Biotin synthase mechanism: on the origin of sulphur

Biotin synthase catalyses the last step of the biosynthesis of biotin in microorganisms and plants. The active protein isolated from Bacillus sphaericusand Escherichia colicontains an iron‐sulphur (FeS) cluster. The native enzymes were depleted of their iron and inorganic sulphide and the resulting apoenzymes were chemically reconstituted with FeCl3and Na2[34S] to give labelled (Fe34S) enzymes. These enzymes were functional and when assayed in vitro produced labelled biotin containing about 65% of 34S. These data strongly support the hypothesis that the sulphur of biotin is derived from the (FeS) centre of the enzyme.