Unprecedented Mechanism Employed by the Salmonella enterica EutT ATP: Co(I)rrinoid Adenosyltransferase Precludes Adenosylation of Incomplete Co(II)rrinoids

Abstract

Three distinct families of ATP: corrinoid adenosyltransferases (ACATs) exist that are capable of converting vitamin B-12 derivatives into coenzyme B-12 by catalyzing the thermodynamically challenging reduction of Co(II)rrinoids to form "supernucleophilic" Co-I intermediates. While the structures and mechanisms of two of the ACAT families have been studied extensively, little is known about the EutT enzymes beyond the fact that they exhibit a unique requirement for a divalent metal cofactor for enzymatic activity. In this study we have obtained compelling evidence that EutT converts cob(II) alamin into an effectively four-coordinate Co-II species so as to facilitate Co-II -> Co-I reduction. Intriguingly, EutT fails to promote axial ligand dissociation from the substrate analogue cob(II) inamide, a natural precursor of cob(II) alamin. This unique substrate specificity of EutT has important physiological implications.