots of adenosyltransferase, which provided unexpectedly rich structural insights into how mobile loops construct a dynamic pocket for B12, controlling binding affinity and coordination geometry and thereby, cofactor reactivity. Protein dynamics promote transfer of the solvent exposed active cofactor to the mutase while averting transfer of the inactive and buried cofactor form. While X-ray crystallography typically captures thermodynamically stable states of a protein, in this instance, it provided unexpected new insights into protein and cofactor dynamics. The study also revealed an important role for the dimethylbenzimidazole tail in the cofactor for its translocation between protein active sites.

Mobile loop dynamics in adenosyltransferase control binding and reactivity of coenzyme B12

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.