Mössbauer studies of Escherichia coli biotin synthase: evidence for reversible interconversion between [2Fe-2S](2+) and [4Fe-4S](2+) clusters.

The nature and properties of the iron-sulphur (Fe-S) cluster in as-prepared and reduced biotin synthase of Escherichia coli have been investigated by Mössbauer spectroscopy. Our data clearly demonstrate that in the as-prepared sample, the cluster is present as [2Fe-2S](2+) with isomer shift, delta = 0.29 mm/s and quadrupole splitting, DeltaE(Q) = 0.53 mm/s, indicating incomplete cysteinyl-S coordination. Anaerobic reduction by dithionite in the presence of 55% (v/v) glycerol converts this form to [4Fe-4S](2+) (delta = 0.45 mm/s and DeltaE(Q) = 1.11 mm/s) and is accompanied by some destruction to Fe(2+). This cluster conversion is reversible and when exposed to air, the [4Fe-4S](2+) cluster is quantitatively reconverted to the [2Fe-2S](2+) cluster without any further cluster degradation.