Sulfoxide synthases are non-hemei ron enzymes that participatei nt he biosynthesis of thiohistidines, such as ergothioneine ando vothiol A. The sulfoxide synthase EgtB from Chloracidobacterium thermophilum (CthEgtB) catalyzes oxidative coupling between the side chains of N-a-trimethyl histidine( TMH) and cysteine (Cys) in ar eaction that entails complete reduction of molecular oxygen,c arbon-sulfur (CÀ S) and sulfur-oxygen (SÀO) bond formationa sw ell as carbon-hydrogen (CÀH) bond cleavage. In this report, we show that CthEgtB and other bacterial sulfoxide synthases cannote fficiently accept selenocysteine (SeCys) as as ubstrate in place of cysteine. In contrast, the sulfoxide synthase from the filamentous fungus Chaetomium thermophilum (CthEgt1) catalyzes CÀSa nd CÀSe bond formation at almost equale fficiency.W ed iscuss evidence suggesting that this functional differenceb etween bacterial and fungal sulfoxide synthases emerges from different modes of oxygen activation.[a] K.

No abstract