The last enzyme (LytB) of the methylerythritol phosphate pathway for isoprenoid biosynthesis catalyzes the reduction of (E)-4-hydroxy-3-methylbut-2-enyl diphosphate into isopentenyl diphosphate and dimethylallyl diphosphate. This enzyme possesses a dioxygen-sensitive [4Fe^4S] cluster. This prosthetic group was characterized in the Escherichia coli enzyme by UV/visible and electron paramagnetic resonance spectroscopy after reconstitution of the puri¢ed protein. Enzymatic activity required the presence of a reducing system such as £avodoxin/£avodoxin reductase/reduced nicotinamide adenine dinucleotide phosphate or the photoreduced deaza£avin radical.

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Biotin synthase catalyses the last step of the biosynthesis of biotin in microorganisms and plants. The active protein isolated from Bacillus sphaericus and Escherichia coli contains an iron-sulphur (FeS) cluster. The native enzymes were depleted of their iron and inorganic sulphide and the resulting apoenzymes were chemically reconstituted with FeCl3 and 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.