Biotin synthase (BioB), an iron-sulfur enzyme, catalyzes the last step of the biotin biosynthesis pathway. The reaction consists in the introduction of a sulfur atom into two non-activated C-H bonds of dethiobiotin. Substrate radical activation is initiated by the reductive cleavage of S-adenosylmethionine (AdoMet) into a 5'-deoxyadenosyl radical. The recently described pyridoxal 5'-phosphate-bound enzyme was used to show that only one molecule of AdoMet, and not two, is required for the formation of one molecule of biotin. Furthermore 5'-deoxyadenosine, a product of the reaction, strongly inhibited biotin formation, an observation that may explain why BioB is not able to make more than one turnover. However this enzyme inactivation is not irreversible.