Structural basis for substrate binding and catalytic mechanism of the Se-glycosyltransferase SenB in the biosynthesis of selenoneine

Abstract

Abstract Selenium is a multi-functional trace element essential for diverse organisms. SenB is a Se-glycosyltransferase that incorporates selenium into small molecules in the selenoneine biosynthesis pathway and is also the only known Se-glycosyltransferase in nature. Although the biochemical function of SenB has been investigated, its substrate specificity, structure, and catalytic mechanism remain unclear. Here, we revealed that SenB exhibits sugar donor specificity and promiscuity and can utilize six UDP-sugars to generate selenosugars. The crystal structures of SenB complexed with four different UDP-sugars were solved. The residues N20, T23, and E231 were proven as the key elements that determine the sugar donor promiscuity of SenB. Structure-guided mutagenesis further revealed a novel catalytic triad H58/D86/K158 in SenB, which accounts for the C-Se glycosidic bond formation and Se-P bond cleavage during the Se-glycosylation process. Furthermore, we mined, functionally and structurally characterized two other novel Se-glycosyltransferase, CbSenB and RsSenB, which also exhibit sugar donor promiscuity.