Further studies of the membrane-bound iron-sulfur proteins and P700 in a photosystem I subchloroplast particle

The denaturation by urea-ferricyanide of the membrane-bound iron-sulfur proteins in a spinach photosystem I particle was investigated further. We observed the following results: (i) During the course of the denaturation, the conversion of labile sulfide to zerovalence sulfur proceeds with an equivalent loss of nonheme iron. (ii) Particles depleted of nonheme iron and labile sulfide lack electron spin resonance Centers A and B after chemical or photochemical reduction. (iii) The inorganic sulfide, previously shown to be regenerated in the inactivated particle by dithiothreitol (J. H. Golbeck, S. Lien, and A. San Pietro, 1977, Arch. Biochem. Biophys. 178, 140-150), becomes bound to the particle only if inorganic iron is present. (iv) This binding to the particle is probably in the form of an artificial iron-sulfur cluster, inasmuch as electron spin resonance and photochemical assays failed to reveal recovery of activity. (v) During the course of urea-ferricyanide denaturation, the relationship between the remaining bound iron-sulfur protein and the flash-induced P700 oxidation proved to be strictly linear. This implies that all 10-12 Fe-S pairs which are present in each photosystem I unit show equal sensitivity to oxidation by urea-ferricyanide. Consequently, we suggest the possibility that the bound iron-sulfur centers are located on a single protein.