Formation of both C-S bonds (71, 78, 80, 99, 100). Investigations of the mechanism of LipA sulfur insertion were guided by the extensive prior work on biotin synthase (BioB), the enzyme that inserts the sulfur atom of the biotin thiophene ring. BioB makes two C-S bonds to a single sulfur atom by the replacement of two unactivated hydrogen atoms. LipA also makes two C-S bonds but to different sulfu