Core extrusion of the bound iron-sulfur centers from spinach Photosystem I showed the presence of [2Fe-2S] clusters as well as [4Fe-4S] clusters among FA, FB and FX. Extrusion of the iron-sulfur ensemble was not quantitative; however, the presence of [2Fe-2S] clusters correlated with higher concentration of unfolding solvent. Since FX is highly resistant to denaturation, and since FA and FB are known to contain [4Fe-4S] clusters, the [2Fe-2S] clusters are assigned to FX. The presence of [2Fe-2S] clusters in Photosystem I has significance in the structure and organization of FX on the reaction center. Since four cysteinyl ligands are assumed to hold an iron-sulfur cluster, a Photosystem I subunit may consist of two approx. 64-kDa proteins bridged by a single [2Fe-2S] cluster. The complete reaction center would consist of two subunits positioned so that two [2Fe-2S] clusters are in magnetic interaction, thereby constituting FX.
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