Purification and properties of the intact P-700 and F _x-containing Photosystem I core protein

The intact Photosystem I core protein, containing the psaA and psaB polypeptides, and electron transfer components P-700 through F_x, was isolated from cyanobacterial and higher plant Photosystem I complexes with chaotropic agents followed by sucrose density ultracentrifugation. The concentrations of NaClO₄, NaSCN, NaI, NaBr or urea required for the functional removal of the 8.9 kDa, F_A/F_B polypeptide was shown to be inversely related to the strength of the chaotrope. The Photosystem I core protein, which was purified to homogeniety, contains 4 mol of acid-labile sulfide and has the following properties: (i) the F_x-containing core consists of the 82 and 83 kDa reaction center polypeptides but is totally devoid of the low-molecular-mass polypeptides; (ii) methyl viologen and other bipyridilium dyes have the ability to accept electrons directly from F_x; (iii) the difference spectrum of F_x from 400 to 900 nm is characteristic of an iron-sulfur cluster; (iv) the midpoint potential of F_x, determined optically at room temperature, is 60 mV more positive than in the control; (v) there is indication by ESR spectroscopy of low-temperature heterogeniety within F_x; and (vi) the heterogeneity is seen by optical spectroscopy as inefficiency in low-temperature electron flow to F_x. The constraints imposed by the amount of non-heme iron and labile sulfide in the Photosystem I core protein, the cysteine content of the psaA and psaB polypeptides, and the stoichiometry of high-molecular-mass polypeptides, cause us to re-examine the possibility that F_x is a [4Fe-4S] rather than a [2Fe-2S] cluster ligated by homologous cysteine residues on the psaA and psaB heterodimer.