Analysis of the iron-sulfur cluster of aconitase by natural and magnetic circular dichroism

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Abstract
The Fe-S cluster of [pig heart] aconitase, a high-potential Fe-S protein, was examined by absorption, circular dichroism (CD) and magnetic circular dichroism (MCD) spectroscopy. The MCD spectrum of unactivated aconitase, which is presumably oxidized, is similar to those of reduced 2 Fe-2 sulfide ferredoxins but distinct from the MCD of known 4 Fe-4 sulfide proteins. The magnitude of the natural CD of unactivated aconitase also suggests the absence of 4 Fe-4 S clusters. Reduction of the enzyme with dithionite and activation with the cysteine-ascorbate-ferrous ion activation mixture generate spectra which are significantly different from those of any Fe-S protein seen to date. These results are interpreted as indicating that aconitase does not contain a 4 Fe-4 S cluster generally thought to be characteristic of high-potential Fe-S proteins. It could contain a 2 Fe-2 S center or some other center such as a cyclic 3 Fe 3-S center.

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