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

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Abstract
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, δ=0.29 mm/s and quadrupole splitting, ΔE 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+ (δ=0.45 mm/s and ΔE Q=1.11 mm/s) and is accompanied by some destruction to Fe2+. 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.