Mössbauer study of the native, reduced and substrate‐reacted Desulfovibrio gigas aldehyde oxido‐reductase

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Abstract
The Desulfovibrio gigas aldehyde oxido‐reductase contains molybdenum and iron‐sulfur clusters. Mössbauer spectroscopy was used to characterize the iron‐sulfur clusters. Spectra of the enzyme in its oxidized, partially reduced and benzaldehyde‐reacted states were recorded at different temperatures and applied magnetic fields. All the iron atoms in D. gigas aldehyde oxido‐reductase are organized as [2Fe‐2S] clusters. In the oxydized enzyme, the clusters are diamagnetic and exhibit a single quadrupole doublet with parameters (ΔEQ= 0.62 ± 0.02 mm/s and δ= 0.27 ± 0.01 mm/s) typical for the [2Fe‐2S]2+ state. Mössbauer spectra of the reduced clusters also show the characteristics of a [2Fe‐2S]1+ cluster and can be explained by a spin‐coupling model proposed for the [2Fe‐2S] cluster where a high‐spin ferrous ion (S= 2) is antiferromagnetically coupled to a high‐spin ferric ion (S= 5/2) to form a S= 1/2 system. Two ferrous sites with different ΔEQ values (3.42 mm/s and 2.93 mm/s at 85 K) are observed for the reduced enzyme, indicating the presence of two types of [2Fe‐2S] clusters in the D. gigas enzyme. Taking this observation together with the re‐evaluated value of iron content (3.5 ± 0.1 Fe/molecule), it is concluded that, similar to other Mo‐hydroxylases, the D. gigas aldehyde oxido‐reductase also contains two spectroscopically distinguishable [2Fe‐2S] clusters.

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