On the role of the 2Fe-2S cluster in the formation of the structure of spinach ferredoxin

The conformational changes of spinach ferredoxin in the apoprotein ⇌ holoprotein conversion have been studied by the far-ultraviolet circular dichroic technique. Apoferredoxin shows a largely disordered structure in solution. Evidence is presented, based on the effects either of chemical removal or of enzymic insertion of the 2Fe-2S center, that the cluster itself acts as the major force in determining the structure of spinach ferredoxin. The presence of sodium chloride cannot reverse the loss of structure consequent to the removal of the cluster, but stabilizes the residual structure once the cluster has been removed. Upon urea treatment of the apoferredoxin all the residual structural elements are lost even in the presence of sodium chloride.