Biological radical sulfur insertion reactions

Two recent discoveries gave new insights into the understanding of sulfur metabolism and mechanisms by which sulfur atoms are incorporated into biomolecules in biosynthetic reactions. First, it is established that cysteine is the common sulfur source from which sulfur atoms can be mobilized by cysteine desulfurases. Second, a number of enzymes such as BioB, LipA, and MiaB, catalyzing sulfur atom insertion reactions, are members of the radical-SAM enzyme superfamily. The most intriguing issue concerns the nature of the direct sulfur donor to the radical intermediates. Based on the discovery of the involvement of cysteine desulfurase activities in biotin synthase, it is suggested that persulfides play an important role as reactive intermediates also in radical sulfur insertion reactions.