REVERSIBLE, NONDENATURING METAL SUBSTITUTION IN BOVINE ADRENODOXIN AND SPINACH FERREDOXIN AND THE DIFFERENT REACTIVITIES OF [2FE-2S]-CLUSTER-CONTAINING PROTEINS

The non-denaturing substitution of cluster iron by other metals was st udied in spinach ferredoxin and in bovine adrenodoxin. Only some of se veral metal species tested (Cd2+, Zn2+, VO2+, Mn2+, Co2+, Ni2+) caused bleaching of the residual visible absorbance and of the EPR signals o f the reduced ferredoxins. No formation of mixed-metal fluster was obs erved. The most reactive metal species were Cd2+ and Zn2+, and Cd2+ wa s found to react also with oxidized adrenodoxin. Metal-treated protein s were resolved into a mixture of apoprotein, metal-substituted protei n and unreacted holoprotein. Their biological activity was proportiona l to the residual holoprotein concentration, Spinach ferredoxin and ad renodoxin were found to differ substantially with regard to their meta l-substitution reactivity under oxidizing and reducing conditions, rea ction time, and formation of apoprotein, which was more pronounced for spinach ferredoxin. Exchange of cluster iron with Cd2+ in adrenodoxin generated stable species containing 2 mol sulfide/mol protein, and 2 or 5 mol cadmium/mol protein, respectively, The relative amount of the two substitution products depended on the experimental conditions. CD and NMR data on all the cadmium-substituted proteins suggest that iro n replacement led to a significant structural rearrangement. Neverthel ess, all the metal-substituted proteins could be re-converted into the native iron-containing form upon incubation with iron in the absence of reductants, of denaturing agents, and of an external source of sulf ide, The different reactivity of the two proteins is discussed in term s of the cluster environment, along with the possible physiological re levance of these findings.